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  • American Institute of Physics (AIP)  (81,592)
  • Nature Publishing Group  (77,305)
  • Blackwell Publishing Ltd  (68,183)
  • American Association for the Advancement of Science  (63,629)
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  • 1
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    Nature Publishing Group
    In:  EPIC3London, United Kingdom, Nature Publishing Group
    Publication Date: 2016-01-19
    Repository Name: EPIC Alfred Wegener Institut
    Type: PANGAEA Documentation , notRev
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  • 2
    Publication Date: 2022-04-01
    Description: We present a workflow to estimate geostatistical aquifer parameters from pumping test data using the Python package welltestpy. The procedure of pumping test analysis is exemplified for two data sets from the Horkheimer Insel site and from the Lauswiesen site, Germany. The analysis is based on a semi‐analytical drawdown solution from the upscaling approach Radial Coarse Graining, which enables to infer log‐transmissivity variance and horizontal correlation length, beside mean transmissivity, and storativity, from pumping test data. We estimate these parameters of aquifer heterogeneity from type‐curve analysis and determine their sensitivity. This procedure, implemented in welltestpy, is a template for analyzing any pumping test. It goes beyond the possibilities of standard methods, for example, based on Theis' equation, which are limited to mean transmissivity and storativity. A sensitivity study showed the impact of observation well positions on the parameter estimation quality. The insights of this study help to optimize future test setups for geostatistical aquifer analysis and provides guidance for investigating pumping tests with regard to aquifer statistics using the open‐source software package welltestpy.
    Description: Article impact statement: We present a workflow to infer parameters of subsurface heterogeneity from pumping test data exemplified at two sites using welltestpy.
    Description: German Federal Environmental Foundation (DBU) http://dx.doi.org/10.13039/100007636
    Keywords: ddc:551.49
    Language: English
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  • 3
    Publication Date: 2022-04-01
    Description: In designed experiments, different sources of variability and an adequate scale of measurement need to be considered, but not all approaches in common usage are equally valid. In order to elucidate the importance of sources of variability and choice of scale, we conducted an experiment where the effects of biochar and slurry applications on soil properties related to soil fertility were studied for different designs: (a) for a field‐scale sampling design with either a model soil (without natural variability) as an internal control or with composited soils, (b) for a design with a focus on amendment variabilities, and (c) for three individual field‐scale designs with true field replication and a combined analysis representative of the population of loess‐derived soils. Three silty loam sites in Germany were sampled and the soil macroaggregates were crushed. For each design, six treatments (0, 0.15 and 0.30 g slurry‐N kg−1 with and without 30 g biochar kg−1) were applied before incubating the units under constant soil moisture conditions for 78 days. CO2 fluxes were monitored and soils were analysed for macroaggregate yields and associated organic carbon (C). Mixed‐effects models were used to describe the effects. For all soil properties, results for the loess sites differed with respect to significant contributions of fixed effects for at least one site, suggesting the need for a general inclusion of different sites. Analysis using a multilevel model allowed generalizations for loess soils to be made and showed that site:slurry:biochar and site:slurry interactions were not negligible for macroaggregate yields. The use of a model soil as an internal control enabled observation of variabilities other than those related to soils or amendments. Experiments incorporating natural variability in soils or amendments resulted in partially different outcomes, indicating the need to include all important sources of variability. Highlights Effects of biochar and slurry applications were studied for different designs and mixed‐effects models were used to describe the effects. Including an internal control allowed observation of, e.g., methodological and analytical variabilities. The results suggested the need for a general inclusion of different sites. Analysis using a multilevel model allowed generalizations for loess soils. The results indicated the need to include all important sources of variability.
    Keywords: ddc:631.4
    Language: English
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  • 4
    Publication Date: 2022-04-01
    Description: Temperate forest soils are often considered as an important sink for atmospheric carbon (C), thereby buffering anthropogenic CO2 emissions. However, the effect of tree species composition on the magnitude of this sink is unclear. We resampled a tree species common garden experiment (six sites) a decade after initial sampling to evaluate whether forest floor (FF) and topsoil organic carbon (Corg) and total nitrogen (Nt) stocks changed in dependence of tree species (Norway spruce—Picea abies L., European beech—Fagus sylvatica L., pedunculate oak—Quercus robur L., sycamore maple—Acer pseudoplatanus L., European ash—Fraxinus excelsior L. and small‐leaved lime—Tilia cordata L.). Two groups of species were identified in terms of Corg and Nt distribution: (1) Spruce with high Corg and Nt stocks in the FF developed as a mor humus layer which tended to have smaller Corg and Nt stocks and a wider Corg:Nt ratio in the mineral topsoil, and (2) the broadleaved species, of which ash and maple distinguished most clearly from spruce by very low Corg and Nt stocks in the FF developed as mull humus layer, had greater Corg and Nt stocks, and narrow Corg:Nt ratios in the mineral topsoil. Over 11 years, FF Corg and Nt stocks increased most under spruce, while small decreases in bulk mineral soil (esp. in 0–15 cm and 0–30 cm depth) Corg and Nt stocks dominated irrespective of species. Observed decadal changes were associated with site‐related and tree species‐mediated soil properties in a way that hinted towards short‐term accumulation and mineralisation dynamics of easily available organic substances. We found no indication for Corg stabilisation. However, results indicated increasing Nt stabilisation with increasing biomass of burrowing earthworms, which were highest under ash, lime and maple and lowest under spruce. Highlights We studied if tree species differences in topsoil Corg and Nt stocks substantiate after a decade. The study is unique in its repeated soil sampling in a multisite common garden experiment. Forest floors increased under spruce, but topsoil stocks decreased irrespective of species. Changes were of short‐term nature. Nitrogen was most stable under arbuscular mycorrhizal species.
    Description: Deutsche Forschungsgemeinschaff (DFG)
    Keywords: ddc:551.9 ; ddc:631.41
    Language: English
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  • 5
    Publication Date: 2022-05-25
    Description: © The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Sadai, S., Condron, A., DeConto, R., & Pollard, D. Future climate response to Antarctic Ice Sheet melt caused by anthropogenic warming. Science Advances, 6(39), (2020): eaaz1169, doi:10.1126/sciadv.aaz1169.
    Description: Meltwater and ice discharge from a retreating Antarctic Ice Sheet could have important impacts on future global climate. Here, we report on multi-century (present–2250) climate simulations performed using a coupled numerical model integrated under future greenhouse-gas emission scenarios IPCC RCP4.5 and RCP8.5, with meltwater and ice discharge provided by a dynamic-thermodynamic ice sheet model. Accounting for Antarctic discharge raises subsurface ocean temperatures by 〉1°C at the ice margin relative to simulations ignoring discharge. In contrast, expanded sea ice and 2° to 10°C cooler surface air and surface ocean temperatures in the Southern Ocean delay the increase of projected global mean anthropogenic warming through 2250. In addition, the projected loss of Arctic winter sea ice and weakening of the Atlantic Meridional Overturning Circulation are delayed by several decades. Our results demonstrate a need to accurately account for meltwater input from ice sheets in order to make confident climate predictions.
    Description: This research was supported by the NSF Office of Polar Programs through NSF grant 1443347, the Biological and Environmental Research (BER) division of the U.S. Department of Energy through grant DE-SC0019263, the NSF through ICER 1664013, and by a grant to the NASA Sea Level Science Team 80NSSC17K0698.
    Repository Name: Woods Hole Open Access Server
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  • 6
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    American Association for the Advancement of Science
    Publication Date: 2022-05-25
    Description: © The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Foukal, N. P., Gelderloos, R., & Pickart, R. S. A continuous pathway for fresh water along the East Greenland shelf. Science Advances, 6(43), (2020): eabc4254, doi:10.1126/sciadv.abc4254.
    Description: Export from the Arctic and meltwater from the Greenland Ice Sheet together form a southward-flowing coastal current along the East Greenland shelf. This current transports enough fresh water to substantially alter the large-scale circulation of the North Atlantic, yet the coastal current’s origin and fate are poorly known due to our lack of knowledge concerning its north-south connectivity. Here, we demonstrate how the current negotiates the complex topography of Denmark Strait using in situ data and output from an ocean circulation model. We determine that the coastal current north of the strait supplies half of the transport to the coastal current south of the strait, while the other half is sourced from offshore via the shelfbreak jet, with little input from the Greenland Ice Sheet. These results indicate that there is a continuous pathway for Arctic-sourced fresh water along the entire East Greenland shelf from Fram Strait to Cape Farewell.
    Description: Funding for this work comes from the NSF under grant numbers OCE-1756361 and OCE-1558742 (N.P.F. and R.S.P.) and grant numbers OCE-1756863 and OAC-1835640 (R.G.).
    Repository Name: Woods Hole Open Access Server
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  • 7
    Publication Date: 2022-10-31
    Description: The statistical properties of seismicity are known to be affected by several factors such as the rheological parameters of rocks. We analysed the earthquake double-couple as a function of the faulting type. Here we show that it impacts the moment tensors of earthquakes: thrust- faulting events are characterized by higher double-couple components with respect to strike- slip- and normal-faulting earthquakes. Our results are coherent with the stress dependence of the scaling exponent of the Gutenberg-Richter law, which is anticorrelated to the double- couple. We suggest that the structural and tectonic control of seismicity may have its origin in the complexity of the seismogenic source marked by the width of the cataclastic damage zone and by the slip of different fault planes during the same seismic event; the sharper and concentrated the slip as along faults, the higher the double-couple. This phenomenon may introduce bias in magnitude estimation, with possible impact on seismic forecasting.
    Description: Published
    Description: 258
    Description: 2T. Deformazione crostale attiva
    Description: JCR Journal
    Keywords: double couple ; damage zone ; different fault type ; seismicity ; tectonics ; fault type ; seismicity ; 04.06. Seismology
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
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  • 8
    Publication Date: 2022-06-20
    Description: Between 2003-2016, the Greenland ice sheet (GrIS) was one of the largest contributors to sea level rise, as it lost about 255 Gt of ice per year. This mass loss slowed in 2017 and 2018 to about 100 Gt yr−1. Here we examine further changes in rate of GrIS mass loss, by analyzing data from the GRACE-FO (Gravity Recovery and Climate Experiment – Follow On) satellite mission, launched in May 2018. Using simulations with regional climate models we show that the mass losses observed in 2017 and 2018 by the GRACE and GRACE-FO missions are lower than in any other two year period between 2003 and 2019, the combined period of the two missions. We find that this reduced ice loss results from two anomalous cold summers in western Greenland, compounded by snow-rich autumn and winter conditions in the east. For 2019, GRACE-FO reveals a return to high melt rates leading to a mass loss of 223 ± 12 Gt month−1 during the month of July alone, and a record annual mass loss of 532 ± 58 Gt yr−1.
    Repository Name: EPIC Alfred Wegener Institut
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  • 9
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    Nature Publishing Group
    In:  EPIC3Nature Climate Change, Nature Publishing Group, 12(3), pp. 249-255
    Publication Date: 2022-06-20
    Repository Name: EPIC Alfred Wegener Institut
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  • 10
    Publication Date: 2022-09-13
    Description: © The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Liu, C.-Z., Dick, H. J. B., Mitchell, R. N., Wei, W., Zhang, Z.-Y., Hofmann, A. W., Yang, J.-F., & Li, Y. Archean cratonic mantle recycled at a mid-ocean ridge. Science Advances, 8(22), (2022): eabn6749, https://doi.org/10.1126/sciadv.abn6749.
    Description: Basalts and mantle peridotites of mid-ocean ridges are thought to sample Earth’s upper mantle. Osmium isotopes of abyssal peridotites uniquely preserve melt extraction events throughout Earth history, but existing records only indicate ages up to ~2 billion years (Ga) ago. Thus, the memory of the suspected large volumes of mantle lithosphere that existed in Archean time (〉2.5 Ga) has apparently been lost somehow. We report abyssal peridotites with melt-depletion ages up to 2.8 Ga, documented by extremely unradiogenic 187Os/188Os ratios (to as low as 0.1095) and refractory major elements that compositionally resemble the deep keels of Archean cratons. These oceanic rocks were thus derived from the once-extensive Archean continental keels that have been dislodged and recycled back into the mantle, the feasibility of which we confirm with numerical modeling. This unexpected connection between young oceanic and ancient continental lithosphere indicates an underappreciated degree of compositional recycling over time.
    Description: This study was financially supported by the National Science Fund for Distinguished Young Scholars 42025201 (to C.-Z.L.), the National Key Research and Development Project of China 2020YFA0714801 (to C.-Z.L.), the Strategic Priority Research Program of the Chinese Academy of Sciences XDA13010106 (to C.-Z.L.), the Strategic Priority Research Program of the Chinese Academy of Sciences XDB42020301 (to C.-Z.L.), and NSF grants 2114652 and 1657983 (to H.J.B.D.).
    Repository Name: Woods Hole Open Access Server
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  • 11
    Publication Date: 2022-05-27
    Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Gomaa, F., Utter, D. R., Powers, C., Beaudoin, D. J., Edgcomb, V. P., Filipsson, H. L., Hansel, C. M., Wankel, S. D., Zhang, Y., & Bernhard, J. M. Multiple integrated metabolic strategies allow foraminiferan protists to thrive in anoxic marine sediments. Science Advances, 7(22), (2021): eabf1586, https://doi.org/10.1126/sciadv.abf1586.
    Description: Oceanic deoxygenation is increasingly affecting marine ecosystems; many taxa will be severely challenged, yet certain nominally aerobic foraminifera (rhizarian protists) thrive in oxygen-depleted to anoxic, sometimes sulfidic, sediments uninhabitable to most eukaryotes. Gene expression analyses of foraminifera common to severely hypoxic or anoxic sediments identified metabolic strategies used by this abundant taxon. In field-collected and laboratory-incubated samples, foraminifera expressed denitrification genes regardless of oxygen regime with a putative nitric oxide dismutase, a characteristic enzyme of oxygenic denitrification. A pyruvate:ferredoxin oxidoreductase was highly expressed, indicating the capability for anaerobic energy generation during exposure to hypoxia and anoxia. Near-complete expression of a diatom’s plastid genome in one foraminiferal species suggests kleptoplasty or sequestration of functional plastids, conferring a metabolic advantage despite the host living far below the euphotic zone. Through a unique integration of functions largely unrecognized among “typical” eukaryotes, benthic foraminifera represent winning microeukaryotes in the face of ongoing oceanic deoxygenation.
    Description: his project was funded by the U.S. NSF IOS 1557430 and 1557566. H.L.F. acknowledges support from the Swedish Research Council VR (grant number 2017-04190).
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  • 12
    Publication Date: 2022-05-27
    Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Le Roux, V., Urann, B. M., Brunelli, D., Bonatti, E., Cipriani, A., Demouchy, S., & Monteleone, B. D. Postmelting hydrogen enrichment in the oceanic lithosphere. Science Advances, 7(24), (2021): eabf6071, https://doi.org/10.1126/sciadv.abf6071.
    Description: The large range of H2O contents recorded in minerals from exhumed mantle rocks has been challenging to interpret, as it often records a combination of melting, metasomatism, and diffusional processes in spatially isolated samples. Here, we determine the temporal variations of H2O contents in pyroxenes from a 24-Ma time series of abyssal peridotites exposed along the Vema fracture zone (Atlantic Ocean). The H2O contents of pyroxenes correlate with both crustal ages and pyroxene chemistry and increase toward younger and more refractory peridotites. These variations are inconsistent with residual values after melting and opposite to trends often observed in mantle xenoliths. Postmelting hydrogen enrichment occurred by ionic diffusion during cryptic metasomatism of peridotite residues by low-degree, volatile-rich melts and was particularly effective in the most depleted peridotites. The presence of hydrous melts under ridges leads to widespread hydrogen incorporation in the oceanic lithosphere, likely lowering mantle viscosity compared to dry models.
    Description: Funding for this study was supported by NSF EAR-P&G 1524311 and 1839128 to V.L.R. and the Andrew W. Mellon Foundation Award for Innovative Research to V.L.R. A.C. and D.B. were funded by the Italian Programma di Rilevante Interesse Nazionale PRIN 20178LPCPW and PRIN2017KY5ZX8, respectively. Revisions were performed within the duration of a “Visiting Scholar at SCIENCE 2020” award to V.L.R. (University of Copenhagen, Denmark), with support from the Department of Geosciences and Natural Resource Management, Section for Geology.
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  • 13
    Publication Date: 2022-05-27
    Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Clemens, S. C., Yamamoto, M., Thirumalai, K., Giosan, L., Richey, J. N., Nilsson-Kerr, K., Rosenthal, Y., Anand, P., & McGrath, S. M. Remote and local drivers of Pleistocene South Asian summer monsoon precipitation: a test for future predictions. Science Advances, 7(23), (2021): eabg3848, https://doi.org/10.1126/sciadv.abg3848.
    Description: South Asian precipitation amount and extreme variability are predicted to increase due to thermodynamic effects of increased 21st-century greenhouse gases, accompanied by an increased supply of moisture from the southern hemisphere Indian Ocean. We reconstructed South Asian summer monsoon precipitation and runoff into the Bay of Bengal to assess the extent to which these factors also operated in the Pleistocene, a time of large-scale natural changes in carbon dioxide and ice volume. South Asian precipitation and runoff are strongly coherent with, and lag, atmospheric carbon dioxide changes at Earth’s orbital eccentricity, obliquity, and precession bands and are closely tied to cross-equatorial wind strength at the precession band. We find that the projected monsoon response to ongoing, rapid high-latitude ice melt and rising carbon dioxide levels is fully consistent with dynamics of the past 0.9 million years.
    Description: S.C.C. and S.M.M. were supported by U.S. NSF OCE1634774. M.Y. was funded by JSPS grants JPMXS05R2900001 and 19H05595 and JAMSTEC Exp. 353 postcruise study. K.N.-K. and P.A. were supported by UK-IODP, Open University, and NERC (NE/L002493/1), K.T. was supported by the Technology and Research Initiative Fund, Arizona Board of Regents.
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  • 14
    Publication Date: 2022-05-27
    Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Seltzer, A. M., Bekaert, D. V., Barry, P. H., Durkin, K. E., Mace, E. K., Aalseth, C. E., Zappala, J. C., Mueller, P., Jurgens, B., & Kulongoski, J. T. Groundwater residence time estimates obscured by anthropogenic carbonate. Science Advances, 7(17), (2021): eabf3503, https://doi.org/10.1126/sciadv.abf3503.
    Description: Groundwater is an important source of drinking and irrigation water. Dating groundwater informs its vulnerability to contamination and aids in calibrating flow models. Here, we report measurements of multiple age tracers (14C, 3H, 39Ar, and 85Kr) and parameters relevant to dissolved inorganic carbon (DIC) from 17 wells in California’s San Joaquin Valley (SJV), an agricultural region that is heavily reliant on groundwater. We find evidence for a major mid-20th century shift in groundwater DIC input from mostly closed- to mostly open-system carbonate dissolution, which we suggest is driven by input of anthropogenic carbonate soil amendments. Crucially, enhanced open-system dissolution, in which DIC equilibrates with soil CO2, fundamentally affects the initial 14C activity of recently recharged groundwater. Conventional 14C dating of deeper SJV groundwater, assuming an open system, substantially overestimates residence time and thereby underestimates susceptibility to modern contamination. Because carbonate soil amendments are ubiquitous, other groundwater-reliant agricultural regions may be similarly affected.
    Description: his work was conducted as a part of the USGS National Water Quality Assessment Program (NAWQA) Enhanced Trends Project (https://water.usgs.gov/nawqa/studies/gwtrends/). Measurements at Argonne National Laboratory were supported by Department of Energy, Office of Science under contract DE-AC02-06CH11357. Measurements at Pacific Northwest National Laboratory were part of the Ultra-Sensitive Nuclear Measurements Initiative conducted under the Laboratory Directed Research and Development Program. PNNL is operated by Battelle for the U.S. Department of Energy under Contract DE-AC05-76RL01830. This work was also partially supported by NSF award OCE-1923915 (to A.M.S. and P.H.B. at WHOI).
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  • 15
    Publication Date: 2022-05-27
    Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Zhang, Y., Gazel, E., Gaetani, G. A., & Klein, F. Serpentinite-derived slab fluids control the oxidation state of the subarc mantle. Science Advances, 7(48), (2021): eabj2515, https://doi.org/10.1126/sciadv.abj2515.
    Description: Recent geochemical evidence confirms the oxidized nature of arc magmas, but the underlying processes that regulate the redox state of the subarc mantle remain yet to be determined. We established a link between deep subduction-related fluids derived from dehydration of serpentinite ± altered oceanic crust (AOC) using B isotopes and B/Nb as fluid proxies, and the oxidized nature of arc magmas as indicated by Cu enrichment during magma evolution and V/Yb. Our results suggest that arc magmas derived from source regions influenced by a greater serpentinite (±AOC) fluid component record higher oxygen fugacity. The incorporation of this component into the subarc mantle is controlled by the subduction system’s thermodynamic conditions and geometry. Our results suggest that the redox state of the subarc mantle is not homogeneous globally: Primitive arc magmas associated with flat, warm subduction are less oxidized overall than those generated in steep, cold subduction zones.
    Description: Y.Z. acknowledges funding from the National Science Foundation of China (91958213), the Chinese Academy of Sciences (XDB42020402), and the Shandong Provincial Natural Science Foundation, China (ZR2020QD068). This study was supported in part by the U.S. National Science Foundation NSF EAR 1826673 to E.G. and G.A.G. and OCE 1756349 to E.G.
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  • 16
    Publication Date: 2022-05-27
    Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Trembath-Reichert, E., Shah Walter, S. R., Ortiz, M. A. F., Carter, P. D., Girguis, P. R., & Huber, J. A. Multiple carbon incorporation strategies support microbial survival in cold subseafloor crustal fluids. Science Advances, 7(18), (2021): eabg0153, https://doi.org/10.1126/sciadv.abg0153.
    Description: Biogeochemical processes occurring in fluids that permeate oceanic crust make measurable contributions to the marine carbon cycle, but quantitative assessments of microbial impacts on this vast, subsurface carbon pool are lacking. We provide bulk and single-cell estimates of microbial biomass production from carbon and nitrogen substrates in cool, oxic basement fluids from the western flank of the Mid-Atlantic Ridge. The wide range in carbon and nitrogen incorporation rates indicates a microbial community well poised for dynamic conditions, potentially anabolizing carbon and nitrogen at rates ranging from those observed in subsurface sediments to those found in on-axis hydrothermal vent environments. Bicarbonate incorporation rates were highest where fluids are most isolated from recharging bottom seawater, suggesting that anabolism of inorganic carbon may be a potential strategy for supplementing the ancient and recalcitrant dissolved organic carbon that is prevalent in the globally distributed subseafloor crustal environment.
    Description: The Gordon and Betty Moore Foundation sponsored most of the observatory components at North Pond through grant GBMF1609. This work was supported by the National Science Foundation through grants NSF OCE-1745589, OCE-1635208, and OCE-1062006 to J.A.H. and NSF OCE-1635365 to P.R.G. and S.R.S.W.; NASA Postdoctoral Fellowship with the NASA Astrobiology Institute to E.T.-R.; L’Oréal USA For Women in Science Fellowship to E.T.-R.; and Woods Hole Partnership Education Program, sponsored by the Woods Hole Diversity Initiative to M.A.F.O. The Center for Dark Energy Biosphere Investigations (C-DEBI OCE-0939564) also supported the participation of J.A.H. and P.D.C. This is C-DEBI contribution number 564.
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  • 17
    Publication Date: 2022-05-27
    Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Johnson, A. C., Ostrander, C. M., Romaniello, S. J., Reinhard, C. T., Greaney, A. T., Lyons, T. W., & Anbar, A. D. Reconciling evidence of oxidative weathering and atmospheric anoxia on Archean Earth. Science Advances, 7(40), (2021): eabj0108, https://doi.org/10.1126/sciadv.abj0108.
    Description: Evidence continues to emerge for the production and low-level accumulation of molecular oxygen (O2) at Earth’s surface before the Great Oxidation Event. Quantifying this early O2 has proven difficult. Here, we use the distribution and isotopic composition of molybdenum in the ancient sedimentary record to quantify Archean Mo cycling, which allows us to calculate lower limits for atmospheric O2 partial pressures (PO2) and O2 production fluxes during the Archean. We consider two end-member scenarios. First, if O2 was evenly distributed throughout the atmosphere, then PO2 〉 10–6.9 present atmospheric level was required for large periods of time during the Archean eon. Alternatively, if O2 accumulation was instead spatially restricted (e.g., occurring only near the sites of O2 production), then O2 production fluxes 〉0.01 Tmol O2/year were required. Archean O2 levels were vanishingly low according to our calculations but substantially above those predicted for an abiotic Earth system.
    Description: We would like to thank our funding sources, including FESD “Dynamics of Earth System Oxygenation” (NSF EAR 1338810 to A.D.A.), NASA Earth and Space Science Fellowship awarded to A.C.J. (80NSSC17K0498), NSF EAR PF to A.C.J. (1952809), and WHOI Postdoctoral Fellowship to C.M.O. C.T.R. acknowledges support from the NASA Astrobiology Institute. We also acknowledge support from the Metal Utilization and Selection across Eons (MUSE) Interdisciplinary Consortium for Astrobiology Research, sponsored by the National Aeronautics and Space Administration Science Mission Directorate (19-ICAR19_2-0007).
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  • 18
    Publication Date: 2022-05-27
    Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Bekaert, D. V., Auro, M., Shollenberger, Q. R., Liu, M.-C., Marschall, H., Burton, K. W., Jacobsen, B., Brennecka, G. A., McPherson, G. J., von Mutius, R., Sarafian, A., & Nielsen, S. G. Fossil records of early solar irradiation and cosmolocation of the CAI factory: a reappraisal. Science Advances, 7(40), (2021): eabg8329, https://doi.org/10.1126/sciadv.abg8329.
    Description: Calcium-aluminum–rich inclusions (CAIs) in meteorites carry crucial information about the environmental conditions of the nascent Solar System prior to planet formation. Based on models of 50V–10Be co-production by in-situ irradiation, CAIs are considered to have formed within ~0.1 AU from the proto-Sun. Here, we present vanadium (V) and strontium (Sr) isotopic co-variations in fine- and coarse-grained CAIs and demonstrate that kinetic isotope effects during partial condensation and evaporation best explain V isotope anomalies previously attributed to solar particle irradiation. We also report initial excesses of 10Be and argue that CV CAIs possess essentially a homogeneous level of 10Be, inherited during their formation. Based on numerical modeling of 50V–10Be co-production by irradiation, we show that CAI formation during protoplanetary disk build-up likely occurred at greater heliocentric distances than previously considered, up to planet-forming regions (~1AU), where solar particle fluxes were sufficiently low to avoid substantial in-situ irradiation of CAIs.
    Description: This study was funded by NASA Emerging Worlds grant NNX16AD36G to S.G.N. and prepared by LLNL under contract DE-AC52-07NA27344 with release number LLNL-JRNL-819045. M.C.L acknowledges the support by the NASA grant 80NSSC20K0759. The UCLA ion microprobe facility is partially supported by a grant from the NSF Instrumentation and Facilities program.
    Repository Name: Woods Hole Open Access Server
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  • 19
    Publication Date: 2022-09-27
    Description: Little research attention has been given to validating clusters obtained from the groundwater geochemistry of the waterworks' capture zone with a prevailing lake‐groundwater exchange. To address this knowledge gap, we proposed a new scheme whereby Gaussian finite mixture modeling (GFMM) and Spike‐and‐Slab Bayesian (SSB) algorithms were utilized to cluster the groundwater geochemistry while quantifying the probability of the resulting cluster membership against each other. We applied GFMM and SSB to 13 geochemical parameters collected during different sampling periods at 13 observation points across the Barnim Highlands plateau located in the northeast of Berlin, Germany; this included 10 observation wells, two lakes, and a gallery of drinking production wells. The cluster analysis of GFMM yielded nine clusters, either with a probability ≥0.8, while the SSB produced three hierarchical clusters with a probability of cluster membership varying from 〈0.2 to 〉0.8. The findings demonstrated that the clustering results of GFMM were in good agreement with the classification as per the principal component analysis and Piper diagram. By superimposing the parameter clustering onto the observation clustering, we could identify discrepancies that exist among the parameters of a certain cluster. This enables the identification of different factors that may control the geochemistry of a certain cluster, although parameters of that cluster share a strong similarity. The GFMM results have shown that from 2002, there has been active groundwater inflow from the lakes towards the capture zone. This means that it is necessary to adopt appropriate measures to reverse the inflow towards the lakes.
    Description: Article impact statement: The probability of cluster membership quantified using an algorithm should be validated against another probabilistic‐based classifier.
    Description: Federal Ministry of Education and Research http://dx.doi.org/10.13039/501100002347
    Keywords: ddc:551.9 ; ddc:551.49
    Language: English
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  • 20
    Publication Date: 2022-10-01
    Description: Copper (Cu) is an essential element for plants and microorganisms and at larger concentrations a toxic pollutant. A number of factors controlling Cu dynamics have been reported, but information on quantitative relationships is scarce. We aimed to (i) quantitatively describe and predict soil Cu concentrations (CuAR) in aqua regia considering site‐specific effects and effects of pH, soil organic carbon (SOC) and cation exchange capacity (CEC), and (ii) study the suitability of mixed‐effects modelling and rule‐based models for the analysis of long‐term soil monitoring data. Thirteen uncontaminated long‐term monitoring soil profiles in southern Germany were analysed. Since there was no measurable trend of increasing CuAR concentrations with time in the respective depth ranges of the sites, data from different sampling dates were combined and horizon‐specific regression analyses including model simplifications were carried out for 10 horizons. Fixed‐ and mixed‐effects models with the site as a random effect were useful for the different horizons and significant contributions (either of main effects or interactions) of SOC, CEC and pH were present for 9, 8 and 7 horizons, respectively. Horizon‐specific rule‐based cubist models described the CuAR data similarly well. Validations of cubist models and mixed‐effects models for the CuAR concentrations in A horizons were successful for the given population after random splitting into calibration and validation samples, but not after independent validations with random splitting according to sites. Overall, site, CEC, SOC and pH provide important information for a description of CuAR concentrations using the different regression approaches. Highlights: Information on quantitative relationships for factors controlling Cu dynamics is scarce. Site, CEC, SOC and pH provide important information for a description of Cu concentrations. Validations of cubist models and mixed‐effects models for A horizons were successful for a closed population of sites.
    Description: Bavarian State Ministry of the Environment and Consumer Protection http://dx.doi.org/10.13039/501100010219
    Description: Ministry of Agriculture and Environment Mecklenburg‐Western Pomerania
    Keywords: ddc:631.4
    Language: English
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  • 21
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    American Association for the Advancement of Science
    In:  EPIC3Science, American Association for the Advancement of Science, 371(6531), pp. 811-818
    Publication Date: 2022-10-01
    Description: Geological archives record multiple reversals of Earth's magnetic poles, but the global impacts of these events, if any, remain unclear. Uncertain radiocarbon calibration has limited investigation of the potential effects of the last major magnetic inversion, known as the Laschamps Excursion 41 to 42 thousand years ago (ka). We use ancient New Zealand kauri trees (Agathis australis) to develop a detailed record of atmospheric radiocarbon levels across the Laschamps Excursion. We precisely characterize the geomagnetic reversal and perform global chemistry-climate modeling and detailed radiocarbon dating of paleoenvironmental records to investigate impacts. We find that geomagnetic field minima ~42 ka, in combination with Grand Solar Minima, caused substantial changes in atmospheric ozone concentration and circulation, driving synchronous global climate shifts that caused major environmental changes, extinction events, and transformations in the archaeological record.
    Repository Name: EPIC Alfred Wegener Institut
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  • 22
    Publication Date: 2022-09-30
    Description: In recent years, German cities were heavily impacted by pluvial flooding and related damage is projected to increase due to climate change and urbanisation. It is important to ask how to improve urban pluvial flood risk management. To understand the current state of property level adaptation, a survey was conducted in four municipalities that had recently been impacted by pluvial flooding. A hybrid framework based on the Protection Motivation Theory (PMT) and the Protection Action Decision Model (PADM) was used to investigate drivers of adaptive behaviour through both descriptive and regression analyses. Descriptive statistics revealed that participants tended to instal more low‐ and medium‐cost measures than high‐cost measures. Regression analyses showed that coping appraisal increased protection motivation, but that the adaptive behaviour also depends on framing factors, particularly homeownership. We further found that, while threat appraisal solely affects protection motivation and responsibility appraisal affects solely maladaptive thinking, coping appraisal affects both. Our results indicate that PMT is a solid starting point to study adaptive behaviours in the context of pluvial flooding, but we need to go beyond that by, for instance, considering factors of the PADM, such as responsibility, ownership, or respondent age, to fully understand this complex decision‐making process.
    Description: Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347
    Keywords: ddc:551.489 ; ddc:363.34
    Language: English
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  • 23
    Publication Date: 2022-10-04
    Description: Soil aeration is a critical factor for oxygen‐limited subsoil processes, as transport by diffusion and advection is restricted by the long distance to the free atmosphere. Oxygen transport into the soil matrix is highly dependent on its connectivity to larger pore channels like earthworm and root colonised biopores. Here we hypothesize that the soil matrix around biopores represents different connectivity depending on biopore genesis and actual coloniser. We analysed the soil pore system of undisturbed soil core samples around biopores generated or colonised by roots and earthworms and compared them with the pore system of soil, not in the immediacy of a biopore. Oxygen partial pressure profiles and gas relative diffusion was measured in the rhizosphere and drilosphere from the biopore wall into the bulk soil with microelectrodes. The measurements were linked with structural features such as porosity and connectivity obtained from X‐ray tomography and image analysis. Aeration was enhanced in the soil matrix surrounding biopores in comparison to the bulk soil, shown by higher oxygen concentrations and higher relative diffusion coefficients. Biopores colonised by roots presented more connected lateral pores than earthworm colonised ones, which resulted in enhanced aeration of the rhizosphere compared to the drilosphere. This has influenced biotic processes (microbial turnover/mineralization or root respiration) at biopore interfaces and highlights the importance of microstructural features for soil processes and their dependency on the biopore's coloniser.
    Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Keywords: ddc:631.4
    Language: English
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  • 24
    Publication Date: 2022-08-15
    Description: Anaerobic oxidation of ammonium (anammox) in oxygen minimum zones (OMZs) is a major pathway of oceanic nitrogen loss. Ammonium released from sinking particles has been suggested to fuel this process. During cruises to the Peruvian OMZ in April–June 2017 we found that anammox rates are strongly correlated with the volume of small particles (128–512 µm), even though anammox bacteria were not directly associated with particles. This suggests that the relationship between anammox rates and particles is related to the ammonium released from particles by remineralization. To investigate this, ammonium release from particles was modelled and theoretical encounters of free-living anammox bacteria with ammonium in the particle boundary layer were calculated. These results indicated that small sinking particles could be responsible for ~75% of ammonium release in anoxic waters and that free-living anammox bacteria frequently encounter ammonium in the vicinity of smaller particles. This indicates a so far underestimated role of abundant, slow-sinking small particles in controlling oceanic nutrient budgets, and furthermore implies that observations of the volume of small particles could be used to estimate N-loss across large areas.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 25
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    Unknown
    American Association for the Advancement of Science
    Publication Date: 2022-06-09
    Description: © The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Freeman, D. H., & Ward, C. P. Sunlight-driven dissolution is a major fate of oil at sea. Science Advances, 8(7), (2022): eabl7605, https://doi.org/10.1126/sciadv.abl7605.
    Description: Oxygenation reactions initiated by sunlight can transform insoluble components of crude oil at sea into water-soluble products, a process called photo-dissolution. First reported a half century ago, photo-dissolution has never been included in spill models because key parameters required for rate modeling were unknown, including the wavelength and photon dose dependence. Here, we experimentally quantified photo-dissolution as a function of wavelength and photon dose, making possible a sensitivity analysis of environmental variables in hypothetical spill scenarios and a mass balance assessment for the 2010 Deepwater Horizon (DwH) spill. The sensitivity analysis revealed that rates were most sensitive to oil slick thickness, season/latitude, and wavelength and less sensitive to photon dose. We estimate that 3 to 17% (best estimate 8%) of DwH surface oil was subject to photo-dissolution, comparable in magnitude to other widely recognized fate processes. Our findings invite a critical reevaluation of surface oil budgets for both DwH and future spills at sea.
    Description: This work was supported by the Fisheries and Oceans Canada Multi-Partner Research Initiative award to C.P.W. (project #1.06), the NSF Graduate Research Fellowship awarded to D.H.F. (award #174530), and NSF-OCE grant #1841092 to C.P.W.
    Repository Name: Woods Hole Open Access Server
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  • 26
    Publication Date: 2022-08-11
    Description: The methanogenic degradation of oil hydrocarbons can proceed through syntrophic partnerships of hydrocarbon-degrading bacteria and methanogenic archaea1,2,3. However, recent culture-independent studies have suggested that the archaeon ‘Candidatus Methanoliparum’ alone can combine the degradation of long-chain alkanes with methanogenesis4,5. Here we cultured Ca. Methanoliparum from a subsurface oil reservoir. Molecular analyses revealed that Ca. Methanoliparum contains and overexpresses genes encoding alkyl-coenzyme M reductases and methyl-coenzyme M reductases, the marker genes for archaeal multicarbon alkane and methane metabolism. Incubation experiments with different substrates and mass spectrometric detection of coenzyme-M-bound intermediates confirm that Ca. Methanoliparum thrives not only on a variety of long-chain alkanes, but also on n-alkylcyclohexanes and n-alkylbenzenes with long n-alkyl (C≥13) moieties. By contrast, short-chain alkanes (such as ethane to octane) or aromatics with short alkyl chains (C≤12) were not consumed. The wide distribution of Ca. Methanoliparum4,5,6 in oil-rich environments indicates that this alkylotrophic methanogen may have a crucial role in the transformation of hydrocarbons into methane.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 27
    Publication Date: 2022-07-25
    Description: © The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Peng, Q., Xie, S.-P., Wang, D., Huang, R. X., Chen, G., Shu, Y., Shi, J.-R., & Liu, W. Surface warming-induced global acceleration of upper ocean currents. Science Advances, 8(16), (2022): eabj8394, https://doi.org/10.1126/sciadv.abj8394.
    Description: How the ocean circulation changes in a warming climate is an important but poorly understood problem. Using a global ocean model, we decompose the problem into distinct responses to changes in sea surface temperature, salinity, and wind. Our results show that the surface warming effect, a robust feature of anthropogenic climate change, dominates and accelerates the upper ocean currents in 77% of the global ocean. Specifically, the increased vertical stratification intensifies the upper subtropical gyres and equatorial currents by shoaling these systems, while the differential warming between the Southern Ocean upwelling zone and the region to the north accelerates surface zonal currents in the Southern Ocean. In comparison, the wind stress and surface salinity changes affect regional current systems. Our study points a way forward for investigating ocean circulation change and evaluating the uncertainty.
    Description: Q.P. is supported by the National Natural Science Foundation of China (42005035), the Science and Technology Planning Project of Guangzhou (202102020935), and the Independent Research Project Program of State Key Laboratory of Tropical Oceanography (LTOZZ2102). D.W. is supported by the National Natural Science Foundation of China (92158204), and the Innovation Group Project of Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai) (311020004). S.-P.X. is supported by the National Science Foundation (AGS-1934392). Y.S. is supported by the National Key Research and Development Program of China (2016YFC1401702). G.C. is supported by National Natural Science Foundation of China (41822602). The numerical simulation is supported by the High-Performance Computing Division and HPC managers of W. Zhou and D. Sui in the South China Sea Institute of Oceanology.
    Repository Name: Woods Hole Open Access Server
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  • 28
    Publication Date: 2022-07-26
    Description: Application of farmyard manure (FYM) is common practice to improve physical and chemical properties of arable soil and crop yields. However, studies on effects of FYM application mainly focussed on topsoils, whereas subsoils have rarely been addressed so far. We, therefore, investigated the effects of 36‐year FYM application with different rates of annual organic carbon (OC) addition (0, 469, 938 and 1875 g C m−2 a−1) on OC contents of a Chernozem in 0–30 cm (topsoil) and 35–45 cm (subsoil) depth. We also investigated its effects on soil structure and hydraulic properties in subsoil. X‐ray computed tomography was used to analyse the response of the subsoil macropore system (≥19 μm) and the distribution of particulate organic matter (POM) to different FYM applications, which were related to contents in total OC (TOC) and water‐extractable OC (WEOC). We show that FYM‐C application of 469 g C m−2 a−1 caused increases in TOC and WEOC contents only in the topsoil, whereas rates of ≥938 g C m−2 a−1 were necessary for TOC enrichment also in the subsoil. At this depth, the subdivision of TOC into different OC sources shows that most of the increase was due to fresh POM, likely by the stimulation of root growth and bioturbation. The increase in subsoil TOC went along with increases in macroporosity and macropore connectivity. We neither observed increases in plant‐available water capacity nor in unsaturated hydraulic conductivity. In conclusion, only very high application of FYM over long periods can increase OC content of subsoil at our study site, but this increase is largely based on fresh, easily degradable POM and likely accompanied by high C losses when considering the discrepancy between OC addition rate by FYM and TOC response in soil. Highlights A new image processing procedure to distinguish fresh and decomposed POM. The increase of subsoil C stock based to a large extend on fresh, labile POM. Potential of arable subsoils for long‐term C storage by large FYM application rates is limited. The increase in TOC has no effect on hydraulic properties of the subsoil.
    Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Keywords: ddc:631.4
    Language: English
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  • 29
    Publication Date: 2022-05-26
    Description: © The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in DiBenedetto, M., Qin, Z., & Suckale, J. Crystal aggregates record the pre-eruptive flow field in the volcanic conduit at Kilauea, Hawaii. Science Advances, 6(49), (2020): eabd4850, doi:10.1126/sciadv.abd4850.
    Description: Developing reliable, quantitative conduit models that capture the physical processes governing eruptions is hindered by our inability to observe conduit flow directly. The closest we get to direct evidence is testimony imprinted on individual crystals or bubbles in the conduit and preserved by quenching during the eruption. For example, small crystal aggregates in products of the 1959 eruption of Kīlauea Iki, Hawaii contain overgrown olivines separated by large, hydrodynamically unfavorable angles. The common occurrence of these aggregates calls for a flow mechanism that creates this crystal misorientation. Here, we show that the observed aggregates are the result of exposure to a steady wave field in the conduit through a customized, process-based model at the scale of individual crystals. We use this model to infer quantitative attributes of the flow at the time of aggregate formation; notably, the formation of misoriented aggregates is only reproduced in bidirectional, not unidirectional, conduit flow.
    Description: M.D. acknowledges support the Stanford Gerald J. Lieberman Fellowship and the Postdoctoral Scholarship from Woods Hole Oceanographic Institution.
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  • 30
    Publication Date: 2022-05-26
    Description: © The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Praetorius, S. K., Condron, A., Mix, A. C., Walczak, M. H., McKay, J. L., & Du, J. The role of northeast pacific meltwater events in deglacial climate change. Science Advances, 6(9), (2020): eaay2915, doi:10.1126/sciadv.aay2915.
    Description: Columbia River megafloods occurred repeatedly during the last deglaciation, but the impacts of this fresh water on Pacific hydrography are largely unknown. To reconstruct changes in ocean circulation during this period, we used a numerical model to simulate the flow trajectory of Columbia River megafloods and compiled records of sea surface temperature, paleo-salinity, and deep-water radiocarbon from marine sediment cores in the Northeast Pacific. The North Pacific sea surface cooled and freshened during the early deglacial (19.0-16.5 ka) and Younger Dryas (12.9-11.7 ka) intervals, coincident with the appearance of subsurface water masses depleted in radiocarbon relative to the sea surface. We infer that Pacific meltwater fluxes contributed to net Northern Hemisphere cooling prior to North Atlantic Heinrich Events, and again during the Younger Dryas stadial. Abrupt warming in the Northeast Pacific similarly contributed to hemispheric warming during the Bølling and Holocene transitions. These findings underscore the importance of changes in North Pacific freshwater fluxes and circulation in deglacial climate events.
    Description: The research was partly supported by the NSF through grants ARC-257 1204045 and PLR-1417667. The numerical model simulations used resources from the National Energy Research Scientific Computing Center (NERSC), a U.S. Department of Energy Office of Science User Facility operated under contract no. DE-AC02-05CH11231.
    Repository Name: Woods Hole Open Access Server
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  • 31
    Publication Date: 2022-05-26
    Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Torres, J. P., Lin, Z., Watkins, M., Salcedo, P. F., Baskin, R. P., Elhabian, S., Safavi-Hemami, H., Taylor, D., Tun, J., Concepcion, G. P., Saguil, N., Yanagihara, A. A., Fang, Y., McArthur, J. R., Tae, H. S., Finol-Urdaneta, R. K., Özpolat, B. D., Olivera, B. M., & Schmidt, E. W. Small-molecule mimicry hunting strategy in the imperial cone snail, Conus imperialis. Science Advances, 7(11), (2021): eabf2704, https://doi.org/10.1126/sciadv.abf2704.
    Description: Venomous animals hunt using bioactive peptides, but relatively little is known about venom small molecules and the resulting complex hunting behaviors. Here, we explored the specialized metabolites from the venom of the worm-hunting cone snail, Conus imperialis. Using the model polychaete worm Platynereis dumerilii, we demonstrate that C. imperialis venom contains small molecules that mimic natural polychaete mating pheromones, evoking the mating phenotype in worms. The specialized metabolites from different cone snails are species-specific and structurally diverse, suggesting that the cones may adopt many different prey-hunting strategies enabled by small molecules. Predators sometimes attract prey using the prey’s own pheromones, in a strategy known as aggressive mimicry. Instead, C. imperialis uses metabolically stable mimics of those pheromones, indicating that, in biological mimicry, even the molecules themselves may be disguised, providing a twist on fake news in chemical ecology.
    Description: Research reported in this publication was supported by NIH R35GM12252, with contributions to biological work from NIH Fogarty International Center U19TW008163, NIH P01GM48677, and DOD CDMRP W81XWH-17-1-0413. The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIH.
    Repository Name: Woods Hole Open Access Server
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  • 32
    Publication Date: 2022-05-26
    Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Guillermic, M., Cameron, L. P., De Corte, I., Misra, S., Bijma, J., de Beer, D., Reymond, C. E., Westphal, H., Ries, J. B., & Eagle, R. A. Thermal stress reduces pocilloporid coral resilience to ocean acidification by impairing control over calcifying fluid chemistry. Science Advances, 7(2), (2021): eaba9958, https://doi.org/10.1126/sciadv.aba9958.
    Description: The combination of thermal stress and ocean acidification (OA) can more negatively affect coral calcification than an individual stressors, but the mechanism behind this interaction is unknown. We used two independent methods (microelectrode and boron geochemistry) to measure calcifying fluid pH (pHcf) and carbonate chemistry of the corals Pocillopora damicornis and Stylophora pistillata grown under various temperature and pCO2 conditions. Although these approaches demonstrate that they record pHcf over different time scales, they reveal that both species can cope with OA under optimal temperatures (28°C) by elevating pHcf and aragonite saturation state (Ωcf) in support of calcification. At 31°C, neither species elevated these parameters as they did at 28°C and, likewise, could not maintain substantially positive calcification rates under any pH treatment. These results reveal a previously uncharacterized influence of temperature on coral pHcf regulation—the apparent mechanism behind the negative interaction between thermal stress and OA on coral calcification.
    Description: R.A.E. and J.B.R. acknowledge support from National Science Foundation grants OCE-1437166 and OCE-1437371. The work was also supported by the “Laboratoire d’Excellence” LabexMER (ANR-10-LABX-19), cofunded by a grant from the French government under the program “Investissements d’Avenir,” and an IAGC student grant 2017. R.A.E. acknowledges financial and logistical support from the Pritzker Endowment to UCLA IoES, and J.B.R. acknowledges support from the ZMT and the Hanse-Wissenschaftskolleg Fellowship Program and the NSF OCE award #1437371.
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  • 33
    Publication Date: 2022-05-26
    Description: © The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Law, K. L., Starr, N., Siegler, T. R., Jambeck, J. R., Mallos, N. J., & Leonard, G. H. The United States' contribution of plastic waste to land and ocean. Science Advances, 6(44), (2020): eabd0288, doi:10.1126/sciadv.abd0288.
    Description: Plastic waste affects environmental quality and ecosystem health. In 2010, an estimated 5 to 13 million metric tons (Mt) of plastic waste entered the ocean from both developing countries with insufficient solid waste infrastructure and high-income countries with very high waste generation. We demonstrate that, in 2016, the United States generated the largest amount of plastic waste of any country in the world (42.0 Mt). Between 0.14 and 0.41 Mt of this waste was illegally dumped in the United States, and 0.15 to 0.99 Mt was inadequately managed in countries that imported materials collected in the United States for recycling. Accounting for these contributions, the amount of plastic waste generated in the United States estimated to enter the coastal environment in 2016 was up to five times larger than that estimated for 2010, rendering the United States’ contribution among the highest in the world.
    Description: This work was funded by Ocean Conservancy through support from the Arthur Vining Davis Foundations.
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  • 34
    Publication Date: 2022-05-26
    Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Skinner, C., Mill, A. C., Fox, M. D., Newman, S. P., Zhu, Y., Kuhl, A., & Polunin, N. V. C. Offshore pelagic subsidies dominate carbon inputs to coral reef predators. Science Advances, 7(8), (2021): eabf3792, https://doi.org/10.1126/sciadv.abf3792.
    Description: Coral reefs were traditionally perceived as productive hot spots in oligotrophic waters. While modern evidence indicates that many coral reef food webs are heavily subsidized by planktonic production, the pathways through which this occurs remain unresolved. We used the analytical power of carbon isotope analysis of essential amino acids to distinguish between alternative carbon pathways supporting four key reef predators across an oceanic atoll. This technique separates benthic versus planktonic inputs, further identifying two distinct planktonic pathways (nearshore reef-associated plankton and offshore pelagic plankton), and revealing that these reef predators are overwhelmingly sustained by offshore pelagic sources rather than by reef sources (including reef-associated plankton). Notably, pelagic reliance did not vary between species or reef habitats, emphasizing that allochthonous energetic subsidies may have system-wide importance. These results help explain how coral reefs maintain exceptional productivity in apparently nutrient-poor tropical settings, but also emphasize their susceptibility to future ocean productivity fluctuations.
    Description: Sample analysis funding was provided by NERC LSMSF grant BRIS/102/0717 and BRIS/125/1418. C.S. was supported by a Newcastle University SAgE DTA studentship and a cooperative agreement with Banyan Tree.
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  • 35
    Publication Date: 2022-05-26
    Description: © The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Richter, M., Nebel, O., Maas, R., Mather, B., Nebel-Jacobsen, Y., Capitanio, F. A., Dick, H. J. B., & Cawood, P. A. An early cretaceous subduction-modified mantle underneath the ultraslow spreading Gakkel Ridge, Arctic Ocean. Science Advances, 6(44), (2020): eabb4340, doi:10.1126/sciadv.abb4340.
    Description: Earth’s upper mantle, as sampled by mid-ocean ridge basalts (MORBs) at oceanic spreading centers, has developed chemical and isotopic heterogeneity over billions of years through focused melt extraction and re-enrichment by recycled crustal components. Chemical and isotopic heterogeneity of MORB is dwarfed by the large compositional spectrum of lavas at convergent margins, identifying subduction zones as the major site for crustal recycling into and modification of the mantle. The fate of subduction-modified mantle and if this heterogeneity transmits into MORB chemistry remains elusive. Here, we investigate the origin of upper mantle chemical heterogeneity underneath the Western Gakkel Ridge region in the Arctic Ocean through MORB geochemistry and tectonic plate reconstruction. We find that seafloor lavas from the Western Gakkel Ridge region mirror geochemical signatures of an Early Cretaceous, paleo-subduction zone, and conclude that the upper mantle can preserve a long-lived, stationary geochemical memory of past geodynamic processes.
    Description: O.N. was supported by the Australian Research Council (grant FT140101062). P.A.C. was supported by the Australian Research Council (grant FL160100168). H.J.B.D. was supported by the NSF (grants PLR 9912162, PLR 0327591, OCE 0930487, and OCE 1434452). M.R. was supported by a graduate scholarship of Monash University and the SEAE.
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  • 36
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    The Journal of Chemical Physics 91 (1989), S. 4455-4461 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Intermolecular potentials for Ar and Kr interacting with HBr are obtained by least-squares fitting of potential parameters to data obtained from the molecular-beam microwave spectra of the Ar–HBr and Kr–HBr van der Waals complexes. The equilibrium geometry is linear Rg–H–Br in each case, but there are substantial secondary minima at the linear Rg–Br–H geometries; for Ar–HBr, the secondary minimum is only about 5 cm−1 shallower than the primary minimum. This potential feature is found to explain the anomalous H/D isotope effects in centrifugal distortion constants that have been observed for the Rg–HBr complexes. It is predicted that Ar–HBr will have a very low-energy bending state, only 11 cm−1 above the ground state, arising from the secondary minimum.
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  • 37
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    The Journal of Chemical Physics 91 (1989), S. 4477-4484 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Excited rotational level dependence of the external magnetic field effects both on intensity and on decay of fluorescence of pyrazine vapor has been carefully examined for the zero-point vibrational level in S1 with a field strength of 0–170 G. The magnetic quenching of the slow fluorescence becomes more effective with increasing rotational quantum number J' of the excited level, and the field strength at which the amount of fluorescence quenching becomes one-half of the total amount of quenching at the saturated fields is roughly proportional to (2J'+1)−1. The magnetic quenching is also found to depend on K' of the excited level. The rotational level dependence of the magnetic quenching of the slow fluorescence is related to a difference in the number of the triplet levels coupled to the optically excited singlet rovibronic level, based on the spin decoupling mechanism of the singlet–triplet mixed level.
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  • 38
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    The Journal of Chemical Physics 91 (1989), S. 4499-4503 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: We have observed many collision-induced-dipole (CID) absorption bands arising from the transitions between quasimolecular ground and high-lying (n≤10) states in the strontium–rare-gas systems. For each absorption band, we have measured the energy shift of the absorption peak from the energy of the correlating atomic forbidden transition and the effective oscillator strength per unit perturber density fCID/Np. The shift is roughly proportional to the electron scattering length L0 for each rare-gas atom, whereas the fCID/Np is roughly proportional to L20. The shift decreases in general as the principal quantum number n increases, and increases as one goes from the s state to the d state, and to the degenerate manifold state with l≥3. These general features of the shift and fCID/Np are consistent with the predictions by a simple Fermi-potential model, suggesting the important role of the interaction between a Rydberg electron and a rare-gas atom in the CID absorption processes.
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  • 39
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    The Journal of Chemical Physics 91 (1989), S. 4582-4586 
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    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ν2 band of the silylene SiH2 molecule in X˜ 1 A1 was observed for the first time in the gas phase by using infrared diode laser kinetic spectroscopy. Silylene molecules were generated by the photolysis of phenylsilane at 193 nm. The observed spectrum was analyzed to determine the rotational and centrifugal distortion constants in the ground and v2 =1 states and the band origin ν0 =998.6241(3) cm−1 with one standard deviation in parentheses. The significance of the derived parameters is discussed in detail.
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  • 40
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    The Journal of Chemical Physics 91 (1989), S. 4597-4602 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Using translational spectroscopy we have studied the d 1Πg and C 3 Πg Rydberg states of O2 . The dissociation of the vibrational levels v=0–2 to all energetically accessible dissociation limits has been followed. The dissociation pathways directly reflect the predissociation mechanisms involved. For the d 1 Πg Rydberg state competition between predissociation by a 3 Πg valence state, due to singlet–triplet mixing, and by a 1 Πg valence state has been observed. Using the Fermi golden rule the observed vibrationally dependent competition is reproduced, corroborating the positions of the lower 1 Πg and 3 Πg valence state curves and yielding various coupling strengths.
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  • 41
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    The Journal of Chemical Physics 91 (1989), S. 4636-4642 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Rotationally inelastic scattering of carbon dioxide by translationally hot H, D, and Cl atoms was studied by time-resolved diode laser absorption. The high J rotational distribution falls off quite rapidly between J=60 and J=80. D atom collisions have roughly twice the excitation cross section versus H atom collisions, with the H*/D* ratio decreasing with increasing J. These results are consistent with a constraint on the total reagent orbital angular momentum available for rotational excitation. Transient Doppler profiles measured immediately after hot atom/CO2 collisions indicate that CO2 molecules excited to high J levels have a larger recoil velocity than molecules excited to lower J levels. This result is consistent with predictions based on a simple model which treats the CO2 potential as a hard shell ellipsoid.
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  • 42
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    The Journal of Chemical Physics 91 (1989), S. 4643-4650 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The molecular dynamics simulation method is extended to study a model of diffusion-controlled reactions. This allows a molecular description of solvent at an equal footing of reactants. Nondiffusional dynamic behavior of reactive molecules is found at short times. It enhances the rate of reactive encounter in comparison to the prediction of Smoluchowski theory. The model studied in this work can be regarded as a theoretical prototype of fluorescence quenching. In this context it is shown that the nondiffusional dynamics is mainly responsible for the discrepancy between Stern–Volmer plots measured in a continuous excitation experiment or obtained by integrating the time resolved fluorescence intensity. The other aspects such as the long-time behavior of survival probability, solvent effect as well as competing effect from finite concentration of one reactive species are also studied in some detail.
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  • 43
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    The Journal of Chemical Physics 91 (1989), S. 4714-4727 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The reactions of neutral iron clusters Fe7–27 with water are studied in a laser-vaporization cluster source coupled to a continuous-flow reactor. Reaction products are detected via laser ionization and time-of-flight mass spectrometry. The reactions of room-temperature clusters with H2O show adsorbate decomposition and hydrogen desorption, as do the reactions with D2O at elevated temperatures. The room-temperature reaction with D2O appears not to involve any decomposition, and is at equilibrium under the conditions of these experiments. The dependence of reaction extent on D2O pressure yields equilibrium constants for the addition of the first and second D2O molecules. The analysis is complicated by the presence of two-photon ionization processes that are treated quantitatively with a rate-equation model. This treatment also yields estimates for cluster photoabsorption cross sections, which are found to be approximately linear in cluster size, having a magnitude of 2.3×10−17 cm2 per iron atom. From the derived equilibrium constants and estimated adsorption entropies, approximate D2O–cluster binding energies are determined. They range from 0.42 to 0.59 eV, and their dependence on cluster size shows a remarkable similarity to the dependence of the rate constants for reaction of iron clusters with H2. The implications of this similarity are discussed.
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  • 44
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    The Journal of Chemical Physics 91 (1989), S. 4738-4744 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Relativistic effect Erel upon the total electronic energy of an atom is discussed with particular reference to obtaining the nonrelativistic total energy Eexact from the experimental total energy. Numerical values of this effect obtained by various authors by different nonempirical methods are compared for neutral atoms of rare-gas elements. It is shown that methods either of a Hartree–Fock-type or of a Dirac–Hartree–Fock-type give much the same Erel value for He through Ar. It is pointed out that Erel calculated with Hartree–Fock wave functions is not adequate for use in obtaining Eexact from the experimental total energy and that the Erel value calculated with wave functions including electron correlation should work well, although an actual demonstration can be done only for two-electron systems for lack of data. A semiempirical formula is therefore proposed, which is useful for least-squares fit of experimental total energies of isoelectronic series of atoms to extract nonrelativistic total energies along with the relativistic effect. From nonrelativistic energies thus derived, semiempirical values of correlation energies of atoms are obtained. The results thus obtained are in reasonable agreement with correlation energies derived by Clementi along somewhat different lines. The power series expansion in Z of the fitted formula for the He series shows that numerical values of expansion coefficients agree reasonably well with the corresponding values obtained by accurate relativistic and nonrelativistic Z expansion-type calculations.
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  • 45
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    The Journal of Chemical Physics 91 (1989), S. 4785-4792 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The ground and low-lying states of CuF2 and CuCl2 have been studied using the single and doubles configuration interaction (SDCI) and coupled pair functional (CPF) methods in a large Gaussian basis set including up to g-type functions. The results include a determination of the bond distances for all the ligand field states (the three states with one hole in the 3d shell) and charge transfer (CT) states, and force constants for the ligand field states. Overall the SDCI (with or without the Davidson correction for higher excitations) and CPF results are in good agreement. The CPF results, which we believe to be the most accurate, can be summarized as follows. The computed value for the asymmetric stretching frequency in the 2Σ+g ground state of CuF2 is 740 cm−1, compared to the experimental value of 765 cm−1. The d–d transition energies for CuF2 are 2500 and 10 800 cm−1. The two lowest charge transfer states are at 31 200 and 33 900 cm−1, respectively. CuCl2 has been found to have a ground state which is an almost equal mixture of 2Σ+g and 2Πg when an estimate of the spin–orbit interaction is included. Three d–d transitions are found: 1600 cm−1 ( J=1/2), 7500 cm−1 (J=3/2), and 9700 cm−1 (J=5/2). The lowest charge transfer states have been computed to lie at 16 700 and 19 600 cm−1. Two bands have been found in the gas-phase spectrum of CuCl2 at 9000 and 19 000 cm−1, in good agreement with the present results. The computed f value for the CT band is 0.017, to be compared to the experimental value of 0.014.
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  • 46
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    The Journal of Chemical Physics 91 (1989), S. 4909-4911 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The dissociation equilibrium AlBr=Al+Br was studied by effusion beam mass spectrometry over the range 1970 to 2260 K and the dissociation energy D00(AlBr) was derived as 4.41±0.06 eV. This value is in general agreement with other fragmentary thermochemical results, but it is lower than a value derived from a short extrapolation of vibrational levels in the excited 1π state, doubtless because of a potential maximum of about 0.22 eV in that state. A Birge–Sponer extrapolation of the ground state vibrational levels, when corrected for degree of ionicity, yields a D00 value in close accord with the experimental result, but an electrostatic model calculation falls short by 0.45 eV.
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  • 47
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    The Journal of Chemical Physics 91 (1989), S. 4920-4925 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: A new scheme of experimental criterion is proposed for estimating the carrier mobility and main-chain conformation of an electroactive polymer chain by the technique of transient electric birefringence (TEB). The rise response Δnr (t) and the decay response Δnd (t) in the TEB satisfy in the Kerr regime (i) Δnr (t)=Δnst -Δnd (t) for a polymer molecule of arbitrary conformation with an induced dipole moment alone arising from carriers highly mobile along the main chain, (ii) Δnr (t)=Δnst -(3/2)Δnd (t/3)+(1/2)Δnd (t) for a rodlike polymer molecule with a permanent dipole moment alone originating from carriers with low mobility, and (iii) Δnr(t)=Δnst-2Δnd (t/2)+Δnd (t) for a random-coiled polymer with a permanent dipole moment alone due to low-mobility carriers, where Δnst =limt→∞ Δnr(t) . Then, comparison of the TEB rise and decay responses gives us information on the carrier mobility and main-chain conformation. This criterion is valid also for polydispersed polymer samples. By applying the criterion to the exemplifying data of the TEB responses for soluble urethane-substituted polydiacetylenes, it is found that the polydiacetylene molecules take a random-coil conformation with a permanent dipole moment in a good solvent, while in a poor solvent the carrier mobility depends crucially on the solvent condition.
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  • 48
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    The Journal of Chemical Physics 91 (1989), S. 4942-4948 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The rate of catalytic carbon monoxide oxidation on a Pt(100) single crystal surface under isothermal, low-pressure conditions exhibits for certain ranges of parameters (O2 and CO partial pressures, temperature) sustained temporal oscillations whose mechanism had been explored in previous work. Coupling between reaction and diffusion leads to spatial pattern formation as manifested by patches with different work function on the intrinsically homogeneous surface. Imaging is performed by means of the novel technique of scanning photoemission microscopy. Typically, nuclei with dimensions of a few microns, as determined by the instrumental resolution, are formed spontaneously and expand with sharp fronts and velocities of about 0.5 mm/min (at 480 K) up to sizes ≥1 mm. Waves with even more extended fronts propagating with somewhat higher velocities across the sample surface are responsible for the occurrence of large amplitude temporal oscillations of the integral reaction rate.
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  • 49
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    The Journal of Chemical Physics 91 (1989), S. 4338-4345 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Carbon K edge near edge x-ray absorption fine structure (NEXAFS) spectra of adsorbed acetonitrile and reactive intermediates derived therefrom on Ag(110) were studied in order to determine their orientation and bonding to the surface. For both the multilayer and the adsorbed monolayer there is excellent agreement between the spectra of the adsorbed acetonitrile and the gas phase species. For the multilayer coverage the orientation of the acetonitrile molecules is not completely random; the molecules prefer an orientation with their bond axes closer to parallel than perpendicular to the surface. At monolayer coverage acetonitrile bound to the surface is unrehybridized. It is bound with its linear molecular skeleton parallel to the surface and randomly oriented azimuthally. Annealing molecular CH3 CN to 325 K in the presence of coadsorbed atomic oxygen produces a linear CH2 CN surface intermediate, the molecular axis of which is parallel to the surface and perpendicular to the close-packed direction. The C–C and C–N bond lengths are essentially unperturbed from their values in gas phase CH3 CN. Bonding appears to occur between the methylene carbon and the metal, as expected. A broadening of the π* resonance for transition into the π* orbital perpendicular to the surface is interpreted in terms of π-donor bonding.
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    The Journal of Chemical Physics 91 (1989), S. 4265-4272 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Attenuated total reflectance (ATR) from principal faces of orthorhombic (and higher symmetry) crystals in spectral regions including strongly polar optic modes is considered. A detailed, quantitative comparison between experimental and calculated ATR infrared (IR) spectra of orthorhombic sodium nitroprusside dihydrate (SNP), Na2[Fe(CN)5NO]⋅2H2O, single crystals in the 500–3800 cm−1 region is presented. Calculated ATR spectra are generated employing the dielectric tensor of SNP obtained from Kramers–Kronig analysis of external reflectance data. The dependence of intensities, profiles, and locations of bands in the spectra upon the relative refractive index of the sample against the ATR prism and the polarization state of the reflected IR beam will be discussed. Both theoretical and experimental evidence of the coupling of the TM-polarized evanescent wave with strong in-plane-polarized optic modes at about their LO frequencies will be given with reference to the case of the strong NO stretching mode of SNP. Finally, the computed and experimental ATR results in SNP single crystals will be critically compared with powder ATR data and reported polarized IR transmittance spectra of SNP single crystals plates.
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    The Journal of Chemical Physics 91 (1989), S. 4299-4306 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: X-ray diffraction data for liquid n-butane near the triple and boiling points are presented. The data are analyzed using scattering factors for −CH3 and −CH2 groups which permits extraction of structure and distribution functions for carbon sites. An intramolecular carbon–carbon distance at approximately 3.1 A(ring) is assigned to a short end-to-end distance in a near-gauche conformation, which is in agreement with the electron diffraction results for gaseous n-butane. The intermolecular pair distribution functions show a large difference in the liquid structure for the low and high temperature states, similar to that found in liquid propane.
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    The Journal of Chemical Physics 91 (1989), S. 4330-4337 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: We report studies of the kinetics of thermal desorption of In from Ga-stabilized GaAs(100) in ultrahigh vacuum. The relative coverage of In was monitored by x-ray photoelectron spectroscopy (XPS), while the substrate temperature was accurately measured using infrared laser interferometric thermometry. The In was deposited on GaAs by dosing, at room temperature, to saturated monolayer coverage with trimethylindium, yielding In ∼2×1014 cm−2. Subsequent heating to 400 °C desorbs all hydrocarbon species without affecting the In coverage. Further heating leads to first-order desorption of In, as confirmed by exponential decay of In(3d 5/2) XPS signal in isothermal desorption experiments at 473 and 503 °C. From temperature programmed desorption studies through the range 450 to 530 °C, differentiation of the In coverage vs time yielded desorption rates from which Arrhenius parameters were extracted. The unit-weighted average values of the preexponential factor and the desorption energy, obtained from three experiments with heating rates from 0.6 to 3.2 °C/min, are: log10A(s−1)=12.2±0.5 and Ed=53.5±1.2 kcal/mol, where the uncertainties are 1σ sample standard deviations. The Arrhenius parameters of desorption of In from Ga-stabilized GaAs(100) are found to be similar to those of vaporization of pure In. This is consistent with the known tendency of In to form islands on GaAs surfaces, but could also reflect the similarity of the local environments of an In atom adsorbed on an In island and an In atom adsorbed on a Ga-terminated surface. The observed first-order kinetics and the constancy of the In(3d 5/2) XPS signal up to the temperature where desorption occurs indicates that if islands are formed they are small (〈10 A(ring) diameter) at the low coverage used here.
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    The Journal of Chemical Physics 91 (1989), S. 4360-4368 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Cooperative diffusion D of polyacrylamide gels in water was investigated by quasielastic light scattering both under the isotropically swollen state and under the uniaxially stretched and swollen state. The concentration (Ce) dependence of D for the isotropic gels having equilibrium degrees of swelling was measured by systematically changing crosslinking density of gels. The results yielded D=(3.4±0.5)×10−6 C0.76±0.03e cm2/s, in accord with a scaling prediction. For each of the gels prepared and having a given Ce, the uniaxial stretching was applied, and the values D were investigated as a function of extension ratio α and direction β with respect to the stretching direction. The results gave D (α,Ce)=(3.4±0.5)×10−6 αx Cye cm2/s, where x=2/3 for β=0° and −1/4 for β=90°, and y=0.76, independent of α and β. The results clearly indicate that upon increasing α, D increases and decreases, respectively, in the direction parallel and perpendicular to the stretching direction, implying suppression and enhancement of the concentration fluctuations as a result of the network being stiffened and softened in the respective directions.
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    The Journal of Chemical Physics 91 (1989), S. 4387-4400 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The discrete chain representation of multiple scattering theory of the concentration dependence of the hydrodynamics of polymer solutions is applied to the calculation of the leading concentration correction to the dynamic structure factor S(k,t) and its first and second cumulants of individual labeled Gaussian chains in theta solutions at nonzero concentrations. Contributions are separated into those from overall translational and internal chain motions as well as couplings between different internal modes and between translation and internal modes, coupling that are introduced by interchain hydrodynamic interactions. The separate contributions are analyzed as a function of k and of t in order to isolate regions where certain contributions are dominant. As expected, short times and larger k tend to favor contributions from internal chain dynamics, while longer times and smaller k make concentration dependent translational effects predominate. Computations for shorter chains are extrapolated to provide the asymptotic long chain behavior.
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    The Journal of Chemical Physics 91 (1989), S. 4418-4419 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The interconversion tunneling frequencies for (HCl)2 and (DCl)2 are obtained from near-infrared absorption spectra of the H(D)Cl stretching transitions, to spectroscopic precision for the mixed 35Cl–37Cl dimers. A phenomenological model of the interconversion process explains several experimental observations, and provides good estimates of the splittings expected for the 35Cl–35Cl and 37Cl–37Cl species.
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    The Journal of Chemical Physics 91 (1989), S. 4425-4426 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The dissociation energy of boron hydride has been calculated ab initio using a very accurate theoretical model, with an expected accuracy of ±0.2 kcal mol−1. The values of 85.7 and 82.4 kcal mol−1 for De and D0, respectively, confirm the conclusion of Curtiss and Pople [J. Chem. Phys. 90, 2522 (1989)], that the predissociation limit of 82.6±0.4 kcal mol−1 might be close to the actual dissociation energy, and that an earlier proposed De of 78.9 kcal mol−1 is in error.
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    The Journal of Chemical Physics 91 (1989), S. 3525-3531 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The ultrafast photodynamics of four-coordinate nickel(II) porphyrins in noncoordinating solvents has been studied using femtosecond time resolved optical spectroscopy. Unambiguous evidence has been found for the formation of a metastable metal (d,d) excited state in less than 350 fs following excitation of the macrocycle. However, the transient absorption spectrum of this ligand-field electronic excited state continues to evolve and reaches the steady-state form only after about 20 ps. This spectral behavior and the attendant complex kinetics can be modeled phenomenologically in terms of a broad distribution of the (d,d) excited states evolving to a narrower distribution. The dynamics are associated with vibrational relaxation. Intramolecular and intermolecular contributions to this process are considered.
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    The Journal of Chemical Physics 91 (1989), S. 3571-3576 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: A new method for the calculation of bound state eigenvalues and eigenfunctions of the Schrödinger equation is presented. The Fourier grid Hamiltonian method is derived from the discrete Fourier transform algorithm. Its implementation and use is extremely simple, requiring the evaluation of the potential only at certain grid points and yielding directly the amplitude of the eigenfunctions at the same grid points.
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    Notes: The evaluation of free energy differences using the perturbation method or thermodynamic integration method requires special caution if multiple rotational isomeric states may exist in the system under investigation. In this article a recently suggested procedure to properly treat rotational isomeric states is illustrated with a molecular dynamics simulation of an aqueous solution of uncomplexed 18-crown-6 crown ether, as an example of a system in which large numbers of isomeric states may exist. By using very long molecular dynamics simulations, thermodynamic perturbation methods and symmetry arguments, the free energy of host organization into the conformation required to form the complex with K+ is estimated to be 2.6 kJ mol−1. It has also been found that the lowest energy conformations of 18-crown-6 in vacuo do not necessarily correspond to the most populated conformations in aqueous solution.
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    The Journal of Chemical Physics 91 (1989), S. 3689-3699 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Pulsed neutron diffraction measurements have been made on liquid 1,2-dichloroethane-d4 (DCE). The wide momentum-transfer range (∼0.3–50 A(ring)−1) available has been used to further refine previously measured molecular structure parameters as well as to test the validity of the inelasticity corrections applied. A measurement using chlorine isotopes on a steady (reactor) source served to partially separate the chlorine–chlorine and the chlorine–carbon plus chlorine–deuterium correlations. The isotopic difference curves were then analyzed and the relevant features of the distribution of internal dihedral angles [P(τ)] obtained by adequate inversion of the experimental difference–functions. The intermolecular pair correlation function was then derived and both sets of functions (from pulsed and steady sources) are compared and tentatively assigned.
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    The Journal of Chemical Physics 91 (1989), S. 3721-3723 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Brownian dynamics computer simulations are used to investigate the shape of the mean-squared end-to-end distance distribution function in the three regimes: (1) excluded volume; (2) θ; (3) collapsed. It is found that Mazur's function fits regime (1), that the Gaussian function fits regime (2), and that neither of these appear to describe the collapsed state. The implications of these results to theories of ring formation during polymerization is pointed out.
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  • 62
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    The Journal of Chemical Physics 91 (1989), S. 5154-5159 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Using a purely ab initio minimum energy path for the trans-tunneling motion in the HF dimer, the energy levels for the K-type rotation and trans-tunneling motion for (HF)2 and (DF)2 are calculated with a one-dimensional semirigid bender Hamiltonian and no adjustable parameters. The transition moments for rotation-tunneling transitions are calculated, using our ab initio value for the dipole moment of an isolated HF molecule, and we also calculate B¯ values. The energy levels we obtain are in close agreement with experiment; for example, the K=0 tunneling splitting in (HF)2 is calculated as 0.65 cm−1 compared to the experimental value of 0.658 69 cm−1. As well as showing that our ab initio minimum energy path is very good, the calculation demonstrates that the semirigid bender formalism is able to account quantitatively for the unusual K dependence of the rotational energies resulting from the quasilinear behavior, and that the trans-tunneling motion is separable from the other degrees of freedom. We use the results to predict the locations, and transition moments, of the ΔK=0 and ±1 subbands in the tunneling spectra of (HF)2 and (DF)2, many of which have not yet been observed.
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    The Journal of Chemical Physics 91 (1989), S. 5139-5153 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The problem of classical vs nonclassical structure of protonated acetylene (vinyl cation) C2H+3 has been studied using high resolution infrared spectroscopy. The spectrum has been observed in the 3.2 μm region in air-cooled and water-cooled plasmas using C2H2:H2:He mixtures and in liquid nitrogen-cooled plasmas using CH4:H2:He mixtures. The difference frequency spectrometer with the velocity modulation method has been used to conduct the Doppler-limited, high sensitivity spectroscopy.The observed vibration–rotation pattern with the band origin at 3142.2 cm−1 has been identified as due to the antisymmetric CH stretching ν6 band of the C2H+3 ion with the nonclassical (bridged) structure. The observed spectral pattern was anomalous, but definitive assignments could be made for a part of the spectrum using the ground state combination differences which fit to the usual asymmetric rotor pattern. The discrimination between the classical and nonclassical structures is based on the observed spectral intensity pattern due to spin statistical weights. Agreement of vibrational band patterns and the rotational constants with ab initio values gives supporting evidence. The anomaly of the spectrum is at least partly ascribed to the small energy difference between the classical and nonclassical structures and possible rearrangement between them, the idea used by organic chemists over the years in wet chemistry. Systematic splittings with the intensity ratio of 2:1 have been noticed in some parts of the spectrum indicating that the protons tunnel between the apex and the two end equilibrium positions of the bridged structure. Using a simplified internal rotation model proposed by Hougen, the barrier height of the tunneling has been estimated. Chemical kinetics in plasmas related to C2H+3 is also discussed.We conclude that (1) the nonclassical structure is lower in energy than the classical structure, and (2) the apex proton and the two end protons exchange their positions with a measurable time scale.
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  • 64
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    The Journal of Chemical Physics 91 (1989), S. 5164-5169 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The inelastic neutron scattering (INS) spectra of some ammonium halides and ammonium nitrate are analyzed in terms of the internal vibrations of the NH+4 ion convoluted with multiphonon lattice modes up to the eighth term. Due to the low mass of this ion, most of the intensity of the internal-mode region of the INS spectra is in the "phonon wings,'' differences between the salts in this spectral region being mainly due to differences in their lattice-mode spectra. Refinement of the lattice Debye–Waller factor is included in the profile refinement of interatomic force constants.
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  • 65
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    The Journal of Chemical Physics 91 (1989), S. 5160-5163 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: A new upper bound for the dissociation energy of acetylene, D00(HCC–H) =529.89(±0.01) kJ/mol, has been determined by Stark anticrossing spectroscopy. The zero-pressure extrapolated (unimolecular) decay rates of levels of S1 (A˜ 1Au) v'3=2 and 3 (quanta of the trans-bending vibration) increase upon application of an electric field of 113 kV/cm. We attribute this increase in molecular decay rate to predissociation rather than any other Stark-induced nonradiative or radiative phenomenon. The lowest level (v'3 =2, J'=2, K'=1) from which we have observed an increase in decay rate (i.e., predissociation) has an internal energy of 44 295.65 cm−1 relative to v=0, J=0 of S0 (X˜ 1∑+g). This corresponds to a value about 24 kJ/mol lower then the consensus upper bound result from four different recent experimental determinations (including one of ours) as well as current ab initio results. The new value agrees, however, with earlier work and with recent modeling studies of acetylene pyrolysis kinetics.
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    The Journal of Chemical Physics 91 (1989), S. 3792-3793 
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    The Journal of Chemical Physics 91 (1989), S. 2776-2778 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Electron paramagnetic resonance (EPR) studies performed at room tempertaure in Mn2+ doped Cs2NaYCl6 single crystals (elpasolite) showed that the Mn2+ ion occupies a tetragonal symmetry site in the cubic crystalline lattice. In contrast with previous results on elpasolite, where trivalent rare earth impurities occupy Oh sites, and divalent europium impurities occupy trigonal sites, this work reports, the first evidence of an impurity in a tetragonal symmetry site in this host with principal axes along 〈100〉 crystallographic directions. The unusually large total splitting of the EPR spectrum (∼10 000×10−4 cm−1) is discussed in order to establish a tentative model for the site.
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    The Journal of Chemical Physics 91 (1989), S. 3596-3602 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Novel rigorous upper and lower bounds, at primitive level, to general electron-repulsion integrals (ERIs) involving Gaussian basis sets have been derived and interconnections with the earlier works in the literature are brought out. New optimal strategies for a preemptive elimination of insignificant ERIs at atom and contraction levels are discussed and tested, resulting in a significant reduction in CPU time. Similar analysis is carried out for the computation of the molecular electrostatic potential for the first time in the literature, leading to a marked savings in computer time.
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    The Journal of Chemical Physics 91 (1989), S. 5185-5200 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Multiphoton ionization spectra have been obtained and analyzed for excitation in the 215–360 nm region from the X 3Σ−g, a 1Δg, and b 1Σ+g states of O2. The 0–0 band of the C 1Πg state is reported for the first time. Measurements of other vibrational bands terminating in the C 3Πg and d 1Πg states are in good agreement with determinations by other groups. Several vibrational levels (v'=0–5) of the 3dπg Rydberg complex have been assigned on the basis of (1) an analysis of the spin–orbit couplings between the (Λ,S) basis-set states, (2) spectral simulation, and (3) the behavior of the states when the excitation radiation is changed from linear to circular polarization.
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    The Journal of Chemical Physics 91 (1989), S. 5201-5207 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Diode laser transient absorption/gain spectroscopy is used to monitor time-dependent populations of high rotational levels in OD (v=0) produced in the reaction of O(1D)+D2. Pure rotational transitions on species with large dipole moments offer good sensitivity, full state resolution and μs time resolution in the present apparatus. Measured nascent populations of OD in the four highest rotational levels thermodynamically accessible in this reaction are in reasonable agreement with the reported results of earlier laser-induced-fluorescence measurements, in which corrections for transition moments and predissociation introduce increasing uncertainties at high rotational levels. The relaxation kinetics of the highest rotational levels are not hopelessly complex, and evidence is presented for strong, but not complete propensity for conservation of Λ doublet symmetry during rotational relaxation.
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    The Journal of Chemical Physics 91 (1989), S. 3647-3661 
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    Notes: A consistent simulation of ionic or strongly polar solutes in polar solvents presents a major challenge from both fundamental and practical aspects. The frequently used method of periodic boundary conditions (PBC) does not correctly take into account the symmetry of the solute field. Instead of using PBC, it is natural to model this type of system as a sphere (with the solute at the origin), but the boundary conditions to be used in such a model are not obvious. Early calculations performed with our surface constrained soft sphere dipoles (SCSSD) model indicated that the dipoles near the surface of the sphere will show unusual orientational preferences (they will overpolarize) unless a corrective force is included in the model, and thus we implemented polarization constraints in this spherical model of polar solutions. More recent approaches that treated the surface with stochastic dynamics, but did not take into account the surface polarization effects, were also found to exhibit these nonphysical orientational preferences. The present work develops a surface constrained all-atom solvent (SCAAS) model in order to consistently treat the surface polarization effects in all-atom molecular dynamics simulations. The SCAAS model, which was presented in a preliminary way in previous works, introduces surface constraints as boundary conditions in order to make the necessarily finite system behave as if it was part of an infinite system. The performance of the model with regard to various properties of bulk water is examined by comparing its results to those obtained by PBC simulations. The results obtained from SCAAS models of different sizes are found to be similar to each other and to the corresponding PBC results. The performance of the model in simulations of solvated ions is emphasized and a comparison of the results obtained with spheres of different sizes demonstrates that the model does not possess significant size dependence. This indicates that the model can be used with a relatively small number of solvent molecules for convergent simulation of structure, energetics, and dynamics of polar solutions. The much simpler fixedcenter Langevin dipoles (FCLD) model is also examined and found to provide a powerful tool for estimating solvation free energies. Finally, a preliminary study of the dielectric properties of the SCAAS model is reported and the potential of this model for exploring the correct implementation of the solvent reaction field is discussed.
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    The Journal of Chemical Physics 91 (1989), S. 3700-3706 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The first-order phase transition of the ferromagnetic Ising model driven by the magnetic field at temperatures below criticality is studied by Monte Carlo methods for a two-dimensional thin film geometry, L×M with two free boundaries of length M(very-much-greater-than)L, at which boundary fields act. This model study is relevant, in particular, for phase transitions in monolayers adsorbed at stepped surfaces. While in the bulk geometry (L→∞) this transition occurs for zero field in the present model, with the system "jumping'' from a state with uniformly positive magnetization to a state with uniformly negative magnetization, in the thin film geometry the transition occurs at a critical field H*∼L−1, and the two states between which the transition occurs are characterized by strongly nonuniform magnetization profiles across the film. These findings are in agreement with the scaling theory of Fisher and Nakanishi.
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    The Journal of Chemical Physics 91 (1989), S. 3724-3728 
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    Notes: A simplified stability condition for the Hartree–Fock (HF) solution giving the self-consistent field crystal orbitals (SCF-CO) of the infinite one-dimensional system is derived. Since the present formulation, particularly for the systems having nearly or entirely degenerated highest occupied and the lowest unoccupied COs, contains only two physical parameters, that is, the density of states and the Coulomb repulsion integrals both at the Fermi level, it is tractable to examine the stability of the HF solutions of such polymers as well as the ordinary molecular systems. An example of its application to metallic trans-type polyacetylene is also shown.
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    The Journal of Chemical Physics 91 (1989), S. 3744-3761 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: An ensemble of distributed donor molecules that undergoes rotational transitions into a discrete excimer state has been analyzed, in an attempt to model distributed electronic relaxation and nonexponential fluorescence of aromatic polymers in presence of rotational sampling processes. In case of irreversible trapping, the donor survival function has been formulated in terms of the one-sided Laplace transform and specified for a modified Gaussian distribution to yield a closed-form expression for the donor decay. The formalism permits a time-dependent rate function to be derived that makes possible the construction of the excimer excitation function by means of the convolution theorem. In case of reversible constraints, a generalized treatment based upon time-dependent transition and transform matrices has been given which applies a perturbation technique for approximately solving the system of nonautonomous differential equations in the time domain. In the limit of weak coupling, the method develops approximate Mth order expressions (M=2,4,6, and 8) to the fluorescence response functions of donor and excimer. The perturbational solutions are well behaved up to relatively long time scales and they prove useful for providing the typical nonexponential time behavior of such a system affected by dispersion and dissociation. The physical restriction of this mathematical analysis (weak reversibility) has been addressed and the implications of distributed event times in future analyses of polymer fluorescence have been discussed.
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    The Journal of Chemical Physics 91 (1989), S. 2771-2775 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: 5,6-Dihydro-2H-thiopyran, CH2CH2CH=CHCH2S, has been synthesized and its far-infrared and Raman spectra recorded. Two series of sharp bands were observed originating from 139 and 235 cm−1 in the infrared spectrum for the out-of-plane ring-bending and the ring-twisting vibrations, respectively. A detailed energy level diagram including numerous excited states was determined for the two coupled vibrations. The two-dimensional potential energy surface, which satisfactorily fits more than two dozen observed transitions, was calculated to be V=2.431×104 x41 −0.383×104x21 +2.258×104x42 −1.966×104 x22 +1.026×105x21 x22 , where x1 is the ring-bending coordinate and x2 is the ring-twisting coordinate. The minimum energy on the potential surface corresponds to a twisting angle of 37.8° (the half-chair conformation). The lowest energy bent (boat) conformation corresponds to a saddle point 4130 cm−1 above the twisted conformation on the potential energy surface. The results are compared to analogous molecules and to molecular mechanics calculations.
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    The Journal of Chemical Physics 91 (1989), S. 2808-2813 
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    Notes: Photoabsorption cross sections of thiirane and thietane vapors have been studied in the 110–240 nm region using synchrotron radiation. A number of peaks from thiirane and thietane are arranged into four and five Rydberg series, respectively, converging to the first ionization potential. Many Rydberg peaks from both molecules are observed to possess vibronic structures. The vibrational progressions from thiirane are assigned to the CH2 wagging (ν4) and CS symmetry stretching (ν5) modes. For thietane, the CH2 wagging (ν5) and CS symmetry stretching (ν7) modes of the excited states are found to be active. Molecular geometries and vibrational frequencies for the excited states are discussed on the basis of ab initio calculations.
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    The Journal of Chemical Physics 91 (1989), S. 2840-2847 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Exact close-coupling calculations are used to evaluate the effectiveness of the coupled states approximation in the interpretation of low energy, noble gas methane collisions. The effect of the higher order angle dependent terms of the potential on the inelastic rotational transition was investigated. Calculations using only open channel basis functions were clearly different from converged calculations, but the differences were small enough to be negligible in comparison with experiment. It was shown that open channel coupled states calculations are of sufficient accuracy to evaluate the agreement of the potential model with experiment. The use of the coupled states approximation does not fully account for discrepancies between theory and experiment observed earlier.
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    The Journal of Chemical Physics 91 (1989), S. 2892-2897 
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    Notes: A modified general model for radiationless energy transfer is examined. The model does not rely on selection criteria and is intended to apply over the range of diffusion and resonance energy transfer parameters normally encountered in quenching experiments. Experimental data is compared with theoretical predictions and found to confirm the model's validity for a wide range of physical conditions.
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    The Journal of Chemical Physics 91 (1989), S. 2912-2921 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The photodissociation of aluminum clusters, Al+n (n=7–17), has been studied over a broad energy range (1.88–6.99 eV). Measurements of the lifetimes of the photoexcited clusters are described. Dissociation energies have been determined by comparing the measured lifetimes with the predictions of a simple RRKM model. The dissociation energies show an overall increase with cluster size, but there are substantial oscillations around n=7–8 and n=13–15. Cluster cohesive energies are derived from these results and from previous measurements of the dissociation energies of the smaller clusters. The cohesive energies of the larger clusters (n〉6) are in good agreement with the predictions of a simple model based on the bulk cohesive energy and the cluster surface energy. However, the cohesive energies are substantially larger than the results of recent ab initio calculations. The photodissociation spectrum of Al+8 has been measured and shows a broad absorption feature with a maximum ∼470 nm.
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    The Journal of Chemical Physics 91 (1989), S. 2102-2107 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The microwave spectrum of dichlorosilylene SiCl2 has been observed to characterize this molecule of chemical interest. The molecule was generated by the thermal reaction between silicon powder and tetrachlorosilane at about 1000 °C. The rotational constants and the centrifugal distortion constants were determined for the three isotopic species Si35Cl2, Si35Cl37Cl, and Si37Cl2. The nuclear quadrupole coupling constants were determined from triplet hyperfine splittings observed for several transitions. The asymmetry of the Cl nuclear quadrupole coupling tensor was found to be very large and was accounted for by π electron backdonation from Cl to Si.
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    The Journal of Chemical Physics 91 (1989), S. 2971-2979 
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    Notes: The structures and stabilities of N2 and its 15 possible first-row isoelectronic analogs (CO, BF, BeNe, NO+, CF+, BNe+, O2+2, NF2+, CNe2+, OF3+, NNe3+, ONe4+, F4+2, FNe5+, and Ne6+2) have been examined using ab initio molecular orbital theory. Equilibrium structures have been obtained at a variety of levels of theory including MP3/6-311G(d) and ST4CCD/6-311+G(2df ) and dissociation energies determined at the MP4/6-311+G(3d2f ) level. Full potential energy curves for dissociation, including dissociation barriers, have been obtained at the CASSCF/6-311G(d) level. Spectroscopic constants have also been determined at this level. For the neutral and monocation analogs of N2, the calculated equilibrium geometries, dissociation energies, and spectroscopic constants are in good agreement with the experimental values. The dication analogs of N2, namely O2+2, NF2+, and CNe2+, are all found to be kinetically stable species lying in deep potential wells. In particular, the hitherto unobserved NF2+ dication is predicted to have a short equilibrium bond length (1.102 A(ring)) and a large barrier (445 kJ mol−1) to dissociation to N++F+. Thus NF2+ should be experimentally accessible in the gas phase. The (experimentally known) O2+2 dication is predicted to contain the shortest bond between any two heavy atoms, our best estimate of the bond length being 1.052 A(ring). The first excited state (A 3Σ+u) of O2+2 is predicted to be unbound, and observed metastable decomposition processes are reinterpreted in terms of the ground-state (X 1Σ+g) potential surface. In agreement with previous theoretical studies, we find that CNe2+ is a kinetically stable species, albeit with a relatively long C–Ne bond length. The OF3+ trication is calculated to have a relatively short bond but lies in a well of depth only 23 kJ mol−1. The potential energy curves of the other highly charged species are found to be purely repulsive.
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    The Journal of Chemical Physics 91 (1989), S. 3002-3011 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The relative efficacy of using low order trucations with large reference space vs high order methods with small reference space is tested by comparing quasidegenerate many-body perturbation theory (QDMBPT) calculations of potential curves for the five lowest electronic states of LiH with the multireference coupled cluster calculations of Ben-Shlomo and Kaldor [J. Chem. Phys. 89, 956 (1988)]. The infinite order coupled cluster calculations use two configurational reference spaces involving the 2σ, 3σ, and 1π orbitals, while the QDMBPT computations are truncated at either second or third orders and employ the full active reference space formed either from the 2σ, 3σ, and 1π or from the 2σ, 3σ, 4σ, and 1π orbitals. This gives us the opportunity of testing the dependence of QDMBPT computations on the size of reference space, the available freedom in choosing valence orbitals and orbital energies, and the order of truncation. Second order, four valence orbital space QDMBPT calculations provide good agreement with the repulsive portion of the coupled cluster potentials, but yield a separated atom limit that is too high and that therefore distorts the remainder of the potential. Third order improves the separated atom limit considerably, providing good agreement with the coupled cluster calculations. The "full chemical'' five orbital reference space, on the other hand, yields very good agreement with coupled cluster potentials when using only the simpler second order QDMBPT calculations, and third order corrections in this case are very small but generally improve agreement with coupled cluster potentials. The five orbital reference space calculations are quite insensitive to a wide range of different choices of valence orbitals and orbital energies, demonstrating a robustness to the QDMBPT formalism used.
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    The Journal of Chemical Physics 91 (1989), S. 3045-3055 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The hypernetted-chain (HNC) approximation is solved for fluids of dipolar hard ellipsoids and the structural, thermodynamic, and dielectric properties of the isotropic phase are discussed in some detail. Both prolate and oblate particles are considered and the isotropic–nematic transition is investigated using density functional theory (DFT). For fluids of prolate particles dipolar forces are found to have a significant effect upon the isotropic–nematic transition which occurs at lower densities as the dipole moment is increased. For the oblate case the dipolar interactions have only a very small, if any, influence upon the isotropic–nematic transition density. For both prolate and oblate particles the present HNC/DFT calculations do not predict ferroelectric nematic phases.
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    The Journal of Chemical Physics 91 (1989), S. 5313-5315 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The ν3 (O–Si stretch) fundamental bands of the HOSi+ and DOSi+ molecular ions in the 9 μm region have been detected for the first time, using a tunable infrared diode laser spectrometer and a hollow cathode discharge cell. Analysis of the results yielded accurate values for the molecular rotational and centrifugal distortion parameters, as well as for the band origins, which are 1127.009 cm−1 for HOSi+ and 1103.112 cm−1 for DOSi+ . The ground vibrational state parameters are in excellent agreement with those determined from the ν1 bands of the two isotopes.
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    The Journal of Chemical Physics 91 (1989), S. 5324-5328 
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    Notes: Fragment ions from N2 in the states of (K)−1(nl)1 have been measured at 54.7° with respect to the polarization direction of the incident photon by using a time-of-flight (TOF) mass spectrometer. Simulation fitting calculation to the measured TOF spectra gave distributions of the kinetic energy for these ions. Ion yield spectra observed showed that no singly charged molecular ion was produced from the states of (K)−1(nl)1 in contrast to that of (K)−1(1πg)1. These results are discussed in connection to the dissociation pathway of N2 in these states.
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    The Journal of Chemical Physics 91 (1989), S. 5329-5334 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Dissociation of O2 stimulated by soft x-ray absorption has been studied by using a monochromatized synchrotron radiation and a time-of-flight mass spectrometer. The parent molecular ion O+2 was formed only at 531 eV (excitation of 1s→1πg), and a fragment ion O+ had the highest intensity in all the energies giving core–hole states. The measured time-of-flight spectra were reproduced by a simulation calculation, which provided kinetic energy distributions of O+ and O++. The dissociation pathways from the core–hole states of O2 were discussed using the obtained kinetic energy distribution and ion intensity ratios as well as Auger electron spectra in the literature.
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    The Journal of Chemical Physics 91 (1989), S. 5316-5323 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The general theory for inelastic scattering of molecules 1Δ electronic states is outlined and applied to the specific case of 1Δ states arising from a π2 electron occupancy, e.g., NH(a 1Δ). Integral cross sections for rotational transitions out of the lowest rotational level (J=2) of NH(a 1Δ) v=0 are reported for several targets. A pulsed beam of rotationally cold NH(a 1Δ) was produced by 193 nm photolysis of a dilute mixture of hydrazoic acid in nitrogen seed gas at the tip of a nozzle. The target beam was also prepared as a pulsed supersonic beam. The final rotational state distribution was measured in the collision zone by laser fluorescence excitation. The state-to-state cross sections were found to decrease significantly with increasing final rotational quantum number J'. The magnitude of the J=2→J'=3 cross sections were compared for the different targets. Isotopic scrambling in NH(a 1Δ)–D2 collisions was also searched for but not observed.
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  • 88
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    The Journal of Chemical Physics 91 (1989), S. 5335-5342 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The yield of metastable oxygen atoms through dissociative recombination of O+2 ions with electrons has been studied using a plasma flow tube experiment. For O+2 with high vibrational excitation (around v=9) it was found that half of the oxygen atoms are formed on the O(1D) state and that the branching ratio toward O(1S) is large (∼0.4). Using Xe+ instead of Ar+ as precursor ions, it was shown that the O(1S) yield is much less for ions with low vibrational excitation. However, the present experimental results are not compatible with the extremely low theoretical value of this yield which was reported recently for O+2 (v=0).
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  • 89
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    The Journal of Chemical Physics 91 (1989), S. 5343-5351 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Quenching and vibrational energy transfer in the B2Π state of the NS free radical have been studied using temporally and spectrally resolved laser-induced fluorescence in a low-pressure discharge flow reactor. The collision partners were He, Ar, H2, N2, O2, SF6, CO2, and N2O. Total removal cross sections show an oscillatory behavior with v' in the range of unperturbed levels, v'=4–7, for all colliders studied save O2 where a nearly monotonic increase is seen. Vibrational transfer occurs for H2 and the polyatomics; the rates vary little with v'. Δv=2 transfer occurs with the polyatomic colliders for v'=5. Fluorescence decay traces from the perturbed v'=3 and 8 levels differ from the unperturbed levels and from each other. v'=3 is perturbed by quartets and shows "gateway'' level behavior whereas v'=8 is perturbed by a doublet and shows efficient interelectronic transfer for all rotational levels.
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  • 90
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    The Journal of Chemical Physics 91 (1989), S. 5359-5364 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: State-to-state charge–transfer cross sections have been computed for N+2(X;v=0,1,2) +Ar at 12 collision energies between 1.2 and 320 eV. A classical path method is used, whereby the vibronic degrees of freedom are treated quantum mechanically as the system moves along a classical trajectory. The calculations use the potential energy surfaces computed by Archirel and Levy. Comparison is made with experimental results for this system, including the recent work from Ng's laboratory. In most cases the agreement is quite good. There is, however, a significant difference in the charge–transfer branching ratios to produce Ar+(2P3/2) or Ar+(2P1/2) products. Possible explanations of the discrepancy are discussed. As expected, the cross sections obey the Franck–Condon principle at energies above 200 eV.
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  • 91
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    The Journal of Chemical Physics 91 (1989), S. 5352-5358 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Second virial coefficients as function of temperature are computed for the title molecular systems interacting with He, Ne, and Ar. The relevant anisotropic forces are obtained via accurate potential functions tested earlier through the analysis of several, different properties of the various systems. The relevant quantum corrections are also computed, in addition to the classical results, and their effects analyzed vis à vis the available experimental data. The influence of such corrections on the very low-T behavior of the virial coefficients and on the determination of the Boyle temperatures is also shown and discussed. All examined potential functions are found to yield B(T) values in rather good accord with experiments, in spite of their marked differences in anisotropic behavior and in the shape of their potential well regions.
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  • 92
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    The Journal of Chemical Physics 91 (1989), S. 5365-5373 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reaction between ground state atomic oxygen and acetylene was studied using the crossed molecular beam method with an average collision energy of 6 kcal/mol. The two major primary reaction channels are (a) formation of CH2 and CO and (b) formation of HCCO and H. Product angular distributions and time-of-flight spectra were measured and the translational energy release was determined for each channel. The reaction proceeds primarily on the triplet surface through a long-lived intermediate. For both channels the translational energy distributions were found to peak at about 30% of the total available energy, indicating the existence of an exit channel barrier in each case. The branching ratio between channel (a) and (b) was found to be 1.4±0.5.
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  • 93
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    The Journal of Chemical Physics 91 (1989), S. 5387-5391 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A pump-and-probe technique is utilized to yield a population distribution over the rotational quantum states of the nascent product MgH in the reaction of Mg(1P1) and H2. The resulting normalized profile of the MgH bimodal distribution at 693 K coincides with that at 733 K, as well as with the results obtained at 380 K by Breckenridge and co-workers. This temperature dependence demonstrates that the bimodality actually results from the insertive reaction alone. This conclusion is consistent with the isotopic effect.
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  • 94
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    The Journal of Chemical Physics 91 (1989), S. 5374-5386 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Oscillations with large amplitudes (〉500 K), high frequencies (〉8 Hz), and good reproducibility are observed in methylamine decomposition on electrically heated Pt, Rh, and Ir wires at pressures of 0.25–8.0 Torr and temperatures between 900 and 1500 K. The major reaction involves formation of HCN which is endothermic by 37 kcal/mol, so that the reaction strongly cools the wire, which counteracts resistive heating. A photodiode array was used to monitor spatial and temporal oscillations with resolutions of 1 mm and 30 Hz, respectively. On Pt, the frequency increased with increasing pressure and with decreasing wire diameter, while amplitude was a weak function of these parameters. On Ir, oscillations were more complex with regions of the wire often oscillating independently and more variability between experiments. However, the overall behavior resembles that on Pt. On Rh, frequencies were much lower (〈0.03 Hz), oscillations occurred over a smaller temperature range and halves of the wire frequently oscillated out of phase with each other. Oscillatory behavior of methylamine decomposition on Pt was modeled by a simple lumped model which gives semiquantitative agreement with observations. The model assumes a simple unimolecular reaction with CH3NH2 adsorption blocked at low temperatures by formation of a strongly adsorbed CN which is known to be very stable on Pt (Ed (approximately-equal-to)50 kcal/mol). Calculated frequencies, pressure dependencies, amplitudes, and temperature range of oscillations agree quite well with experiments using reasonable values for adsorption and reaction rate parameters, and wire geometry and properties.
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  • 95
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    The Journal of Chemical Physics 91 (1989), S. 5402-5411 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A method to deconvolute experimental differential cross sections is presented. The procedure removes the effect of the angular divergence of the primary beam, the finite size of the detector, and multiple scattering. The resulting cross section is the optimal experimental result. The method also determines ε=nlQ0, the average number of times each beam particle is scattered. Here n is the number density of scattering gas, l is the length of the scattering region, and Q0 is the true absolute cross section. If the product nl is known, Q0 can be determined. Since the effect of the finite angular resolution of the apparatus has been removed, this method should yield accurate total cross sections. This, in turn, allows the differential cross section to be put on an absolute scale.
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  • 96
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    The Journal of Chemical Physics 91 (1989), S. 5392-5401 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Quenching of highly excited vibrational states of NO2 in baths of CO2, N2O, and CO has been investigated. Dilute NO2 mixtures were excited by a pulse from an excimer pumped dye laser operating at 495 nm. The ν3 antisymmetric stretching modes of CO2 (2349 cm−1) and N2O (2223 cm−1) and the v=1 level of CO (2143 cm−1) were probed with continuous wave IR diode lasers. The amount of energy transferred from excited NO2 to the ν3 modes of both CO2 and N2O was found to be 3%±1% of the original excitation energy. On the other hand, a smaller amount of energy (0.9%±0.3%) was deposited into the CO vibrational mode for NO2 contained in a dilute CO bath. These results support a picture of very inefficient transfer from high energy states of donor molecules to the high-frequency vibrational modes of small bath molecules. Differences in the efficiency of energy reception by the high-frequency modes of these three molecules scale roughly as the infrared transition moments for the bath states indicating the importance of resonance effects in the energy transfer process, but a combination of short- and long-range force mechanisms is probably necessary to account for the observed differences.
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  • 97
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    The Journal of Chemical Physics 91 (1989), S. 5412-5424 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The rate constant of a bimolecular gas-phase chemical reaction is determined by the energy distribution of the reactants and the cross section for the reaction. Systems in which photolysis and other nonequilibrium processes occur may have energy distributions which differ significantly from the equilibrium distribution. Analytical solutions are derived for the rate constant as a function of temperature for certain systems in which nonequilibium processes occur. The analytical solutions are compared with the numerical solutions and the agreement is found to be excellent, confirming the validity of the approximations that were made. Chemical reactions are simulated with the Monte Carlo method and the results again confirm the validity of the analytical solutions. Under certain nonequilibrium conditions, it is shown that the complex expression for the rate constant as a function of temperature reduces to a simple form, which resembles the Arrhenius equation. In this special case, the dependence of the results on the shape of the reaction cross section is investigated.
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  • 98
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    The Journal of Chemical Physics 91 (1989), S. 5440-5443 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In a recent paper it was shown that rare gas ions form bound molecular states which decay by emission of radiation in the VUV. They are potential candidates as storage media for VUV lasers. In the present investigation the ions Kr+K, Kr+Cs, Xe+K, and Xe+Cs are produced by electron beam excitation of suitable gas mixtures with He as buffer gas. Studies of the reaction kinetics were performed for Kr+K. The rate constant for the reaction Kr++K+He was determined to be 2.8×10−30 cm3 s−1. Kr+K is strongly quenched by plasma electrons and by reaction with Kr. The fluorescence yield at intense excitation was determined to be about 5×10−2 VUV quanta per He+ ion.
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  • 99
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    The Journal of Chemical Physics 91 (1989), S. 2124-2139 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Raman scattering in liquid (and in some cases in solid) isotopic mixtures of HC1 and DC1 is analyzed to prove recent theories by Bratos and Tarjus [Phys. Rev. A 32, 2431 (1985)], Logan [Mol. Phys. 58, 97 (1986)], and Knapp [J. Chem. Phys. 81, 643 (1984)] on vibrational line broadening in liquids. The concentration and temperature dependencies of isotropic [Ji(ω)] and anisotropic [Ja(ω)] line shapes have been studied between triple point (Tt) and critical temperature (Tc). It has been found that in accordance with the Bratos–Tarjus theory, Ji (ω) is much more sensitive of isotopic composition of the liquid than Ja(ω). An analysis of the concentration dependence of the broadening parameters near Tt illustrates the importance of cross correlations between the environmental broadening and the resonant intermolecular coupling. The spectral activity of three-particle resonant transfer also becomes significant. From the change of the maximum of Ji(ω) with isotopic dilution, which is a linear function of mole fraction, the dipole moment derivative δμ/δq is estimated to be more than twice that of its gas phase value. The asymmetry of the isotropic bands of both HC1 and DC1 changes with concentration at constant temperature. With increasing temperature, Ji(ω) of pure and diluted samples narrows as T−0.5 and T−0.3, respectively. Ji(ω) has been found to be intermediate between the slow and the fast modulation limit. From the high frequency wing of Ji (ω) the time constant of the zeroth order memory function was obtained. Its activation energy increases with increasing T. This is in qualitative agreement with the temperature dependence of the Enskog collision time and the spin–rotational correlation time. Taking into account the results of the Bratos–Tarjus theory, orientational correlation times τ(2) are determined from Ja (ω). The Raman method yields τ(2) values which are twice as long as those determined from NMR relaxation.
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    The Journal of Chemical Physics 91 (1989), S. 1734-1753 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Theoretical investigations of the outer valence doubly ionized states of ethylene are presented using ab initio Green's function and configuration interaction methods. The vertical double ionization potentials computed by the Green's function method using the ADC(2) scheme are discussed in connection with the Auger spectrum of ethylene and found to reproduce the experimental spectrum quite accurately. Another main purpose of the present work is the investigation of the potential energy surfaces of selected states via ADC(2) and the study of the nuclear dynamics on them. It is shown that only the ground state of the dication is nonplanar with a torsional angle of 90°. The investigation of the topology of the surfaces reveals a low energy conical intersection of the dicationic ground and first excited states. The associated vibronic coupling problem turns out to include, additionally, the second excited state and thus, we encounter here a three-state problem. In the approximation of linear vibrational and vibronic coupling the static and dynamic aspects of the vibronic interaction are investigated by a model Hamiltonian comprising three nonseparable nondegenerate vibrational modes. The input data for the model are extracted from the ADC(2) results. With the aid of this Hamiltonian we have computed a hypothetical "sudden'' double ionization spectrum of ethylene whose complex structure is analyzed stepwise and by cuts through the pertinent adiabatic and diabatic surfaces. The analysis exhibits that the conical intersection of the two lower lying states dominates the vibronic dynamics and leads to strong vibronic mixing between them. This mixing is indirectly reinforced by the interaction between the intermediate and the upper states. As a consequence, strong nonadiabatic effects occur in the ethylene dication. Furthermore, two of the three surfaces involved in the conical intersection are also involved in a multidimensional avoided crossing. A more detailed analysis presented earlier has exhibited that the states belong to an interesting class where two potential energy surfaces are likely not to coincide although plenty of nuclear degrees of freedom exist, in principle, to allow for an intersection to occur.
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