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  • American Institute of Physics (AIP)  (10.646)
  • Nature Publishing Group
  • 2010-2014  (16)
  • 2000-2004  (10.654)
  • 2010  (16)
  • 2000  (10.654)
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  • 2010-2014  (16)
  • 2000-2004  (10.654)
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  • 1
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    Ancestry of motor innervation to pectoral fin and forelimb (2010)
    Nature Publishing Group
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    Publikationsdatum: 2022-05-25
    Beschreibung: © Macmillan Publishers Limited, 2010. This article is distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works 3.0 Unported License. The definitive version was published in Nature Communications 1 (2010): 49, doi:10.1038/ncomms1045.
    Beschreibung: Motor innervation to the tetrapod forelimb and fish pectoral fin is assumed to share a conserved spinal cord origin, despite major structural and functional innovations of the appendage during the vertebrate water-to-land transition. In this paper, we present anatomical and embryological evidence showing that pectoral motoneurons also originate in the hindbrain among ray-finned fish. New and previous data for lobe-finned fish, a group that includes tetrapods, and more basal cartilaginous fish showed pectoral innervation that was consistent with a hindbrain-spinal origin of motoneurons. Together, these findings support a hindbrain–spinal phenotype as the ancestral vertebrate condition that originated as a postural adaptation for pectoral control of head orientation. A phylogenetic analysis indicated that Hox gene modules were shared in fish and tetrapod pectoral systems. We propose that evolutionary shifts in Hox gene expression along the body axis provided a transcriptional mechanism allowing eventual decoupling of pectoral motoneurons from the hindbrain much like their target appendage gained independence from the head.
    Beschreibung: Th is work was supported by the National Institutes of Health and National Science Foundation.
    Repository-Name: Woods Hole Open Access Server
    Materialart: Article
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  • 2
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    Hydrothermal contribution to the oceanic dissolved iron inventory (2010)
    Nature Publishing Group
    In:  Nature Geoscience, 3 (4). pp. 252-256.
    Details
    Publikationsdatum: 2017-08-28
    Beschreibung: Iron limits phytoplankton growth and hence the biological carbon pump in the Southern Ocean1. Models assessing the impacts of iron on the global carbon cycle generally rely on dust input and sediment resuspension as the predominant sources2, 3. Although it was previously thought that most iron from deep-ocean hydrothermal activity was inaccessible to phytoplankton because of the formation of particulates4, it has been suggested that iron from hydrothermal activity5, 6, 7 may be an important source of oceanic dissolved iron8, 9, 10, 11, 12, 13. Here we use a global ocean model to assess the impacts of an annual dissolved iron flux of approximately 9×108 mol, as estimated from regional observations of hydrothermal activity11, 12, on the dissolved iron inventory of the world’s oceans. We find the response to the input of hydrothermal dissolved iron is greatest in the Southern Hemisphere oceans. In particular, observations of the distribution of dissolved iron in the Southern Ocean3 (Chever et al., manuscript in preparation; Bowie et al., manuscript in preparation) can be replicated in our simulations only when our estimated iron flux from hydrothermal sources is included. As the hydrothermal flux of iron is relatively constant over millennial timescales14, we propose that hydrothermal activity can buffer the oceanic dissolved iron inventory against shorter-term fluctuations in dust deposition.
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  • 3
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    Magnitude of oceanic nitrogen fixation influenced by the nutrient uptake ratio of phytoplankton (2010)
    Nature Publishing Group
    In:  Nature Geoscience, 3 (6). pp. 412-416.
    Details
    Publikationsdatum: 2017-12-21
    Beschreibung: The elemental stoichiometry of sea water and particulate organic matter is remarkably similar. This observation led Redfield to hypothesize that the oceanic ratio of nitrate to phosphate is controlled by the remineralization of phytoplankton biomass1. The Redfield ratio is used universally to quantitatively link the marine nitrogen and phosphorus cycles in numerous biogeochemical applications2,3,4. Yet, empirical and theoretical studies show that the ratio of nitrogen to phosphorus in phytoplankton varies greatly with taxa5,6 and growth conditions7,8,9. Here we present a dynamic five-box ecosystem model showing that non-Redfield utilization of dissolved nitrogen and phosphorus by non-nitrogen-fixing phytoplankton controls the magnitude and distribution of nitrogen fixation. In our simulations, systems dominated by rapidly growing phytoplankton with low nitrogen to phosphorus uptake ratios reduce the phosphorus available for nitrogen fixation. In contrast, in systems dominated by slow-growing phytoplankton with high nitrogen to phosphorus uptake ratios nitrogen deficits are enhanced, and nitrogen fixation is promoted. We show that estimates of nitrogen fixation are up to fourfold too high when non-Redfield uptake stoichiometries are ignored. We suggest that the relative abundance of fast- and slow-growing phytoplankton controls the amount of new nitrogen added to the ocean.
    Materialart: Article , PeerReviewed
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  • 4
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    Regional climate response to solar-radiation management (2010)
    Nature Publishing Group
    In:  Nature Geoscience, 3 (8). pp. 537-541.
    Details
    Publikationsdatum: 2015-06-02
    Beschreibung: Concerns about the slow pace of climate mitigation have led to renewed dialogue about solar-radiation management, which could be achieved by adding reflecting aerosols to the stratosphere1, 2, 3, 4, 5, 6. Modelling studies suggest that solar-radiation management could produce stabilized global temperatures and reduced global precipitation4, 5, 6. Here we present an analysis of regional differences in a climate modified by solar-radiation management, using a large-ensemble modelling experiment that examines the impacts of 54 scenarios for global temperature stabilization. Our results confirm that solar-radiation management would generally lead to less extreme temperature and precipitation anomalies, compared with unmitigated greenhouse gas emissions. However, they also illustrate that it is physically not feasible to stabilize global precipitation and temperature simultaneously as long as atmospheric greenhouse gas concentrations continue to rise. Over time, simulated temperature and precipitation in large regions such as China and India vary significantly with different trajectories for solar-radiation management, and they diverge from historical baselines in different directions. Hence, it may not be possible to stabilize the climate in all regions simultaneously using solar-radiation management. Regional diversity in the response to different levels of solar-radiation management could make consensus about the optimal level of geoengineering difficult, if not impossible, to achieve.
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  • 5
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    Upper-ocean-to-atmosphere radiocarbon offsets imply fast deglacial carbon dioxide release (2010)
    Nature Publishing Group
    In:  Nature, 466 (7310). pp. 1093-1097.
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    Publikationsdatum: 2015-09-07
    Materialart: Article , PeerReviewed
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  • 6
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    Young off-axis volcanism along the ultraslow-spreading Southwest Indian Ridge (2010)
    Nature Publishing Group
    In:  Nature Geoscience, 3 (4). pp. 286-292.
    Details
    Publikationsdatum: 2016-02-16
    Beschreibung: The growth of oceanic plates at mid-ocean ridges, crustal accretion, occurs by a combination of magmatic and tectonic processes. Magmatic processes along ridges spreading at fast, intermediate and slowrates, continually add volcanic material to a centrally located spreading axis. This creates a narrowband of young volcanic rocks. However, at ridges spreading at ultraslow rates, diminished volcanism allows entire blocks of mantle to spread on the sea floor by tectonic processes. Remote imaging has advanced our observational understanding of crustal accretion, but temporal constraints are required to quantitatively understand ultraslow-spreading ridge construction. Here, we use U-series eruption ages of volcanic rocks collected from the ultraslow-spreading Southwest Indian Ridge. Unexpectedly, we find young volcanic eruption ages that are broadly dispersed throughout the rift valley, indicating that crustal accretion of young volcanic rocks is not confined to a narrow central spreading axis. As areas of young volcanism are observed close to distinct fault surfaces, we propose that the widely dispersed volcanism may result frommagma rising along faults. Our results indicate that axial-centric spreading models may not accurately describe crustal accretion at ultraslow-spreading ridges, prompting the re-evaluation of these models.
    Materialart: Article , PeerReviewed
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  • 7
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    Methods for isolation of marine-derived endophytic fungi and their bioactive secondary products (2010)
    Nature Publishing Group
    In:  Nature Protocols, 5 (3). pp. 479-490.
    Details
    Publikationsdatum: 2020-06-24
    Beschreibung: Marine-derived fungi have been shown in recent years to produce a plethora of new bioactive secondary metabolites, some of them featuring new carbon frameworks hitherto unprecedented in nature. These compounds are of interest as new lead structures for medicine as well as for plant protection. The aim of this protocol is to give a detailed description of methods useful for the isolation and cultivation of fungi associated with various marine organisms (sponges, algae and mangrove plants) for the extraction, characterization and structure elucidation of biologically active secondary metabolites produced by these marine-derived endophytic fungi, and for the preliminary evaluation of their pharmacological properties based on rapid 'in house' screening systems. Some results exemplifying the positive outcomes of the protocol are given at the end. From sampling in marine environment to completion of the structure elucidation and bioactivity screening, a period of at least 3 months has to be scheduled.
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  • 8
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    Tropical cyclones and climate change (2010)
    Nature Publishing Group
    In:  Nature Geoscience, 3 (3). pp. 157-163.
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    Publikationsdatum: 2015-09-10
    Beschreibung: Whether the characteristics of tropical cyclones have changed or will change in a warming climate — and if so, how — has been the subject of considerable investigation, often with conflicting results. Large amplitude fluctuations in the frequency and intensity of tropical cyclones greatly complicate both the detection of long-term trends and their attribution to rising levels of atmospheric greenhouse gases. Trend detection is further impeded by substantial limitations in the availability and quality of global historical records of tropical cyclones. Therefore, it remains uncertain whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes. However, future projections based on theory and high-resolution dynamical models consistently indicate that greenhouse warming will cause the globally averaged intensity of tropical cyclones to shift towards stronger storms, with intensity increases of 2–11% by 2100. Existing modelling studies also consistently project decreases in the globally averaged frequency of tropical cyclones, by 6–34%. Balanced against this, higher resolution modelling studies typically project substantial increases in the frequency of the most intense cyclones, and increases of the order of 20% in the precipitation rate within 100 km of the storm centre. For all cyclone parameters, projected changes for individual basins show large variations between different modelling studies.
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  • 9
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    Opposing decadal changes for the North Atlantic meridional overturning circulation (2010)
    Nature Publishing Group
    In:  Nature Geoscience, 3 (10). pp. 728-734.
    Details
    Publikationsdatum: 2015-10-20
    Beschreibung: The hydrographic properties of the North Atlantic Ocean changed significantly from 1950 to 2000: the subtropics warmed and became more saline, whereas the subpolar ocean cooled and freshened. These changes directly affect the storage of heat and fresh water in the ocean, but their consequences for ocean dynamics are determined by the resultant changes in seawater density. Here we use historical hydrographic data to show that the overall seawater density in the North Atlantic basin decreased during this 50-year period. As a result of these density changes, sea-surface heights changed in a spatially varying pattern with typical rates of 2 mm yr−1, in broad agreement with tide-gauge measurements. Melding the observed density fields within a numerical model we find a slight weakening in the overturning of the subtropical gyre by −1.5±1 Sv and a slight strengthening in the overturning of the subpolar gyre by +0.8±0.5 Sv. These gyre-specific changes run counter to the canonical notion of a single, basin-scale overturning cell and probably reflect interannual and decadal trends rather than any long-term climate trend. We conclude that gyre dynamics strongly affect temperature and salinity changes that translate into changes in the meridional overturning circulation.
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  • 10
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    A count in the dark (2010)
    Nature Publishing Group
    In:  Nature Geoscience, 3 (10). pp. 662-663.
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    Publikationsdatum: 2019-09-23
    Beschreibung: The Census of Marine Life has succeeded in raising awareness about marine biodiversity, and contributed much to our understanding of what lives where. But the project has fallen short of its goal to estimate species abundance.
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  • 11
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    Ocean science: Balancing ocean nitrogen (2010)
    Nature Publishing Group
    In:  Nature Geoscience, 3 (6). pp. 383-384.
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    Publikationsdatum: 2017-02-21
    Beschreibung: The ocean's nitrogen budget has escaped quantification. A modelling study shows how a small shift in the nitrate-to-phosphate uptake ratio of phytoplankton has a large effect on calculated nitrogen fixation rates.
    Materialart: Article , PeerReviewed
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  • 12
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    The September 2009 Padang earthquake (2010)
    Nature Publishing Group
    In:  Nature Geoscience, 3 (2). pp. 70-71.
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    Publikationsdatum: 2016-06-22
    Materialart: Article , PeerReviewed
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  • 13
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    A seasonal cycle in the export of bottom water from the Weddell Sea (2010)
    Nature Publishing Group
    In:  Nature Geoscience, 3 (8). pp. 551-556.
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    Publikationsdatum: 2019-09-23
    Beschreibung: Dense water formed over the Antarctic continental shelf rapidly descends into the deep ocean where it spreads throughout the global ocean as Antarctic Bottom Water1, 2. The coldest and most voluminous component of this water mass is Weddell Sea bottom water1, 3, 4, 5, 6, 7. Here we present observations over eight years of the temperature and salinity stratification in the lowermost ocean southeast of the South Orkney Islands, marking the export of Weddell Sea bottom water. We observe a pronounced seasonal cycle in bottom temperatures, with a cold pulse in May/June and a warm one in October/November, but the timing of these phases varies each year. We detect the coldest bottom water in 1999 and 2002, whereas there was no cold phase in 2000. On the basis of current velocities and water mass characteristics, we infer that the pulses originate from the southwest Weddell Sea. We propose that the seasonal fluctuations of Weddell Sea bottom-water properties are governed by the seasonal cycle of the winds over the western margin of the Weddell Sea. Interannual fluctuations are linked to the variability of the wind-driven Weddell Sea gyre and hence to large-scale climate phenomena such as the Southern Annular Mode and El Niño/Southern Oscillation.
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  • 14
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    The key nickel enzyme of methanogenesis catalyses the anaerobic oxidation of methane (2010)
    Nature Publishing Group
    In:  Nature, 465 (7298). pp. 606-608.
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    Publikationsdatum: 2016-10-07
    Beschreibung: Large amounts (estimates range from 70 Tg per year to 300 Tg per year) of the potent greenhouse gas methane are oxidized to carbon dioxide in marine sediments by communities of methanotrophic archaea and sulphate-reducing bacteria1, 2, 3, and thus are prevented from escaping into the atmosphere. Indirect evidence indicates that the anaerobic oxidation of methane might proceed as the reverse of archaeal methanogenesis from carbon dioxide with the nickel-containing methyl-coenzyme M reductase (MCR) as the methane-activating enzyme4, 5. However, experiments showing that MCR can catalyse the endergonic back reaction have been lacking. Here we report that purified MCR from Methanothermobacter marburgensis converts methane into methyl-coenzyme M under equilibrium conditions with apparent Vmax (maximum rate) and Km (Michaelis constant) values consistent with the observed in vivo kinetics of the anaerobic oxidation of methane with sulphate6, 7, 8. This result supports the hypothesis of ‘reverse methanogenesis’4, 9 and is paramount to understanding the still-unknown mechanism of the last step of methanogenesis. The ability of MCR to cleave the particularly strong C–H bond of methane without the involvement of highly reactive oxygen-derived intermediates is directly relevant to catalytic C–H activation, currently an area of great interest in chemistry10, 11, 12, 13.
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  • 15
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    External forcing as a metronome for Atlantic multidecadal variability (2010)
    Nature Publishing Group
    In:  Nature Geoscience, 3 (10). pp. 688-694.
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    Publikationsdatum: 2016-11-01
    Beschreibung: Instrumental records, proxy data and climate modelling show that multidecadal variability is a dominant feature of North Atlantic sea-surface temperature variations1–4, with potential impacts on regional climates. To understand the observed variability and to gauge any potential for climate predictions it is essential to identify the physical mechanisms that lead to this variability, and to explore the spatial and temporal characteristics of multidecadal variability modes. Here we use a coupled ocean–atmosphere general circulation model to show that the phasing of the multidecadal fluctuations in the North Atlantic during the past 600 years is, to a large degree, governed by changes in the external solar and volcanic forcings. We find that volcanoes play a particularly important part in the phasing of the multidecadal variability through their direct influence on tropical sea-surface temperatures, on the leading mode of northern-hemisphere atmosphere circulation and on the Atlantic thermohaline circulation. We suggest that the implications of our findings for decadal climate prediction are twofold: because volcanic eruptions cannot be predicted a decade in advance, longer-term climate predictability may prove challenging, whereas the systematic post-eruption changes in ocean and atmosphere may hold promise for shorter term climate prediction.
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  • 16
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    Closure threat to key museum research facility (2010)
    Nature Publishing Group
    In:  Nature, 465 (7301). p. 1005.
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    Publikationsdatum: 2017-03-06
    Materialart: Article , NonPeerReviewed
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  • 17
    Digitale Medien
    Digitale Medien
    1H NMR study of the local magnetic field gradients in evoluting porous structures. An application to cement gels (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 7621-7626 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: When placed into an external magnetic field inhomogeneous porous structures, like rocks, cement gels, ceramics, etc., exhibit strong local magnetic field gradients (LMFG), which are inherently related with the size, shape, and distribution of their pores. In this paper, we present a method of measuring LMFG in a liquid-filled porous media, provided that the fast exchange model sufficiently describes water motion in the pores, which is based solely on the measurement of the water 1H NMR spin lattice relaxation rates and the knowledge of the unrestricted diffusion coefficient. The method is applied to two hydrating white cement samples with different hydration kinetics, and the results are correlated with the time evolution of the pore structure. It is shown that the measured LMFG have extremely high values, which qualitatively reflect the sharp needlelike morphology of the internal cement gel surface. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1311973
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  • 18
    Digitale Medien
    Digitale Medien
    Triplet states in a series of Pt-containing ethynylenes (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 7627-7634 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: By use of optical steady state and time resolved spectroscopy, we studied the evolution of the triplet excited state in a series of six ethynylenic polymers of the structure [-Pt(PBu3n)2-C(Triple Bond)C-R-C(Triple Bond)C-]n where the spacer unit R is systematically varied to give optical gaps from 1.7–3.0 eV. The inclusion of platinum in the polymer backbone induces a strong spin-orbit coupling such that triplet state emission (phosphorescence) associated with the conjugated system can be detected. Throughout the series we find the S1-T1 (singlet-triplet) energy splitting to be independent of the spacer R, such that the T1 state is always 0.7±0.1 eV below the S1 state. With decreasing optical gap, the intensity and lifetime of the triplet state emission were seen to reduce in accordance with the energy gap law. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1313527
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  • 19
    Digitale Medien
    Digitale Medien
    Implementation and refinement of the modified-conductorlike screening quantum mechanical solvation model at the MP2 level (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 7511-7518 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A modified conductorlike screening continuum solvation model, implemented in the quantum chemistry program GAMESS, has been extended to second order perturbation theory (MP2). Two possible schemes have been considered: (a) the calculation of the MP2 energy using the solvated Hartree–Fock (HF) orbitals, and (b) the implementation of a double-iterative procedure where the HF density is updated with respect to the MP2 surface charges. The influence of the self-consistency of the surface charge distribution with respect to the MP2 density has been analyzed for a small dataset of 21 neutral molecules and 13 ions. In addition, the details of the distribution of surface charge density (σ profiles) and the effects of electron correlation on the accuracy of such distributions is analyzed in terms of the overall concept of deviation of continuum models from dielectric theory, leading to insights into higher order models. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1313789
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  • 20
    Digitale Medien
    Digitale Medien
    Ground state gas and solution phase conformational dynamics of polar processes: Furfural systems (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 7519-7529 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The conductorlike continuum solvation model, modified for ab initio in the quantum chemistry program GAMESS, implemented at the Møller–Plesset Order 2 (MP2) level of theory has been applied to a group of push–pull pyrrole systems to illustrate the effects of donor/acceptor and solvation on the stability and energetics of such systems. The most accurate theoretical gas and solution phase data to date has been presented for the parent furan-2-carbaldehyde (furfural) system, and predictions made for three additional analogues, thiophene-2-carbaldehyde, pyrrole2-carbaldehyde, and, cyclopentadiene-1-carbaldehyde. Solvent effects on internal rotational barriers in all systems were evaluated over six different values of dielectric, using the new method. Calculated electrostatic energies are shown to be highly sensitive to level of theory incorporated. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1313790
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  • 21
    Digitale Medien
    Digitale Medien
    Mesoscopic bead-and-spring model of hard spherical particles in a rubber matrix. I. Hydrodynamic reinforcement (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 7554-7563 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Exploiting an electrostatic analogy, we show that the elastic forces between a set of rigid particles embedded in a phantom polymer network can be represented by a simple bead-and-spring model. The beads represent the particles and the springs the rubber matrix. The model is validated by Monte Carlo simulation of rubbers filled with hard spherical particles, at volume fractions between 0.1 and 0.3. We derive both the moduli and the full stress–strain curves, under uniaxial elongation. The model reproduces and extends previous theoretical results on the so-called hydrodynamic reinforcement effect. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1311972
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  • 22
    Digitale Medien
    Digitale Medien
    Temperature dependence of electron transmission through organized organic thin films (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 7571-7577 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The temperature dependence of electron transmission through various organized organic thin films (OOTFs) was investigated. For most systems a strong dependence of the transmission efficiency on temperature was observed, even for a relatively small temperature range. The well defined structure of the OOTFs and the monitoring of the angular dependence of both the initial and the final velocities of photoelectrons were used to reveal the mechanism behind the temperature effect and further elucidate the transmission mechanism. A simple model, which assumes that the electron transmission yield is much higher along the organic chains than in any other direction, was able to reproduce the experimental observations. We find that the photoelectron transmission yield through OOTFs is extremely sensitive to the structural order in the film. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1313238
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  • 23
    Digitale Medien
    Digitale Medien
    Wave-packet dynamics in a cyanine dye molecule excited with femtosecond chirped pulses (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 7546-7553 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Femtosecond vibrational wave-packet dynamics in a cyanine molecule is observed to be strongly dependent on the chirp direction of the excitation pulse. The slow-decay component associated with an oscillatory structure, which corresponds to the excited-state lifetime and a vibrational mode of 160±10 cm−1, respectively, is measured by the femtosecond time-resolved transmission spectroscopy. The excited-state population is substantially decreased and enhanced in the cases of negatively chirped (NC) and positively chirped (PC) excitations, respectively. A quantum mechanical calculation by means of the split operator scheme is performed to reproduce the wave-packet propagation after the chirped pulse excitation. The calculation shows that the spatial distribution of the wave packet for the NC case is narrower than that for the PC case during the excitation, and that the overlap integral between the excited- and ground-state wave packets determines the efficiency of the population dumping. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1313542
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  • 24
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    Characterization of surface defects on MgO thin films by ultraviolet photoelectron and metastable impact electron spectroscopies (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 7564-7570 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy are used in this study to investigate low-defect and defective MgO(100) thin films. Unlike low-defect films, defective films exhibit a new spectroscopic feature located ∼2 eV above the top of valence band. Exposing the defective film to oxygen quenches the emission of electrons from F centers created on the surface and in the subsurface regions. Extended defects, unseen in the MIES spectra of the clean surface, are detectable using NO titration. MIES and thermal programmed desorption indicate that at ∼100 K NO adsorbs dissociatively on defects, forming N2O. Only a small fraction of the MgO surface becomes covered with N2O at ∼100 K for the low-defect MgO film indicating that N2O molecules preferentially adsorb on the extended defects. The saturation coverage of N2O increases appreciably for the defective sample. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1313239
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  • 25
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    The Yang–Yang relation and the specific heats of propane and carbon dioxide (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 7530-7545 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The Yang–Yang relation expresses the heat capacity at constant volume, CV(T,ρ), of a fluid linearly in terms of the second temperature derivatives of the pressure and the chemical potential, p″(T,ρ) and μ″(T,ρ). At a gas–liquid critical point CV diverges so, on approaching Tc from below, either pσ″(T), or μσ″(T), or both must diverge, where the subscript σ denotes the evaluation of p and μ on the phase boundary or vapor-pressure curve. However, previous theoretical and experimental studies have suggested that μσ″(T) always remains finite. To test these inferences, we present an analysis of extensive two-phase heat capacity data for propane recently published by Abdulagatov and co-workers. By careful interpolation in temperature and subsequently making linear, isothermal fits vs specific volume and vs density, we establish that the divergence is shared almost equally between the derivatives pσ″(T) and μσ″(T). A re-examination of the analysis of Gaddy and White for carbon dioxide leads to similar conclusions although the singular contribution from μσ″(T) is found to be of opposite sign and probably somewhat smaller than in propane. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1308284
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  • 26
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    Optical activity effects in second harmonic generation from anisotropic chiral thin films (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 7578-7581 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Circular-difference effects in second-harmonic generation have been used to study chiral, anisotropic thin films of a helicene derivative. For such samples, these effects arise both from the chirality of the film and from its anisotropy. We show theoretically and experimentally that there is a fundamental difference between a circular-difference effect originating from chirality and anisotropy. A method is described that distinguishes the two contributions. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1311977
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  • 27
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    Two- and N-step correlated models for the analysis of molecular dynamics trajectories of linear molecules in silicalite (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 7588-7592 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Recent molecular dynamics data on the diffusion of linear diatomic and triatomic molecules in the zeolite silicalite are analyzed in terms of a new correlated model [F. Jousse, S. M. Auerbach, and D. P. Vercauteren, J. Chem. Phys. 112, 1531 (2000)] capable to account for both first- and higher-order correlation effects. This "N-step" model reproduces very well our calculated mean square displacements and diffusion coefficients of the molecules considered. The improvements with respect to the results obtained with our previous "two-step" model [P. Demontis, J. Kärger, G. B. Suffritti, and A. Tilocca, Phys. Chem. Chem. Phys. 2, 1455 (2000)] are remarkable for all molecules except chlorine, showing that only in this case the effect of (negative) correlations spanning more than two jumps between channel intersections (∼20 Å) can be neglected. The basic trajectory analysis in terms of single- and two-step models, besides being an useful reference, provides all the input data needed for the application of the N-step model. Indeed, in its silicalite formulation, the N-step model is strongly linked to the two-step one because it calculates the probability of a sequence of jumps in the same channel by means of the correlations between any two consecutive jumps. Finally, the possibility to obtain qualitative insight into the diffusive mechanism through various kind of correlation coefficients is discussed. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1312867
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  • 28
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    Bimolecular recombination quenching in Langmuir Blodgett multilayers (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 7598-7605 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A model is developed that describes bimolecular recombination of photogenerated carriers in two dimensional systems. Carriers are free to diffuse in two dimensions and undergo bimolecular recombination, while drifting under the influence of an electric field in the third dimension. The model describes a competition between carrier loss due to transiting and loss due to bimolecular recombination. This model of recombination quenching is then used to obtain information on microscopic parameters associated with photogeneration efficiency and charge transport in organic quantum wells formed from Langmuir Blodgett films of conjugated molecules. The ratio of the intralayer to interlayer tunneling rates is found along with the quantum efficiency for photocarrier generation for two bis-phthalocyanine amphiphilic molecules. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1312868
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  • 29
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    Effect of charge transfer interaction and disorderness on transport properties of polyaniline systems (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 7593-7597 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Films of pure polyaniline and polyaniline-tetracyanoquinodinodimethane have been prepared. The optical absorption spectra of both the samples in the emeraldine base forms are explained in light of the three-dimensional exciton model. A temperature dependence study of the electrical conductivity of the samples shows an interesting crossover phenomenon (Tc=143 K) which may be considered as competition of the two opposingly directed factors, namely, charge transfer interaction and the disorderness parameter (r). While charge transfer interaction is evident from optical absorption spectroscopy, the extent of disorderness is reflected by the results of CHN microanalysis and temperature dependent electrical conductivity measurement. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1313219
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  • 30
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    A theoretical study of the chemical vapor deposition of (100) diamond: An explanation for the slow growth of the (100) surface (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 7582-7587 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: In this investigation we use B3LYP density functional theory (DFT) to investigate the CVD growth mechanism of (100) diamond. Our results are consistent with the Garrison mechanism in which the dimer-opening step involves simultaneous formation of a surface olefin and dissociation of the dimer. We calculate this step to have a barrier of 9.6 kcal/mol. The olefin is then attacked by a surface radical to form a six-membered ring. We find this reaction to be the rate-limiting step with an activation energy of 13.6 kcal/mol. This is in excellent agreement with the experimental value of 15 kcal/mol obtained by the selective growth method and XPS. The direct ring-opening and ring-closing reaction from adsorbed CH2 radical has an activation energy of 49.4 kcal/mol and does not contribute significantly to the growth rate. The barrier on larger clusters that include the effects of neighboring adsorbed hydrogen increases to 15.6 kcal/mol. Additionally, our calculated vibrational frequencies agree within 2% of experimental IR and HREELS spectra. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1311976
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  • 31
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    Digitale Medien
    Influence of electric field on interwell tunneling rate in quasi two dimensional organic quantum wells (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 7606-7612 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The tunneling rate of photocreated charge carriers between layers in Langmuir–Blodgett multilayer structures is measured indirectly using the novel technique of bimolecular recombination quenching. The tunneling rate is demonstrated to be dependent upon the applied electrostatic potential difference between the layers. This dependence is explored in light of the Marcus theory of charge transfer. That theory was developed to describe redox reactions where the driving force is supplied by a chemical potential difference between two chemically different parts of a more complex system. In the current work the electrostatic potential replaces the chemical potential as the driving potential. The field dependence of the exciton dissociation probability is also determined. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1312869
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  • 32
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    On the non-Arrhenius temperature dependence of the interwell electron tunneling rate in quasi two dimensional organic quantum wells (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 7613-7620 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The tunneling rate of photocreated charge carriers between layers in Langmuir–Blodgett multilayer structures is measured indirectly using the novel technique of bimolecular recombination quenching. The tunneling rate is measured as a function of the applied electrostatic potential difference between the layers as the temperature is varied between 300 and 4 K. This dependence is examined in light of the Marcus theory of charge transfer where the electrostatic potential replaces the chemical potential as the driving potential. The expectations of the Marcus theory are not met and the rate is effectively temperature independent, contrary to expectation. Other mechanisms are explored that may explain the lack of temperature dependence including the role of high frequency vibrations and the role of the zero point energy of those vibrations. The temperature dependence of the exciton dissociation probability is also examined. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1312870
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  • 33
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    Aggregation of water molecules: Atmospheric implications (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6652-6659 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The equilibrium constants for water oligomers ranging from dimers to cyclic hexamers are determined using Wertheim's theory of associating systems. In the present model for water, the pair potential has a spherical hard core, and tetrahedral hydrogen bonds which are represented by an energy parameter and an interaction volume. On the basis of the present theory, one predicts that in earth's troposphere, water dimers and perhaps trimers may contribute to the absorption of solar radiation, but concentrations of higher oligomers are too low to influence the optical properties of the earth's atmosphere. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1310601
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  • 34
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    Direct observation of the steric effect in Penning ionization reaction of Ar*+CHCl3→CHCl2++Cl+e−+Ar (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6673-6676 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Steric effect in the Penning ionization reaction of Ar*(3P2,0)+CHCl3→Ar+CHCl2++Cl+e− was directly observed at an average collision energy of 0.13 eV using the oriented CHCl3 molecular beam. The product CHCl2+ ions are measured for the H-end, the CCl3-end, and sideways orientations. The obtained steric opacity function reveals that the CCl3-end orientation is more favorable than the H-end orientation, and the sideways approach is found to be more favorable than the collinear approaches from both ends of the molecule. Furthermore, we confirm the good correlation between Penning ionization anisotropy and the electron density distribution of the 2a2 HOMO orbital of the CHCl3 molecule, whose electron cloud is mostly localized around the sideways. These results substantiate the electron exchange mechanism which is commonly accepted for the Penning ionization reaction, where the overlap of projectile atomic and target molecular orbital plays a key role in Penning ionization efficiency. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1311614
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  • 35
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    Full configuration interaction benchmarking of coupled-cluster models for the lowest singlet energy surfaces of N2 (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6677-6686 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The potential-energy curves for the X 1Σg+, a 1Πg, a′ 1Σu−, w 1Δu, c3 1Πu, and b 1Πu states of N2 have been investigated in full configuration interaction (FCI) and coupled-cluster response calculations. The equilibrium bond lengths, adiabatic excitation energies, and harmonic frequencies have been obtained with the coupled-cluster singles model (CCS), an approximate coupled-cluster singles and doubles model (CC2), the coupled-cluster singles and doubles model (CCSD), and an approximate coupled-cluster singles, doubles, and triples model (CC3), and subsequently compared to FCI results. The weak and strong features of the coupled-cluster models are discussed and illustrated. Overall, improvements towards FCI are obtained in the hierarchy CCS–CC2–CCSD–CC3. CC3 is always consistently better than CCSD, and for all the considered spectroscopic constants CC3 provides excellent results. Examples where the CC3 model fails are also given. The noniterative triples model, CCSDR(3), is compared to the iterative triples models CC3 and FCI. CCSDR(3) recovers the major part of the CC3 correlation contribution and is thus a cheap alternative to the CC3 model. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1311294
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  • 36
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    Photodissociation and recombination of F2 molecule in Ar54 cluster: Nonadiabatic molecular dynamics simulations (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6660-6672 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Photodissociation and recombination of an F2 molecule embedded in an Ar cluster is investigated. The electronic states involved are described by the valence bond approach for the F(2P)+F(2P) interaction, with spin–orbit coupling included and the anisotropic interactions between F and Ar atoms described by the diatomics-in-molecules (DIM) approach. The potential energy surfaces for 36 electronic states and the nonadiabatic couplings between them are constructed in this basis. The surface hopping method is used for dynamical simulations. The main results are: (i) Spin nonconserving transitions play a crucial role both in the dissociation and in the recombination dynamics. (ii) The ratio between the population of the triplet states and the population of the singlet states reaches the statistical equilibrium value of 3:1 60 fs after the photoexcitation, but the population of specific singlet and triplet states remains nonstatistical for at least 1.5 ps. (iii) Recombination on the only bound excited state (3Πu) becomes significant within 100 fs and builds up to 40% of the trajectories within 1 ps after excitation of the cluster with 4.6 eV. This is in accord with recent experiments on ClF/Ar solid, where strong emission from this state was found. (iv) 3% of recombination on the ground 1Σg state is found as well. (v) For excitation energy of 4.6 eV, the dissociation can be direct or delayed. In delayed dissociation the F photofragments hit the Ar cage more than once before escaping the cage. (vi) For excitation energy of 6.53 eV the yield of dissociation was found to be 100%, and the dissociation is direct only. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1310598
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  • 37
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    Water pair potential of near spectroscopic accuracy. I. Analysis of potential surface and virial coefficients (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6687-6701 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A new ab initio pair potential for water was generated by fitting 2510 interaction energies computed by the use of symmetry-adapted perturbation theory (SAPT). The new site–site functional form, named SAPT-5s, is simple enough to be applied in molecular simulations of condensed phases and at the same time reproduces the computed points with accuracy exceeding that of the elaborate SAPT-pp functional form used earlier [J. Chem. Phys. 107, 4207 (1997)]. SAPT-5s has been shown to quantitatively predict the water dimer spectra, see the following paper (paper II). It also gives the second virial coefficient in excellent agreement with experiment. Features of the water dimer potential energy surface have been analyzed using SAPT-5s. Average values of powers of the intermolecular separation—obtained from the ground-state rovibrational wave function computed in the SAPT-5s potential—have been combined with measured values to obtain a new empirical estimate of the equilibrium O–O separation equal to 5.50±0.01 bohr, significantly shorter than the previously accepted value. The residual errors in the SAPT-5s potential have been estimated by comparison to recent large-scale extrapolated ab initio calculations for water dimer. This estimate—together with the dissociation energy D0 computed from SAPT-5s—leads to a new prediction of the limit value of D0 equal to 1165±54 cm−1, close to but significantly more accurate than the best empirical value. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1311289
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  • 38
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    Water pair potential of near spectroscopic accuracy. II. Vibration–rotation–tunneling levels of the water dimer (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6702-6715 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Nearly exact six-dimensional quantum calculations of the vibration–rotation–tunneling (VRT) levels of the water dimer for values of the rotational quantum numbers J and K ≤2 show that the SAPT-5s water pair potential presented in the preceding paper (paper I) gives a good representation of the experimental high-resolution far-infrared spectrum of the water dimer. After analyzing the sensitivity of the transition frequencies with respect to the linear parameters in the potential we could further improve this potential by using only one of the experimentally determined tunneling splittings of the ground state in (H2O)2. The accuracy of the resulting water pair potential, SAPT-5st, is established by comparison with the spectroscopic data of both (H2O)2 and (D2O)2: ground and excited state tunneling splittings and rotational constants, as well as the frequencies of the intermolecular vibrations. © 2000 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.1311290
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  • 39
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    Theoretical evidence for a bound doubly-excited 1B2(C 1s,n→π*2) state in H2CO below the C 1s ionization threshold (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6716-6723 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The group of three lowest singlet C 1s-excited states of formaldehyde H2CO is studied theoretically. The equilibrium geometries are determined at the restricted open-shell Hartree–Fock (ROHF) level and refined total energies are obtained using the multireference configuration interaction (MRCI) approach. In agreement with an earlier prediction [Chem. Phys. 122, 9 (1988)] the second lowest singlet state, 1B2, is characterized by a doubly excited, "two particle–two hole" (2p–2h), configuration C 1s,n→π*2. Our calculations predict that H2CO in the 1B2(2p–2h) state has a stable pyramidal equilibrium structure with a barrier to inversion of 0.28 eV, the valence angle being close to 107°. The calculated length of the CO bond is 1.390 Å. The 1B2(2p–2h) state is shown to be also bound with respect to all possible dissociation and rearrangement processes. The lowest predicted dissociation energy for the 1B2 state (H2CO*→H2+CO* reaction) is 0.29 eV (6.69 kcal/mol). The rationalization of the great stability of the 1B2(2p–2h) state is the similarity of its electronic structure to that of the first singly-excited state 2A″(n) of nitroxyl radical H2NO⋅. The neighboring states 1B1(C 1s→π*) and 1A1(C 1s→3s) are characterized within the same framework. Spectroscopic implications and possibilities for the experimental identification of the 1B2(2p–2h) state are discussed. © 2000 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.1311296
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  • 40
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    Applications of a variational coupled-electron pair approach to the calculation of intermolecular interaction in the framework of the VB theory: Study of the van der Waals complex He–CH4 (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6724-6735 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A general nonorthogonal coupled electron pair approach for the evaluation of electron correlation contribution is presented in details. The self-consistent field for molecular interactions wave function is used as reference state for a multistructure valence bond (VB) calculation. The central idea of the method is the optimization of the virtual space of the VB wave function by means of a procedure very close to the independent electron pair approach (IEPA) scheme. All the orbitals employed are expanded in the basis set of their fragment so as to exclude the basis set superposition error (BSSE) in a priori fashion. As an example, the application to the study of the van der Waals complex He–CH4 is reported. The equilibrium geometry of the system occurs at a He–C distance of 3.6 Å , with the He atom pointing to the center of one of the faces of the CH4 molecule, with a well depth of 19 cm−1. The potential energy surface of the He–CH4 complex is used to determine the parameters of a potential model which is employed in close-coupling calculations of integral state-to-state cross sections for rotationally inelastic scattering of methane molecules with helium atoms. The predicted values are compared with the available experimental data. © 2000 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.1287274
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  • 41
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    A theoretical approach to the O(1D)+H2O(X 1A1) reaction: Ab initio potential energy surface and quasiclassical trajectory dynamics study (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6736-6747 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: An ab initio study of the ground potential energy surface (PES) of the O(1D)+H2O system has been performed, employing Møller–Plesset methods. From the stationary and additional points calculated, the ground PES has been modeled as a triatomic system, with an OH group of the H2O molecule treated as a single atom of 17.0 amu. The rate constant of reaction (1), O(1D)+H2O→2OH (main reaction channel), estimated from the quasiclassical trajectory (QCT) calculations is reasonably close to the recommended experimental value. For the relative translational energies explored (ET=0.234, 0.303, and 0.443 eV) and H2O at T=300 K, the QCT OH vibrational populations are in good agreement with the experimental values reported for the new OH fragment, but the QCT OH average rotational energies are in general quite larger than the experimental ones. Regarding the stereodynamics, for ET=0.234 eV there is not a clear tendency to a particular rotational alignment of the OH product with respect to the initial relative velocity vector, in agreement with experiments. The QCT results also show that nearly all reactive trajectories leading to reaction (1) take place through an insertion microscopic mechanism, which, even at the highest ET value considered (0.443 eV), is mainly (70%) a nondirect one. The collision complex has an average lifetime of about three rotational periods and a geometry around that of the HO(OH) hydrogen peroxide molecule. The QCT results concerning the microscopic mechanism of reaction (1) are in agreement with the suggested ones by the experimentalists to interpret their results. The present study should be considered as a starting point in the study of reaction (1) from which different aspects on the dynamics may be learned. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1311295
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  • 42
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    Digitale Medien
    Induced-dipole contributions to the conductivity and dielectric response of molten ZnCl2 (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6782-6787 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A molecular dynamics simulation of molten ZnCl2 with a realistic interionic potential is used to evaluate the contribution of interaction-induced dipoles to the dielectric response, or equivalently, to the conductivity. The induced dipoles are included self-consistently in the interionic potential. The contribution is found to be significant across the accessible range of frequencies, modifying various features of the spectrum ordinarily attributed to elementary translations of the ionic charges, and markedly improving agreement with experimental spectra. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1310602
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  • 43
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    Digitale Medien
    Homogeneous nucleation rates of n-pentanol in nitrogen measured in a piston-expansion tube (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6774-6781 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Homogeneous nucleation rates of n-pentanol in nitrogen are presented. They are obtained from a piston-expansion tube (pex-tube) involving the nucleation pulse method which generates a limited number of nuclei that grow into droplets. The detection of the droplets is achieved by a new counting method developed on the basis of a CCD camera in combination with a laser light sheet. Nucleation rates between 104 and 109 cm−3 s−1 are covered for three nucleation temperatures 250, 260, and 270 K. The rates are plotted as isotherms vs supersaturation. Influence of the initial expansion temperature and the nucleation pressure on the nucleation rate is identified. Comparison with results available in literature and classical nucleation theory (CNT) is provided. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1310597
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  • 44
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    Digitale Medien
    Theoretical study of the dynamics, stereodynamics, and microscopic mechanism of the O(1D)+CH4(X 1A1)→OH(X 2Π)+CH3(X 2A2″) reaction (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6748-6759 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A previously reported potential energy surface (PES) and a new barrierless PES (both based on ab initio data and describing the CH3 group as a pseudoatom) were used to study the O(1D)+CH4→OH+CH3 reaction with the quasiclassical trajectory (QCT) method. The new PES accurately reproduces the experimental rate constant values, in contrast to the previous PES. The QCT study was mainly performed at the relative translational energy (ET) resulting from the photodissociation of N2O at 193 nm (〈ET〉=0.403 eV), although the collision energy obtained from the photodissociation of O3 at 248 nm (〈ET〉=0.212 eV) was also considered. Good agreement between theory and experiment was obtained for the OH vibrational populations and for the OH rotational populations for the v′≥2 vibrational levels, while the rotational distributions for v′=0–1 are more excited than in the experiment. The QCT results at ET=0.403 eV satisfactorily reproduce the experimental kk′ angular distribution of the state-specific channel OH(v′=4, N′=8) and the corresponding ET′ distribution. For OH(v′=0, N′=5) the reproduction of these properties is poorer, especially for the ET′ distribution. At 0.403 eV the contribution of the abstraction mechanism to the reaction mode is negligible and two insertion like mechanisms (with fast or slow elimination) are found to be predominant, as suggested experimentally. The discrepancies observed between the QCT and experimental results can be explained on the basis of the defective description of the insertion/slow elimination mechanism provided by the model. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1289823
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  • 45
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    Digitale Medien
    Far-ultraviolet resonance Raman spectroscopy of nitrate ion in solution (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6760-6773 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Resonance Raman spectra are presented for the nitrate anion, NO3−, in water, ethylene glycol, methanol, and acetonitrile solution at six excitation wavelengths from 246 to 204 nm, on resonance with the lowest π→π* excitation. Absolute Raman cross sections for the CH stretches of ethylene glycol and methanol at these wavelengths are also reported. The nitrate spectra in all four solvents are dominated by fundamentals, overtones, and combination bands of the totally symmetric NO stretch (ν1) near 1043 cm−1 and the out-of-phase NO stretches (ν3) at 1340–1400 cm−1, consistent with substantial changes in NO bond length upon π-electron excitation. The intensity in ν3 and the (approximate)60 cm−1 splitting of this nominally degenerate vibration are indicative of pronounced breaking of the isolated molecules D3h symmetry by the local solvent environment. Intensity in the overtone of the out-of-plane mode (ν2) near 830 cm−1 suggests a change in the equilibrium geometry from planar to pyramidal upon electronic excitation. The absorption spectra and absolute Raman cross sections are simulated with a model that considers resonance with two orthogonally polarized electronic states whose degeneracy is broken by the locally asymmetric environment. Both solvent reorganization and geometry changes along the nitrate molecular vibrations make major contributions to the breadth of the absorption band. No differences between resonant and nonresonant linewidths are observed for the ν1 band. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1310615
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  • 46
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    Digitale Medien
    Reorientational disorder of anions in the hydrogen sulfides of sodium and potassium (NaDS and KDS) investigated by neutron diffraction between T=4 K and T=470 K (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6788-6794 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: High resolution neutron powder diffraction experiments on NaDS and KDS were carried out as a function of temperature between T=4 K and T=470 K at ROTAX (ISIS, UK). The probability density function (pdf) of the atoms was studied in detail, in particular that of deuterium. Several forms of temperature dependent reorientational disorder of the anions are the reason for structural polymorphism of these compounds. A monoclinic low (LTM), a rhombohedral middle (MTM), and a cubic high temperature (HTM) modification are the polymorphs. For the structures of the different modifications the parameters were refined by the use of split-atom models, Fourier synthesis, and/or cubic harmonics. As a main result of this study the anisotropy of the pdf of deuterium in NaDS and KDS was evaluated. It is due to large amplitudes of librations of the anions. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1310618
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  • 47
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    Digitale Medien
    Structural and thermodynamic description of supercritical argon with ab initio potentials (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6815-6821 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The self-consistent integral equation method is applied to calculate the structure and the thermodynamic properties of supercritical argon considered in a recent neutron-scattering experiment by Pfleiderer et al. [J. Chem. Phys. 111, 2641 (1999)]. Two kinds of potentials, different in nature, are used for the calculations. One is an empirical standard potential and the other is founded upon ab initio quantum chemical calculations. The small discrepancies between the two approaches are discussed, and the results are compared to the recent measurements for structure and experimental thermodynamic properties as well. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1290131
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  • 48
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    Digitale Medien
    Solid effect in the electron spin dressed state: A new approach for dynamic nuclear polarization (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6795-6802 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We describe a new type of solid effect for dynamic nuclear polarization (DNP) that is based on simultaneous, near resonant microwave (mw) and radio frequency (rf) irradiation of a coupled electron nuclear spin system. The interaction of the electron spin with the mw field is treated as an electron spin dressed state. In contrast to the customary laboratory frame solid effect, it is possible to obtain nuclear polarization with the dressed state solid effect (DSSE) even in the absence of nonsecular hyperfine coupling. Efficient, selective excitation of dressed state transitions generates nuclear polarization in the nuclear laboratory frame on a time scale of tens of μs, depending on the strength of the electron–nuclear coupling, the mw and rf offset and field strength. The experiment employs both pulsed mw and rf irradiation at a repetition rate comparable to T1e−1, where T1e is the electronic spin lattice relaxation time. The DSSE is demonstrated on a perdeuterated BDPA radical in a protonated matrix of polystyrene. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1310599
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  • 49
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    Digitale Medien
    Infrared spectra of cesium chloride aqueous solutions (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6803-6814 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The aqueous solutions of CsCl were studied at room temperature by infrared (IR) spectroscopy in the entire solubility range, 0–1200 g/L, using attenuated total reflection (ATR) sampling. The influence of anomalous dispersion on the IR–ATR spectra was evaluated by calculating the imaginary refractive index, k(ν), of each sample. Factor analysis (FA) was used to determine the number and abundance of species in the solutions. FA applied to both k(ν) spectra and IR-ATR spectra produced two principal spectra with a similar abundance of species. This result indicates that, even at high salt concentration, the optical effects do not influence the chemical analysis of IR–ATR spectra. The spectral modifications related to the salt concentrations are mainly first order. Second order effects were observed, but being weak, were not investigated. The two principal spectra are related to the two species present in the solution: pure water and CsCl–solvated water. From the latter, 2.8±0.4 water molecules were calculated to be associated with each close-bound Cs+/Cl− ion pair. In the case of KCl and NaCl aqueous solutions, both of which showed the same number of species, the number of water molecules associated to an ion pair was 5.0±0.4. That the latter number is different from that of CsCl indicates that the interaction between water molecules and ion pairs is different when cation Na or K in the chloride salt is replaced by Cs. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1290133
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  • 50
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    Digitale Medien
    Thermodynamics of heterogeneous multicomponent condensation on mixed nuclei (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6822-6830 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We consider a nucleating center consisting of both an insoluble core and soluble species and develop the thermodynamics of isothermal formation of a droplet on such a nucleus in a multicomponent vapor mixture. Two different approaches to the derivation of the free energy of droplet formation within the framework of the capillarity approximation are considered. If condensation is not barrierless, the free energy of formation describes a multidimensional free-energy surface having a "well" point and a "saddle" point. It is shown that in a strict theory, taking account of surface enrichment effects, the compositions of droplets corresponding to these two points are equal and can be found without knowing the surface tension of the droplet. For the case of no surfactants in the droplet, we extend the Kuni method of investigating the behavior of the free energy of droplet formation to the case of heterogeneous multicomponent condensation on mixed nuclei, which makes it possible to find out all the main features of the free-energy surface without explicitly knowing the free-energy itself. The theoretical results are illustrated by numerical calculations for the water–methanol condensation on mixed nuclei. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1287615
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  • 51
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    Digitale Medien
    Simulated structure, dynamics, and vibrational spectra of liquid benzene (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6851-6863 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A classical molecular dynamics simulation of liquid benzene is performed, using a potential model which allows for full molecular flexibility. The short range intermolecular radial distribution function is on average reminiscent of the crystalline structure, although practically no preferential orientation can be found for the molecules in the first coordination shell. The average cage lifetime and its vibrational dynamics are obtained from appropriate time correlation functions. The intramolecular vibrations are investigated by calculating the vibrational density of states and the infrared and Raman spectra, achieving an excellent agreement with the experimental data. Finally, the dephasing of the ν1(A1g) ring breathing mode and of the ν6(E2g) in-plane bending mode is analyzed on the basis of the Kubo dephasing function. For ν1 mode the Kubo correlation time of 516 fs agrees with the experimental value, and is consistent with a relaxation mechanism involving the cage reorganization. In contrast, ν6 has a practically pure Lorentzian line shape, with a width of 7.16 cm−1 in perfect agreement with the experimental value of 7.2 cm−1. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1290729
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  • 52
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    Digitale Medien
    Microscopic model of carbon monoxide binding to myoglobin (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6831-6850 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We present a microscopic model of carbon monoxide (CO) binding to myoglobin which reproduces the experimentally observed Arrhenius pre-exponential factor of 109 s−1 and activation enthalpy distribution centered at 12 kJ/mol. The model is based on extensive ab initio calculations of CO interacting with a model heme-imidazole group which we performed using a fully quantum mechanical Hartree–Fock/density functional theory (HF/DFT) hybrid method. We fit the HF/DFT calculated energies, obtained for over 1000 heme-CO structures with varied CO and iron positions and orientations for both high (S=2) and low (S=0) spin states, to a model potential function which includes a bonding interaction in both of the spin states, electrostatic, and anisotropic Lennard-Jones-type interactions. By combining the x-ray determined protein structure with this potential and protein-CO interactions and internal heme interaction potentials obtained from established molecular dynamics literature, we calculate the energy required for the CO to reach the spin crossing from the heme pocket. We find that the transition between the two spin states occurs when CO and iron have activation enthalpies of 8 kJ/mol and 3 kJ/mol, respectively, which are necessary to move CO towards the iron and the iron atom relative to the heme plane Npyr. At the same time we find that 1 kJ/mol is needed to move Nε of His-64 and Cγ of Val-68 relative to the heme group. The requirement that these motions be synchronized reduces the Arrhenius pre-exponential by a factor of 150 from the 1012 s−1 obtained from CO motion across the heme pocket, leaving a factor of ∼ 6 to account for CO orientation and nonadiabaticity of the electronic spin change. The observed width of the enthalpy distribution is reproduced by assuming a Gaussian distribution of the heme positions with a standard deviation of 0.2 Å. We characterize the conformational relaxation by calculating an enthalpy barrier using x-ray structures of myoglobin in both the MbCO photoproduct and deoxy conformations, and we find a small difference, ∼ 5 kJ/mol, between the two conformations. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1309524
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  • 53
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    Digitale Medien
    Potential-dependence of CO adlayer structures on Pt(111) electrodes in acid solution: Evidence for a site selective charge transfer (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6864-6874 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The oxidation of carbon monoxide on a Pt(111) electrode surface is studied under transient and stationary reaction conditions in a 0.1 M HClO4 aqueous solution as a function of the applied electrode potential. The optical response is investigated by optical second harmonic generation (SHG) at 280 nm and infrared absorption spectroscopy (IRAS) starting at low potentials where the electrode is not reactive [e.g., 0.1 V vs the reversible hydrogen electrode (RHE)] towards more positive potentials where CO oxidation is initiated. For transient reaction conditions, in the absence of CO dissolved in solution, the oxidation starts at about 0.5 V vs. RHE and consists of a fast oxidation of about 10% of the adlayer and of a second reaction with slower kinetics involving the removal of the complete CO layer (overlayer stripping). Under steady-state conditions in CO-saturated solution the CO adlayer is stable up to 0.9 V vs RHE. At 0.63 V an overlayer phase transition is indicated by a 20% increase of the isotropic component of the second harmonic (SH) intensity (pp-polarization). The same potential region for this phase transition in the adlayer structure is derived from IRAS spectra after correcting for the effect of the thin layer electrolyte in IRAS measurements. The disappearance of hollow sites, the appearance of bridge sites, as well as an increased occupation of on-top sites at 0.63 V is interpreted as corresponding to the adlayer phase transition. The observations are consistent with a transition from the c(2×2) to the ((square root of)19×(square root of)19) adlayer structure of CO. The high sensitivity of SHG with regard to structural phase transitions of the CO adlayer is explained by distinct charge transfer contributions to the second-order surface susceptibility at different coordination sites. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1310604
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  • 54
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    Digitale Medien
    Transport diffusion of argon in AlPO4-5 from equilibrium molecular dynamics simulations (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6875-6881 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Transport diffusion of argon in the unidirectional channels of the molecular sieve AlPO4-5 has been studied using molecular dynamics simulations. Using the Green–Kubo formalism, this nonequilibrium property is, for the first time, extracted from just one equilibrium simulation. Apart from the computational advantages above nonequilibrium simulations, the new method also provides a way to check the validity of the assumption of linear response, which is at the basis of both methods. The transport diffusion coefficient for argon at 87 K and half the maximum loading is found to be equal to Dt=(1.4±0.1)×10−5 cm2/s, of which approximately 20% can be attributed to correlated, collective motion. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1310663
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  • 55
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    Digitale Medien
    The HCO2 potential energy surface: Stationary point energetics and the HOCO heat of formation (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 5138-5140 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The energies of six stationary points on the OH+CO→HOCO→H+CO2 potential energy surface have been calculated using the G3 and CBS-QB3 methods. An analysis combining ab initio and experimental enthalpies yielded ΔHf298 K (trans-HOCO)=−42.9±1.5 kcal mol−1 (−43.8±1.4 kcal mol−1) at the G3(CBS-QB3) level of theory. These results confirm the revised HOCO heat of formation derived from photoionization spectroscopy and suggest that the HOCO potential well is 8.8 kcal mol−1 shallower than previously thought. We discuss the implications of these results for accurate Rice–Ramsperger–Kassel–Marcus modeling or quantum mechanical scattering calculations of the OH+CO reaction. © 2000 American Institute of Physics.
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    URL: http://dx.doi.org/10.1063/1.1312824
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  • 56
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    Digitale Medien
    Evidence for orientational tunneling of CO intercalated in C60: A nuclear magnetic resonance study (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 5141-5144 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We characterize the low-temperature dynamics of CO intercalated in C60 using NMR spectroscopy. CO in C60 is found to be dynamically inhomogeneous below 30 K: The 13CO line shapes reflect a dynamic disorder to static disorder transition, with only quantum tunneling among equivalent orientations in a local S6 symmetry potential remaining. The increased hindrance of the CO motion cannot be reconciled with common expectations of a homogeneous, thermally activated jumplike reorientation process, but is well accounted for in a model of orientational pinning due to asymmetric distortions of the cage potential. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1312866
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  • 57
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    Accurately solving the electronic Schrödinger equation of atoms and molecules by extrapolating to the basis set limit. I. The helium dimer (He2) (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 5145-5153 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A new formula, EL−E∝(L+3/4)−3, to extrapolate energies, EL (that arise when the basis set is truncated at a finite angular momentum quantum number, L) to the limit, E, is derived and applied to the computation of the pair potential of He. Large basis sets up to d-aug-cc-pV5Z and -6Z are used, and in addition, a new cc-pV7Z set is presented. The full-CI is approximated using the "multireference averaged coupled-pair functional" (MR-ACPF) with 121 references. The calculated molecular constants of He2 are in excellent agreement with those recently obtained with r12-MR-ACPF [R. J. Gdanitz, Mol. Phys. 96, 1423 (1999)], but they agree only fairly with the complete-CI estimate of van Mourik and Dunning [J. Chem. Phys. 111, 9248 (1999)]. The potential of Komasa [J. Chem. Phys. 110, 7909 (1999)] which has been calculated with the "exponentially correlated Gaussians" method does not give a bound state. The sensitivity of the molecular constants 〈R〉 and D0 to errors of the interaction potential at different distances is estimated by perturbing the potential by Gaussian functions. The region of 5(approximately-less-than)R/a0(approximately-less-than)9 is found to be most sensitive. From this analysis, doubts arise that recent calculations (including the present one) are accurate enough to allow the molecular constants to be determined to better than (approximate)10%. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1290001
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  • 58
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    Digitale Medien
    The quantum vibrational dynamics of Cl−(H2O)n clusters (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 5171-5178 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The centroid molecular dynamics technique is applied to the case of chloride–water clusters to estimate their finite temperature quantum vibrational structure. We employ the flexible RWK2 water potential [J. R. Reimers, R. O. Watts, and M. L. Klein, Chem. Phys. 64, 95 (1982)] and the parametrization of a chloride–water interaction potential of Dorsett, Watts and Xantheas [J. Phys. Chem. A 103, 3351 (1999)]. We then investigate the temperature-dependent vibrational structure (infrared spectra). We find that the centroid molecular dynamics technique is capable of recovering a majority of the red shift associated with hydrogen bonding. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1290132
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  • 59
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    Digitale Medien
    CC2 excitation energy calculations on large molecules using the resolution of the identity approximation (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 5154-5161 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A new implementation of the approximate coupled cluster singles and doubles method CC2 is reported, which is suitable for large scale integral-direct calculations. It employs the resolution of the identity (RI) approximation for two-electron integrals to reduce the CPU time needed for calculation and I/O of these integrals. We use a partitioned form of the CC2 equations which eliminates the need to store double excitation cluster amplitudes. In combination with the RI approximation this formulation of the CC2 equations leads to a reduced scaling of memory and disk space requirements with the number of correlated electrons (n) and basis functions (N) to, respectively, O(N2) and O(nN2), compared to O(n2N2) in previous implementations. The reduced CPU, memory and disk space requirements make it possible to perform CC2 calculations with accurate basis sets on large molecules, which would not be accessible with conventional implementations of the CC2 method. We present an application to vertical excitation energies of alkenes C2nH2n+2, for n=1–12, and report results for the lowest lying dipole-allowed transitions for the TZVPP basis sets, which for n=12 contain 1108 basis functions. Comparison with conventional CC2 results for the smaller alkenes show that for CC2 ground state energies and for excitation energies of valence states, the error due to the RI approximation is negligible compared to the usual basis set error, if auxiliary basis sets are used, which have been optimized for MP2 energy calculations. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1290013
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    A four-site model for elongated liquid crystal molecules (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6943-6949 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Molecular dynamics simulation is performed for linear liquid crystal molecules using a four-site model. The model consists of four Lennard-Jones centers which are bonded together via central potentials in a shish-kebab shape. When the molecules are left under their own vapor pressure, they exhibit isotropic and smectic B phases with no intervening nematic phase. A wall consisting of frozen Lennard-Jones centers is shown to anchor the molecules and to widen the range of the smectic regime. This demonstrates that the model can be used for molecular dynamics studies on liquid crystals in various polymer matrices. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1311592
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    Molecular simulation study of water–methanol mixtures in activated carbon pores (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6933-6942 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We report a theoretical study of the adsorption behavior of water–methanol mixtures in slit activated carbon micropores. The adsorption isotherms are obtained for a pore of width 2 nm at a temperature of 298 K from grand canonical ensemble Monte Carlo simulations. The water molecules are modeled using the four point transferable intermolecular potential functions (TIP4P) and methanol by the optimized potentials for liquid simulations (OPLS). Carboxyl (COOH) groups are used as active sites on a structured carbon surface. The effect of the relative contributions from dispersion and hydrogen bonding interactions of adsorbates, and of the chemical activation of adsorbents on adsorption behavior is investigated. The adsorption of the mixture components in activated carbon pores occurs by continuous filling, without the sharp capillary condensation observed in graphite pores. Water is preferentially adsorbed over methanol in activated carbon pores for a wide range of pressures, except at lower pressures. The hydrophilic nature of activated carbon pores results in the complexation of both water and methanol molecules with the active sites on the surfaces, leading to bulklike water behavior over the entire pore width. Solvation forces are also calculated as a function of pore size. The negative values found for the solvation force for all pore sizes reflect the hydrophilic interactions of the mixtures with the activated carbon surfaces. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1309012
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  • 62
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    The temperature and polymerization effects on the relaxation time and conductivity, and the evolution of the localized motions (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6957-6965 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: To examine the manner in which molecular dynamics of a polymerizing liquid (stoichiometric amounts of 4,4′-diaminodicyclohexylamine and diglycidyl ether of bisphenol-A) evolves during thermal cycling from its (molecular) vitreous state to its fully polymerized vitreous state, calorimetry, and dielectric spectrometry were performed simultaneously in real time. The half-width of the relaxation spectrum of the liquid was relatively narrow and became narrower on heating. This was followed by an increase in the characteristic relaxation time and the spectrum became broader as polymerization occurred and reached completion. The dc conductivity initially increased and then decreased. The faster dynamics of the Johari–Goldstein relaxation in the fully polymerized state evolved as polymerization reached completion and the temperature increased. The dielectric polarization associated with this relaxation had a broad spectrum, whose half-width increased with decrease in the temperature. Its relaxation rate followed the Arrhenius equation with an activation energy of 63.4 kJ/mol. The temperature dependence of the faster relaxation did not change with the change in the overall configurational entropy of the liquid, a feature that substantiates the dynamic heterogeneity theories for the structure of the liquid and for the origin of the relaxation. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1311623
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    The writhe distribution in DNA plasmids as derived from the free energy of supercoiling (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6950-6956 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: In theoretical work on the molecule, DNA is often treated, approximately, as a naturally straight, inextensible, isotropic elastic rod of circular cross section. It is shown that, consistent with this level of approximation, there exists a general connection between the free energy of supercoiling of plasmids formed by the DNA, and the writhe distribution in plasmids having a given value of the linking number difference, ΔLk. In particular, the writhe distribution in a collection of torsionally relaxed (ΔLk=0), but non-nicked, plasmids is completely determined once the free energy of supercoiling as a function of ΔLk is known. The writhe distribution in the supercoiled plasmids characterized by any other value of ΔLk, we shall also show, is simply related to the distribution in the relaxed plasmid, and, therefore, it, too, is completely determined. These general results are illustrated for two cases: Large plasmids for which the measured free energy of supercoiling, a quadratic function of ΔLk, implies a normal writhe distribution, and miniplasmids for which a theoretical expression for the free energy of supercoiling involving the frequencies of the normal modes of vibration of a circular elastic ring has recently become available. In this latter case, the writhe distribution for supercoiled plasmids is not normal, but shows a skewness related to a property of elastic rings, namely, the loss of stability of the circular equilibrium configuration of the rings when they are twisted beyond a critical value. Such a skewed writhe distribution for miniplasmids is, according to the model, associated with a free energy of supercoiling which is not, as has been assumed, a rigorously quadratic function of ΔLk. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1310325
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    Kohn–Sham calculations using hybrid exchange-correlation functionals with asymptotically corrected potentials (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 5185-5192 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The theory is presented for asymptotically correcting the potentials of hybrid exchange-correlation functionals, i.e., those that include a fraction of orbital exchange. The Kohn–Sham equations involve a multiplicative potential due to the continuum part of the hybrid functional and a nonmultiplicative term due to the orbital exchange. In asymptotic regions the multiplicative σ-spin potential is corrected to take the form (CX−1)/r+εHOMO,σ+Iσ, where CX is the fraction of orbital exchange; εHOMO,σ is the σ-spin self-consistent highest occupied Kohn–Sham eigenvalue; and Iσ is an approximate ionization energy. For the hydrogen atom, the asymptotic correction leads to a potential that closely resembles the exact potential; the eigenvalue spectrum is intermediate between the Schrödinger and Hartree–Fock eigenvalues, reflecting the presence of orbital exchange. Kohn–Sham orbitals and eigenvalues determined from this procedure have been used to calculate singlet vertical excitation energies for CO, N2, H2CO, C2H4, and C6H6. The correction significantly improves excitation energies to Rydberg states, with mean absolute errors below 0.2 eV. However, despite including orbital exchange, the results do not represent an improvement over the results obtained by asymptotically correcting a recently developed GGA functional. The asymptotic correction is also shown to reduce static isotropic polarizabilities. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1290002
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    Fourier grid Hamiltonian multiconfigurational self-consistent-field: A method to calculate multidimensional hydrogen vibrational wavefunctions (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 5214-5227 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The Fourier Grid Hamiltonian Multiconfigurational Self-Consistent-Field (FGH-MCSCF) method for calculating vibrational wavefunctions is presented. This method is designed to calculate multidimensional hydrogen nuclear wavefunctions for use in mixed quantum/classical molecular dynamics simulations of hydrogen transfer reactions. The FGH-MCSCF approach combines a MCSCF variational method, which describes the vibrational wavefunctions as linear combinations of configurations that are products of one-dimensional wavefunctions, with a Fourier grid method that represents the one-dimensional wavefunctions directly on a grid. In this method a full configuration interaction calculation is carried out in a truncated one-dimensional wavefunction space [analogous to complete active space self-consistent-field (CASSCF) in electronic structure theory]. A state-averaged approach is implemented to obtain a set of orthogonal multidimensional vibrational wavefunctions. The advantages of the FGH-MCSCF method are that it eliminates the costly calculation of multidimensional integrals, treats the entire range of the hydrogen coordinates without bias, avoids the expensive diagonalization of large matrices, and accurately describes ground and excited state hydrogen vibrational wavefunctions. This paper presents the derivation of the FGH-MCSCF method, as well as a series of test calculations on systems comparing its performance with exact diagonalization schemes. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1289528
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    Digitale Medien
    Large amplitude vibrations in the X 2A1 state of C2B (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 5228-5234 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A three-dimensional potential energy function (PEF) of the 2A1 electronic ground state of C2B has been generated by electronic structure calculations. The PEF possesses a minimum in an isosceles triangular structure which lies 2204 cm−1 below two equivalent minima having linear equilibrium geometry. The barrier height between the minima relative to the triangular structure has been calculated to the 2383 cm−1. The nuclear motion problem has been solved variationally in Jacobi coordinates for J=0 and 1. Ten vibrational states of A1 and nine of B2 symmetry are calculated to lie below the linear minima. The permutational splitting between the (000)+ and (000)− states in the linear 12C2 11B has been calculated to be 0.064 cm−1, in 12C13C11B this is 0.530 cm−1. Above the energy of the barrier to linearity there are large amplitude vibrations with triangular structure character. In the dense stack of such states vibrational modes of the linear structure are discernible, including their permutational splittings. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1290011
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    Evidence for 1La fluorescence from jet-cooled 3-methylindole-polar solvent complexes (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 5235-5244 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The ratio of nonresonant two-photon induced fluorescence excitation spectra using circularly and linearly polarized light for jet-cooled 3-methylindole complexed with a series of increasingly basic hydrogen bond acceptors (water, methanol, ethanol, diethylether, diethylamine and triethylamine) is consistent with an avoided crossing of the two lowest excited singlet states, 1La and 1Lb. The dispersed fluorescence of these from this series also reflects the crossing, providing a definitive 1La jet-cooled fluorescence spectrum. The jet-cooled 1La fluorescence spectrum is not broad and redshifted, but has vibronic structure that agrees well with ab initio predictions and is similar to that of 3La phosphorescence. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1290030
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    Linear scaling density matrix search based on sign matrices (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6035-6041 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: This paper presents a new approach to the linear scaling evaluation of density matrices in electronic structure theory. The new approach is based on the iterative computation of a special matrix function, the sign of the matrix and its performance is compared to that of some other methods developed for similar purpose. One particular variant of the sign approach turned out to be very competitive with other linear scaling density matrix evaluation algorithms, in terms of computational time and accuracy. It is also shown that a special damping technique greatly improves the stability of self-consistent field (SCF) calculations when using density matrix purification and sign methods. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1308546
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    Dielectric relaxation of hot water (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6025-6028 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Dielectric relaxation of water for a wide range of temperature (373 K≤T≤973 K) and density (0.07 g cm−3≤d≤0.66 g cm−3) is studied with the molecular dynamics simulation technique. The evolving water electronic structure is explicitly incorporated via the truncated adiabatic basis-set representation. For a given density, polarization dynamics of water tends to become faster with increasing temperature. At reduced densities, it shows a marked deviation from a single exponential decay. Another interesting feature is that at fixed T, water dielectric relaxation becomes, in general, accelerated as the density decreases. This trend is at variance with recent microwave spectroscopy measurements at elevated temperatures and reduced densities. Possible sources for the discrepancy between the simulation and experimental results are discussed. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1315991
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    Multidimensional replica-exchange method for free-energy calculations (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6042-6051 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We have developed a new simulation algorithm for free-energy calculations. The method is a multidimensional extension of the replica-exchange method. While pairs of replicas with different temperatures are exchanged during the simulation in the original replica-exchange method, pairs of replicas with different temperatures and/or different parameters of the potential energy are exchanged in the new algorithm. This greatly enhances the sampling of the conformational space and allows accurate calculations of free energy in a wide temperature range from a single simulation run, using the weighted histogram analysis method. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1308516
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    Effect of the chromophore donor group and ferrocene doping on the dynamic range, gain, and phase shift in photorefractive polymers (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 5439-5447 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We have studied the photorefractive performance of poly(N-vinylcarbazole)-based composites doped with various concentrations of two structurally related dipolar chromophores, at 780 nm. The two chromophores had different electron donor groups, N,N-diethylamine and julolidine, respectively. Complete internal diffraction and gain coefficients 〉130 cm−1 were obtained for polymers doped with these chromophores. The polymers prepared with the chromophore having the strongest electron donor group, the julolidine group, had the largest dynamic range, but proved to be slower and had a smaller photorefractive phase shift. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1289763
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    Surface enhanced sum frequency generation of carbon monoxide adsorbed on platinum nanoparticle arrays (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 5432-5438 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Sum frequency generation (SFG) vibrational spectroscopy is used to study the adsorption of CO at ∼1 atm pressure on Pt nanoparticle arrays and Pt thin films. The SFG signal of CO adsorbed on platinum particles of 45 nm diameter is ∼10 000 times larger than from CO on smooth Pt films. The large enhancement is explained by plasmon resonance and Maxwell–Garnett theory. The Pt arrays are prepared using electron beam lithography to produce particles with uniform spacing and sizes on an oxidized Si(100) wafer. Further, as the Pt coverage increases the SFG signal shows a polarization dependence that is explained considering the dielectric properties of a metal film on a dielectric surface. In addition, SFG permits investigation of the CO adsorbed on the particles at ∼1 atm, which is not possible with most surface analytical techniques, that will allow for the study of the reaction of small molecules on surfaces relevant in heterogeneous catalysis. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1290024
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    Comparative ab initio study of the structures, energetics and spectra of X−⋅(H2O)n=1–4 [X=F, Cl, Br, I] clusters (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 5259-5272 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: X−⋅(H2O)n=1–4 [X=F, Cl, Br, I] have been studied using high level ab initio calculations. This extensive work compares the structures of the different halide water clusters and has found that the predicted minimum energy geometries for different cluster are accompanied by several other structures close to these global minima. Hence the most highly populated structures can change depending on temperature due to the entropy effect. As the potential surfaces are flat, the wide-ranging zero point vibrational effects are important at 0 K, and not only a number of low-lying energy conformers but also large amplitude motions can be important in determining structures, energies, and spectra at finite temperatures. The binding energies, ionization potentials, charge-transfer-to-solvent (CTTS) energies, and the O–H stretching frequencies are reported, and compared with the experimental data available. A distinctive difference between F−⋅(H2O)n and X−⋅(H2O)n (X=Cl, Br, I) is noted, as the former tends to favor internal structures with negligible hydrogen bonding between water molecules, while the latter favors surface structures with significant hydrogen bonding between water molecules. These characteristics are well featured in their O–H spectra of the clusters. However, the spectra are forced to be very sensitive to the temperature, which explains some differences between different spectra. In case of F−⋅(H2O)n, a significant charge transfer is noted in the S0 ground state, which results in much less significant charge transfer in the S1 excited state compared with other hydrated halide clusters which show near full charge transfers in the S1 excited states. Finally, the nature of the stabilization interactions operative in these clusters has been explained in terms of many-body interaction energies. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1290016
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    Multiple dynamical pathways in the O(1D)+CH4 reaction: A comprehensive crossed beam study (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 5287-5301 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: In this report, the O(1D)+CH4 reaction has been reinvestigated using universal crossed molecular beam methods. Angular resolved time-of-flight spectra have been measured for various reaction channels of the title reaction: OH+CH3, H+H2COH/H3CO, and H2+HCOH/H2CO. Different product angular distributions have been observed for these product channels, indicating that these reaction channels occur via distinctive dynamical pathways. This study provides an excellent example of multiple dynamical pathways in a single chemical reaction, which opens enormous opportunities in investigating the dynamics of complicated chemical reactions that are important in combustion and atmospheric chemistry, and also provides a link between kinetics studies and dynamical research. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1289462
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    Reaction dynamics of Mg(4 1S0, 3 1D2) with H2: Harpoon-type mechanism for highly excited states (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 5302-5310 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Using a pump–probe technique, the reactions of Mg(4 1S0 and 3 1D2) with H2 have been measured to yield similar rotational distributions of MgH(v=0 and 1) as that obtained for the reaction of the Mg(3 1P1) state with H2. A series of measurements is conducted to clarify that the reactions are initiated directly by these higher states, rather than occurring from the lower 3 1P1 state following radiative and collisional relaxation. The reactivity of the Mg 4 1S0 state with H2 is found to be comparable to that of the 3 1P1 state, but about three times larger than that of the 3 1D2 state. The Mg(4 1S0, 3 1D2)–H2 reactions proceed via a harpoon-type process, and are closely associated with the Mg(3 1P1)–H2 reaction coordinate through evolution of a series of surface crossings. To support our suggestion that the harpoon mechanism is involved, the cross sections of collisional deactivation by H2 for various excited states are measured. The ratios of cross sections observed for the 3 1P1, 4 1S0, and 5 1S0 state, equal to 1:2.85:4.3, are consistent with the calculated prediction of 1:2.62:4.24. The calculated cross sections are based on a simple hard sphere model with effective radii evaluated differently. Here, the effective radii for the higher states are determined from the crossing of ionic and covalent curves, while the Mg(3 1P1)–H2 radius is estimated from the nonadiabatic crossing between the reactive 1 1B2 state and the ground state. Consistency between observation and prediction confirms that the harpoon mechanism proposed in this work is plausible. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1290125
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    Rotational spectroscopy of H3P(centered ellipsis)BrCl and the systematics of intermolecular electron transfer in the series B(centered ellipsis)BrCl, where B=CO, HCN, H2O, C2H2, C2H4, H2S, NH3, and PH3 (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 5278-5286 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Ground-state rotational spectra of the isotopomers H3P(centered ellipsis)79Br35Cl, H3P(centered ellipsis)81Br35Cl, H3P(centered ellipsis)79Br37Cl, H3P(centered ellipsis)81Br37Cl, D3P(centered ellipsis)79Br35Cl, and D3P(centered ellipsis)81Br35Cl, of the phosphine–bromine monochloride complex were observed by the pulsed-jet, Fourier-transform method, incorporating a mixing nozzle to preclude reaction among the component gases. Each isotopomer exhibited a symmetric-top-type spectrum which yielded accurate values of the spectroscopic constants B0, DJ, DJK, χaa (Br), χaa (Cl), Maa (Br), and Mbb (Br) on analysis. Interpretations of the changes in the B0 values with isotopomer showed that the intermolecular bond involves P and Br, with r(P(centered ellipsis)Br)=2.869(1) Å and that the BrCl bond increases in length by ∼0.04 Å on complex formation. Changes in the halogen nuclear quadrupole coupling constants when H3P(centered ellipsis)BrCl is formed lead, with the aid of the Townes–Dailey model, to the conclusion that a fraction δi=0.100(5) of an electron is transferred from P to Br on complex formation, while the polarization of BrCl by PH3 can be viewed as the transfer of 0.128(2)e from Br to Cl, leading to a net change of −0.028(5)e in the population of the 4pz orbital of Br. The complex is only of moderate strength, with an intermolecular stretching force constant kσ=11.5 Nm−1. Values of δi, similarly determined, for the series B(centered ellipsis)BrCl, where B=CO, HCN, H2O, C2H2, C2H4, H2S, NH3, or PH3, are presented. It is shown that the variation of δi with the ionization energy IB of the Lewis base B can be described by an expression δi=Aexp(−bIB). This behavior is compared with that for the corresponding series B(centered ellipsis)ICl. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1290031
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    Structures of the linear silicon carbides SiC4 and SiC6: Isotopic substitution and Ab Initio theory (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 5311-5320 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The structures of two linear silicon carbides, SiC4 and SiC6, have been determined by a combination of isotopic substitution and large-scale coupled-cluster ab initio calculations, following detection of all of the singly substituted isotopic species in a supersonic molecular beam with a Fourier transform microwave spectrometer. Rotational constants obtained by least-squares fitting transition frequencies were used to derive experimental structures; except for those nearest the center of mass, individual bond lengths for both chains have an error of less than 0.008 Å. Accurate equilibrium structures were derived by converting the experimental rotational constants to equilibrium constants using the vibration–rotation coupling constants from coupled-cluster calculations, including connected triple substitutions. Equilibrium dipole moments and harmonic vibrational frequencies were also calculated for both chains. On the basis of the calculated vibration–rotation and l-type doubling constants, weak rotational satellites from a low-lying vibrational state of SiC4 were assigned to v6, a bending mode calculated to lie about 205 cm−1 above the ground state. A recommended ab initio equilibrium structure for SiC8 has also been established. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1290126
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  • 78
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    Structure and vibrational spectra of H+(H2O)8: Is the excess proton in a symmetrical hydrogen bond? (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 5321-5330 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The energetics, structure, and vibrational spectra of a wide variety of H+(H2O)8 structures are calculated using density functional theory and second-order Møller–Plesset ab initio methods. In these isomers of H+(H2O)8 the local environment of the excess proton sometimes resembles a symmetric H5O2+ structure and sometimes H3O+, but many structures are intermediate between these two limits. We introduce a quantitative measure of the degree to which the excess proton resembles H5O2+ or H3O+. Other bond lengths and, perhaps most useful, the position of certain vibrational bands track this measure of the symmetry in the local structure surrounding the excess proton. The general trend is for the most compact structures to have the lowest energy. However, adding zero-point energy counteracts this trend, making prediction of the most stable isomer impossible at this time. At elevated temperatures corresponding to recent experiments and atmospheric conditions (150–200 K), calculated Gibbs free energies clearly favor the least compact structures, in agreement with recent thermal simulations [Singer, McDonald, and Ojamäe, J. Chem. Phys. 112, 710 (2000)]. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1288918
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  • 79
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    An experimental and computational study of the methyl formate radical ultraviolet spectrum (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6465-6468 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Ab initio molecular orbital theory is coupled with laser flash photolysis experiments to study the UV spectrum and absorption cross section of the methyl formate radical. The vertical excitation energies for four of methyl formate radical's excited states are calculated at the MRCI level of theory. In the region between 220 and 340 nm, two electronic transitions are identified to explain the experimental UV spectrum. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1318741
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  • 80
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    Lattice and spin polarons in two dimensions (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6502-6508 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A variational approach is employed to compute the wave function of a single polaron for a two-dimensional Holstein Hamiltonian with arbitrary forms of linear particle–boson interactions and boson dispersion relations. The Toyozawa ansatz is utilized, and generalizations to multiple polarons are outlined. Applications are made to model superradiance in pseudoisocyanine bromide J-aggregates, and to calculate quasiparticle dispersion of an itinerant hole in a two-dimensional antiferromagnet. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1310326
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  • 81
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    The dynamics of the O(1D)+HD reaction: A quasiclassical trajectory multisurface study (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 5339-5353 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Integral and differential cross sections for the O(1D)+HD reaction have been obtained from adiabatic and nonadiabatic quasiclassical trajectory calculations performed on new ab initio versions of the 1A′, 1A″ and 2A′ potential energy surfaces at the collision energies of 0.089 and 0.196 eV (2.05 and 4.53 kcal/mol, respectively). Results are reported for both the OH+D and OD+H exit channels of reaction. The new data are compared with those from previous theoretical studies employing other potential energy surfaces, and are also used to simulate experimental differential cross sections obtained from recent molecular beam measurements, which are partially resolved in the internal states of the products. The comparison provides evidence that excited electronic states do participate in the title reaction at 0.196 eV, but that their contribution, particularly that of the A″ state, is overestimated by the quasiclassical trajectory (QCT) calculations employing the latest, and most accurate, potential energy surfaces. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1290127
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  • 82
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    Simultaneous optimization of exponents, centers of Gaussian-type basis functions, and geometry with full-configuration interaction wave function: Application to the ground and excited states of hydrogen molecule (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 4942-4950 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We have extended the fully variational molecular orbital (FVMO) method to the full-configuration interaction (CI) wave function (full-CI FVMO). All variational parameters in the full-CI scheme, i.e., exponents and centers in Gaussian-type function (GTF) basis set, and nuclear positions, as well as the CI coefficients, are simultaneously optimized by using their analytical gradients. We have applied the full-CI FVMO method to the ground and electronic excited states of hydrogen molecule. In the ground state, the total energy (−1.174 015 hartree) and the internuclear distance (1.4016 bohr) obtained by the full-CI FVMO calculation with [8s4p2d] GTFs agree very well with the high-level calculation by the 249 term expansion in elliptic coordinates (−1.174 476 hartree and 1.4010 bohr, respectively). The excitation energies to the 1Σu+, 1Πu, 3Σg+, and 3Πu Rydberg states calculated by the full-CI FVMO method with [8s4p2d] GTFs coincide with the experimental values within 52 cm−1. The present result can not be obtained with the conventional basis set approach because of the fact that our full-CI FVMO calculation gives an extremely accurate wave function with a relatively small number of basis functions owing to the extension of flexibility in the variational space. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1288382
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  • 83
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    Digitale Medien
    Thermodynamic properties of H2O and D2O in the critical region (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 4985-5002 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: An analysis is presented of the thermodynamic properties of H2O and D2O in the critical region in terms of a six-term Landau expansion for the Helmholtz-energy density transformed so as to incorporate crossover from Ising-type to mean-field critical behavior. It is shown that the effects of the critical fluctuations on the thermodynamic properties of H2O and D2O satisfy the principle of corresponding states, so that the amplitudes of the asymptotic scaling laws, as well as the parameters that govern the crossover from Ising-type to mean-field critical behavior, are identical for H2O and D2O. For H2O, our results provide information supplementing a formulation for calculating thermodynamic properties of H2O for general and scientific use adopted by the International Association for the Properties of Water and Steam. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1289244
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  • 84
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    Quantized Hamilton dynamics (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6557-6565 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The Hamilton approach to classical dynamics is extended to incorporate quantum effects. Quantization of the Hamilton equations of motion results in a hierarchy of equations that are equivalent to quantum mechanics in the Heisenberg form. Closure of the hierarchy gives approximations to the exact quantum dynamics. A specific dynamics algorithm is presented and tested against model applications that exhibit tunneling and zero point motion effects. The quantized Hamilton approach is found accurate, consistent, flexible, and computationally very efficient. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1290288
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  • 85
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    A test of the dependence of an optimal control field on the number of molecular degrees of freedom: HCN isomerization (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6536-6541 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: This paper reports the results of a study of the robustness of the field required to generate the isomerization reaction HCN→CNH as a function of number of degrees of freedom and the complexity of the description of the dynamics. The particular reduced state representation of the control process that is tested in this paper is the reaction path method proposed by Zhao and Rice. We show that if the description of the system dynamics includes vibrational motions perpendicular to the one-dimensional reaction path and both the interactions between those vibrations and between them and the reaction path, the fields found by the conventional computational scheme represent local optima, and none of these correspond to generating a transfer of 100% of the population from the ground vibrational state of HCN to a mixture of vibrational states of CNH. Moreover, it is very difficult to find fields that will efficiently transfer population from the ground vibrational state of HCN to particular vibrational states of CNH. Comparing the optimized control fields reported in this paper with those previously obtained using simplified versions of the reaction path reduction, one finds that the complexity (measured by the power spectra) of optimal control fields increases as the dynamical description includes more degrees of freedom and then the interactions between all of the degrees of freedom. The optimal control field generated using a simpler dynamical description is not a good guide to the optimal control field associated with a more complex dynamical description. We conclude that the reaction path method of reduction of the complexity of calculation of the optimal field required to drive a particular reaction is not likely to be useful for the design of fields with which to actively control reactions of polyatomic molecules. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1311615
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  • 86
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    Three novel high-resolution nonlinear methods for fast signal processing (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6542-6556 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Three novel nonlinear parameter estimators are devised and implemented for accurate and fast processing of experimentally measured or theoretically generated time signals of arbitrary length. The new techniques can also be used as powerful tools for diagonalization of large matrices that are customarily encountered in quantum chemistry and elsewhere. The key to the success and the common denominator of the proposed methods is a considerably reduced dimensionality of the original data matrix. This is achieved in a preprocessing stage called beamspace windowing or band-limited decimation. The methods are decimated signal diagonalization (DSD), decimated linear predictor (DLP), and decimated Padé approximant (DPA). Their mutual equivalence is shown for the signals that are modeled by a linear combination of time-dependent damped exponentials with stationary amplitudes. The ability to obtain all the peak parameters first and construct the required spectra afterwards enables the present methods to phase correct the absorption mode. Additionally, a new noise reduction technique, based upon the stabilization method from resonance scattering theory, is proposed. The results obtained using both synthesized and experimental time signals show that DSD/DLP/DPA exhibit an enhanced resolution power relative to the standard fast Fourier transform. Of the three methods, DPA is found to be the most efficient computationally. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1310612
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  • 87
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    A method whereby Onsager coefficients may be evaluated (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6573-6578 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Much of our understanding of transport phenomena is based on linear irreversible thermodynamics formalized most notably by Onsager in 1931. As with any theory based on an a priori linear assumption, coefficients appear that may not be calculated from within the theory. In the case of transport these coefficients are usually found empirically. To correctly apply a linear theory the criteria for linearity should be evaluated. In this paper, a general theory of nonequilibrium thermodynamics, statistical rate theory, is used to outline a method whereby Onsager coefficients may be evaluated for a given circumstance. For a particular osmotic transport example, expressions for the Onsager coefficients are found which are shown to satisfy Onsager's reciprocity hypothesis. Most importantly, criteria for linearity are explicitly formulated. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1289464
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  • 88
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    Reactions of iron clusters with oxygen and ethylene: Observation of particularly stable species (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6579-6584 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Iron clusters have been produced by CO2-laser-induced decomposition of iron pentacarbonyl in a flow reactor. The absorption of CO2 laser photons was achieved by using SF6 as a sensitizer. By adding an oxidizing gas, N2O, or a hydrocarbon, C2H4, molecules which are also dissociated in the laser field, the iron clusters may react with several radicals. The as-synthesized species are extracted from the reaction zone by a conical nozzle and expanded into the source chamber of a cluster beam apparatus where they are analyzed with a time-of-flight mass spectrometer. In the experiment with N2O, we observe a magic peak at m=856 amu which can be readily assigned to the particularly stable Fe13O8 cluster. If C2H4 is added to the reactant gas, the mass spectrum reveals a magic peak at mass m=884 amu. Using deuterated ethylene, the magic peak shifts by 12 amu to larger masses, indicating that the magic cluster contains 12 hydrogen atoms. With the given restrictions, we readily derive the molecular formula Fe13C12H12. Chemical stability and symmetry considerations suggest that the detailed chemical formula of the magic cluster is Fe13(C2H2)6 and that its structure corresponds to a Fe13 icosahedron with six HC(Double Bond)CH or C(Double Bond)CH2 groups bound to six pairs of the 12 iron surface atoms. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1310596
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  • 89
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    Quantum calculations of the effect of bend excitation in methane on the HCl rotational distribution in the reaction CH4+Cl→CH3+HCl (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 4495-4497 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We report novel, reduced dimensionality quantum calculations of the effect of bend excitation in methane on the rotational distribution of HCl in the reaction CH4+Cl→CH3+HCl. We use a modification of a semiempirical potential developed by Yu and Nyman in these calculations. Reaction probabilities for zero total angular momentum are obtained using an L2 wave packet method for selected HCl rotational states (in the ground vibrational state) and summed over final CH bend/stretch states in CH4. The calculations are done in two ranges of the total energy. In the low energy range only the ground bend/stretch state of CH4 is open, and in the higher energy range that state and the second excited bend state are energetically open. With just the ground state of CH4 open we find a cold, unimodal rotational distribution of HCl, in qualitative agreement with experiment. With the opening of the excited bend state we find a multimodal HCl rotational distribution. We rationalize these results using a simple Franck–Condon argument. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1308049
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  • 90
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    Fast simulation of dynamic two-dimensional nuclear magnetic resonance spectra for systems with many spins or exchange sites (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 4505-4514 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: An efficient sparse-matrix-based numerical method is constructed to simulate two-dimensional nuclear magnetic resonance spectra of many-spin systems including the effects of chemical exchange and/or relaxation. The method employs efficient numerical time propagation requiring O(22n) operations in the case of an n spin 1/2 system. Pulses are treated with a fast implementation algorithm achieving O(n22n) scaling (case of n spins 1/2). The method is tested in simulations of double-quantum-filter correlation spectroscopy and exchange spectroscopy experiments on five- and seven-spin systems with two sites. Observed scaling is consistent with the analytic predictions. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1288607
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  • 91
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    An approximate short time Laplace transform inversion method (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 4533-4548 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The "standard" numerical methods used for inverting the Laplace transform are based on a regularization of an exact inversion formula. They are very sensitive to noise in the Laplace transformed function. In this article we suggest a different strategy. The inversion formula we use is an approximate one, but it is stable with respect to noise. The new approximate expression is obtained from a short time expansion of the Bromwich inversion formula. We show that this approximate result can be significantly improved when iterated, while remaining stable with respect to noise. The iterated method is exact for the class of functions of type EmeaE. The method is applied to a harmonic model of the stilbene molecule, to a truncated exponent series, and to the flux–flux correlation function for the parabolic barrier. These examples demonstrate the utility of the method for application to problems of interest in molecular dynamics. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1288790
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  • 92
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    Multidimensional reactive rate calculations in dissipative chaotic systems (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 4558-4564 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Using concepts from transient chaos and stochastic dynamics, we develop a perturbative solution for multidimensional activated rate processes. The solution is applicable to the underdamped regime where system dynamics prevails over bath fluctuations. The baseline of the method is the partition of the multidimensional reactive flux in a chaotic system to a sum of independent fluxes in one-dimensional systems. The partition is based on the underlying dynamics of the multidimensional system. The method is fast and explains the high and low temperature dependence of multidimensional reaction rates. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1288686
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  • 93
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    The direct production of CO(v=1–9) in the reaction of O(3P) with the ethyl radical (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 4572-4580 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A new product channel that yields vibrationally excited CO(v=1–9) in the reaction of the ethyl radical with O(3P) is experimentally observed by time-resolved Fourier transform infrared emission spectroscopy. The branching ratios for the different vibrational states are estimated to be 0.21±0.06, 0.27±0.03, 0.14±0.02, 0.08±0.02, 0.07±0.02, 0.07±0.02, 0.06±0.02, 0.05±0.02, and 0.05±0.02 for v=1–9, respectively. Previously, only the CH3+H2CO, CH3CHO+H, and C2H4+OH channels were known. Kinetics tests are provided to verify that the CO is produced directly in the reaction and not from secondary chemistry. The two possible new product channels are CO+CH4+H and CO+CH3+H2. The implications of this previously unexplored reaction channel for combustion chemistry and the possible mechanisms for this reaction are discussed. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1288791
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  • 94
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    An accurate global ab initio potential energy surface for the X 1A′ electronic state of HOBr (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 4598-4612 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A global, analytical potential energy surface for the ground electronic state of HOBr has been determined using highly correlated multireference configuration interaction wave functions and explicit basis set extrapolations of large correlation consistent basis sets. The ab initio data have been fit to an analytical functional form that accurately includes both the HOBr and HBrO minima, as well as all dissociation asymptotes. Small adjustments to this surface are made based on the limited experimental data available and by indirectly taking into account the effects of spin–orbit coupling on the OH+Br dissociation channel. Vibrational energy levels are calculated variationally for both HOBr and HBrO up to the OH+Br dissociation limit using a truncation/recoupling method. The HOBr isomer is calculated to contain 708 bound vibrational energy levels, while the HBrO minimum lies above the OH+Br dissociation limit but is calculated to have 74 "quasibound," localized eigenstates. Infrared intensities for all of these vibrational transitions are also calculated using MRCI dipole moment functions. The assignment of the HOBr states is complicated by strong stretch–bend resonances even at relatively low energies. In contrast to the HOCl case, these state mixings made it particularly difficult to assign the relatively intense OH overtone bands above v1=2. The vibrational density of states of HOBr at the OH+Br dissociation limit is determined to be 0.16 states/cm−1. Comparisons to recent work on HOCl using similar methods are made throughout. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1288913
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  • 95
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    Anharmonic treatment of the lowest-energy conformers of glycine: A theoretical study (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 4629-4635 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The structure and energetics of the four lowest-energy conformers of glycine were determined at the MP2/aug-cc-pVDZ level of theory. The optimized structural parameters for these conformers agree with previous theoretical results obtained by highly correlated ab initio methods and with available experimental data. The only structure with planar heavy atom arrangement is conformer I (global minimum), the other conformers have nonplanar heavy atom arrangements. In accordance with temperature dependence studies of the vibrational spectra in various rare gas environments, conformers III and IV have small interconversion barriers to conformer I (940 and 740 cm−1). Our calculations have shown that full-dimensional anharmonic treatment is required for an accurate description of the vibrational modes in various glycine conformers. The most pronounced effect has been observed for conformer II with the intramolecular O–H⋅⋅⋅N bond. The theoretical results obtained at the MP2/aug-cc-pVDZ level reproduce quantitatively the argon matrix experiments. The calculation uses the quartic force field approximation in the framework of second-order perturbation theory. An estimate of the higher-order correction is also given. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1288914
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  • 96
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    Metastable states of negative carbon clusters: Cn−, n=2–6 (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 4662-4667 
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    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Time-dependent photodetachment spectra for small electronically and vibrationally excited negatively charged carbon clusters Cn− (n=2–6) are measured using an electrostatic ion trap. The time dependence demonstrates the presence of metastable electronic states with lifetimes in the range of 10 to 200 ms. Comparison is made with available data and theoretical calculations. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1286978
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  • 97
    Digitale Medien
    Digitale Medien
    Glass transition in chlorobenzene–decalin under pressure (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 4701-4706 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We have investigated the dielectric properties of a 17.2 mol % mixture of chlorobenzene and decalin in the supercooled and in the glassy state under hydrostatic pressure up to 400 MPa. The dielectric loss shows the well-known primary (α) relaxation and a secondary (β) relaxation the origin of which is still being debated. The glass transition temperature Tg increases linearly with pressure. The fragility is pressure independent. With increasing pressure the relaxation strength of the β process rises at the expense of the α process. Above around 160 K, where devitrification occurs at atmospheric pressure, the activation energy of the secondary relaxation process is roughly twice that at lower temperatures. Both energies are proportional to Tg, indicating a strong correlation between α and β processes. A possible origin of the secondary relaxation is rationalized by critical clusters in the melt. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1288904
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  • 98
    Digitale Medien
    Digitale Medien
    The best structural data of liquid ammonia based on the pair approximation: First-principles Monte Carlo simulation (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 4707-4712 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Monte Carlo simulations have been performed for liquid ammonia at 277 K and 1 atm based on the pair approximation with and without adjusted analytical pair potentials. The NH3–NH3 potential function used in the first simulation has been developed on the basis of ab initio dimer calculations at the Hartree–Fock level with triple zeta plus polarization function basis sets. For the second run without the pair potential, the pair interactions have been calculated directly during the simulation using the first-principles ab initio method with the same basis sets. The nitrogen–nitrogen radial distribution function (RDF) obtained from the latter case, which is considered the best structural data based on the pair approximation, shows a first peak at 3.4 Å followed by a broad shoulder ranging from 4.2 to 4.8 Å. This shoulder has been observed for the first time, theoretically, in addition to that reported experimentally at 3.7 Å. Furthermore, energetic error due to three-body effects has been examined. Its effects on the N–N RDF at short distances has been clearly detected. Sensitivity of the structural properties of the solution on the intermolecular (both pair and three-body) interactions has been extensively discussed. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1289145
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  • 99
    Digitale Medien
    Digitale Medien
    Angular distribution of ionic fragments in the dissociation of SO22+ (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6634-6639 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The photoion–photoion coincidence technique has been used to measure the angular distributions of the O++S+ and O++SO+ fragments in the dissociative double photoionization of SO2 in the 37–120 eV range with linearly polarized synchrotron radiation. The present work shows that the obtained values of the asymmetry parameter β are close to zero (−0.2≤β≤0.2) over the whole energy range of photoexcitation, indicating that the fragmentation is almost isotropic in both cases of O++S+ and O++SO+ channels. It is found that the symmetry analysis which has been partially successful for explaining the behavior of β values observed for linear molecules or for molecules with cylindrical symmetry is unsatisfactory to account for the observed energy dependence of β values in the dissociation processes of SO22+. For more quantitative understanding of β values, the dynamical effects such as asymmetric bending or stretching motion of the SO2 molecule prior to the ionic fragmentation, rather than the static effects such as electron configuration of parent ions, may provide a satisfactory model for the observed β values. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1310616
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  • 100
    Digitale Medien
    Digitale Medien
    A statistical approach for the study of singlet–triplet interactions in small polyatomic molecules (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 6640-6651 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Surface electron ejection by laser excited metastable (SEELEM) and laser induced fluorescence (LIF) spectroscopies are complementary techniques that can be employed to provide both qualitative and quantitative insights into the mechanisms of singlet–triplet coupling in small polyatomic molecules. Two qualitatively distinct singlet–triplet coupling mechanisms are examined here in order to reveal the statistical signatures by which they can best be distinguished. These interaction mechanisms are: direct coupling between a "bright" S1 state and an entire background manifold of "dark" triplet states; and "doorway-mediated" indirect coupling in which the bright state couples to the manifold of dark states through the intermediary of one doorway state of unique vibronic character. Our goal in this paper is to present and explain the basis for the effectiveness of statistical methods with which complex LIF/SEELEM spectra may be processed so as to distinguish qualitatively between these two mechanistic possibilities. The trends obtained from these statistical methods are also shown to encode information about some characteristic quantitative features of the triplet perturbers, such as their coupling strength to the bright state S1. The development of the statistical methods described in this paper was motivated by our experiments on acetylene. Acetylene has certain useful dynamical features which make it a good model system for this study. The statistical measures developed distinguish conclusively between the direct and doorway-mediated coupling schemes, because each scheme is shown here to give rise to characteristic statistical signatures in the SEELEM and LIF spectra. Qualitative results from a preliminary real data set analyzed using the statistical approach proposed here are also presented in order to demonstrate the effectiveness of these statistical measures. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1310614
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