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  • Artikel  (3.694)
  • American Association for the Advancement of Science (AAAS)  (2.156)
  • American Institute of Physics (AIP)  (1.437)
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  • 1990-1994  (3.694)
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  • Artikel  (3.694)
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  • 1
    Digitale Medien
    Digitale Medien
    The quantum mechanics of clusters: The low-temperature equilibrium and dynamical behavior of rare-gas systems (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 6658-6667 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We consider in the present paper the quantum-mechanical effects on the equilibrium and dynamical behavior of low-temperature rare-gas clusters. Using a combination of ground-state and finite-temperature Monte Carlo methods, we examine the properties of small (2–7 particles) neon clusters. We find that the magnitude of the equilibrium quantum-mechanical effects in these systems is significant. The present studies also suggest that the low-temperature dynamics of these rare-gas systems is appreciably nonclassical.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.461536
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  • 2
    Digitale Medien
    Digitale Medien
    Extending J walking to quantum systems: Applications to atomic clusters (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 5713-5731 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The J-walking (or jump-walking) method is extended to quantum systems by incorporating it into the Fourier path integral Monte Carlo methodology. J walking can greatly reduce systematic errors due to quasiergodicity, or the incomplete sampling of configuration space in Monte Carlo simulations. As in the classical case, quantum J walking uses a jumping scheme to overcome configurational barriers. It couples the usual Metropolis sampling to a distribution generated at a higher temperature where the sampling is sufficiently ergodic. The J-walker distributions used in quantum J walking can be either quantum or classical, with classical distributions having the advantage of lower storage requirements, but the disadvantage of being slightly more computationally intensive and having a more limited useful temperature range. The basic techniques are illustrated first on a simple one-dimensional double well potential based on a quartic polynomial. The suitability of J walking for typical multidimensional quantum Monte Carlo systems is then shown by applying the method to a multiparticle cluster system consisting of rare gas atoms bound by pairwise Lennard-Jones potentials. Different degrees of quantum behavior are considered by examining both argon and neon clusters. Remarkable improvements in the convergence rate for the cluster energy and heat capacity, analogous to those found in classical systems, are found for temperatures near the cluster transition regions.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.463756
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  • 3
    Digitale Medien
    Digitale Medien
    Reducing quasi-ergodic behavior in Monte Carlo simulations by J-walking: Applications to atomic clusters (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 2769-2784 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A method is introduced that is easy to implement and greatly reduces the systematic error resulting from quasi-ergodicity, or incomplete sampling of configuration space, in Monte Carlo simulations of systems containing large potential energy barriers. The method makes possible the jumping over these barriers by coupling the usual Metropolis sampling to the Boltzmann distribution generated by another random walker at a higher temperature. The basic techniques are illustrated on some simple classical systems, beginning for heuristic purposes with a simple one-dimensional double well potential based on a quartic polynomial. The method's suitability for typical multidimensional Monte Carlo systems is demonstrated by extending the double well potential to several dimensions, and then by applying the method to a multiparticle cluster system consisting of argon atoms bound by pairwise Lennard-Jones potentials. Remarkable improvements are demonstrated in the convergence rate for the cluster configuration energy, and especially for the heat capacity, at temperatures near the cluster melting transition region. Moreover, these improvements can be obtained even in the worst-case scenario where the clusters are initialized from random configurations.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.458863
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  • 4
    Digitale Medien
    Digitale Medien
    Photodissociation dynamics of the N3 radical (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 2616-2631 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The dissociation dynamics of the B˜ 2Σu+ state of N3 were investigated using fast radical beam photodissociation coupled with a new coincidence wedge-and-strip-anode particle detector. With this detector, detailed photofragment kinetic energy and angular distributions can be measured as a function of excitation energy. Calibration of the detector by studies of the predissociation of the O2 B 3Σu− state is discussed. Methods of extracting the center-of-mass kinetic energy and angular distributions are presented. The photodissociation results for N3 show that from the vibrationless level of the B˜ 2Σu+ state in N3, both spin-allowed [N3→N(2D)+N2(1Σg+)] and spin-forbidden [N3→N(4S)+N2(1Σg+)] dissociation processes occur. Bend excitation in the B˜ 2Σu+ state, however, enhances the spin-allowed dissociation process considerably. The kinetic energy distributions reveal partially resolved vibrational structure in the N2 fragment, and indicate substantial rotational excitation of the N2. This implies that bent geometries play a major role in the dissociation of the N3 B˜ state. Possible mechanisms for the spin-forbidden and spin-allowed dissociation channels are discussed.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.465224
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  • 5
    Digitale Medien
    Digitale Medien
    Fast beam photodissociation of the CH2NO2 radical (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 8751-8764 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The photodissociation of the nitromethyl radical, CH2NO2, has been studied using a fast beam photofragment translational spectrometer. In these experiments, a fast beam of mass selected, internally cold nitromethyl radicals is formed via negative ion photodetachment of CH2NO−2 and subsequently dissociated. The recoiling photofragments are detected in coincidence using a microchannel plate detector equipped with a time- and position-sensing anode. Two dissociation product channels are observed at each of three dissociation wavelengths investigated in the range 240–270 nm and are identified as (I) CH2NO2→CH2NO+O and (II) CH2NO2→H2CO+NO. In marked contrast to the ultraviolet photodissociation of CH3NO2, no evidence is found for simple C–N bond fission to give (III) CH2NO2→CH2+NO2. Translational energy and angular distributions were obtained for the two observed channels. The translational energy distribution of channel (I) peaks at only 5–8 kcal/mol, while the distribution for channel (II) peaks at ∼60 kcal/mol. The angular distributions for both channels are largely isotropic. The nature of the electronic excitation and dissociation dynamics are considered at length. The upper state in the electronic transition is assigned to the 1 2B1 state. Results of attempts to model various aspects of the dissociation dynamics as statistical processes on the ground state surface indicate this mechanism is very unlikely. Instead, both dissociation channels are believed to occur primarily on excited state surfaces, and mechanisms for these processes are proposed.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.465597
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  • 6
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    Interpretation of snow-climate feedback as produced by 17 general circulation models (1991)
    American Association for the Advancement of Science (AAAS)
    Details
    Publikationsdatum: 1991-08-23
    Beschreibung: Snow feedback is expected to amplify global warming caused by increasing concentrations of atmospheric greenhouse gases. The conventional explanation is that a warmer Earth will have less snow cover, resulting in a darker planet that absorbs more solar radiation. An intercomparison of 17 general circulation models, for which perturbations of sea surface temperature were used as a surrogate climate change, suggests that this explanation is overly simplistic. The results instead indicate that additional amplification or moderation may be caused both by cloud interactions and longwave radiation. One measure of this net effect of snow feedback was found to differ markedly among the 17 climate models, ranging from weak negative feedback in some models to strong positive feedback in others.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Cess, R D -- Potter, G L -- Zhang, M H -- Blanchet, J P -- Chalita, S -- Colman, R -- Dazlich, D A -- Genio, A D -- Dymnikov, V -- Galin, V -- Jerrett, D -- Keup, E -- Lacis, A A -- LE Treut, H -- Liang, X Z -- Mahfouf, J F -- McAvaney, B J -- Meleshko, V P -- Mitchell, J F -- Morcrette, J J -- Norris, P M -- Randall, D A -- Rikus, L -- Roeckner, E -- Royer, J F -- Schlese, U -- Sheinin, D A -- Slingo, J M -- Sokolov, A S -- Taylor, K E -- Washington, W M -- Wetherald, R T -- Yagai, I -- New York, N.Y. -- Science. 1991 Aug 23;253(5022):888-92.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/17751825" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Digitale ISSN: 1095-9203
    Thema: Biologie , Chemie und Pharmazie , Informatik , Medizin , Allgemeine Naturwissenschaft , Physik
    Publiziert von American Association for the Advancement of Science (AAAS)
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  • 7
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    Isolation of sequences that span the fragile X and identification of a fragile X-related CpG island (1991)
    American Association for the Advancement of Science (AAAS)
    Details
    Publikationsdatum: 1991-03-08
    Beschreibung: Yeast artificial chromosomes (YACs) were obtained from a 550-kilobase region that contains three probes previously mapped as very close to the locus of the fragile X syndrome. These YACs spanned the fragile site in Xq27.3 as shown by fluorescent in situ hybridization. An internal 200-kilobase segment contained four chromosomal breakpoints generated by induction of fragile X expression. A single CpG island was identified in the cloned region between markers DXS463 and DXS465 that appears methylated in mentally retarded fragile X males, but not in nonexpressing male carriers of the mutation nor in normal males. This CpG island may indicate the presence of a gene involved in the clinical phenotype of the syndrome.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Heitz, D -- Rousseau, F -- Devys, D -- Saccone, S -- Abderrahim, H -- Le Paslier, D -- Cohen, D -- Vincent, A -- Toniolo, D -- Della Valle, G -- New York, N.Y. -- Science. 1991 Mar 8;251(4998):1236-9.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Laboratoire de Genetique Moleculaire des Eucaryotes du CNRS, Institut de Chimie Biologique, Faculte de Medecine, Strasbourg, France.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/2006411" target="_blank"〉PubMed〈/a〉
    Schlagwort(e): Base Sequence ; Chromosomes, Fungal ; Cloning, Molecular ; DNA Probes ; *Dinucleoside Phosphates ; Fragile X Syndrome/*genetics ; Humans ; Male ; Molecular Sequence Data ; Mutation ; Nucleic Acid Hybridization ; Oligonucleotide Probes ; Polymerase Chain Reaction ; Reference Values ; Restriction Mapping ; Saccharomyces cerevisiae/genetics ; *X Chromosome
    Print ISSN: 0036-8075
    Digitale ISSN: 1095-9203
    Thema: Biologie , Chemie und Pharmazie , Informatik , Medizin , Allgemeine Naturwissenschaft , Physik
    Publiziert von American Association for the Advancement of Science (AAAS)
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  • 8
    Digitale Medien
    Digitale Medien
    Fast beam studies of NCO free radical photodissociation (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 4937-4947 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The spectroscopy and dissociation dynamics of the NCO radical have been investigated by applying fast radical beam photodissociation spectroscopy to the B˜ 2Π←X˜ 2 Π electronic transition. Measurements of the photodissociation cross section as a function of dissociation wavelength show that even the lowest vibrational levels of the B˜ 2Π state predissociate. Analysis of fragment kinetic energy release reveals that the spin-forbidden N(4S)+CO(1Σ+) products are produced exclusively until 20.3 kcal/mol above the origin, at which point, the spin-allowed N(2D)+CO product channel becomes energetically accessible. The spin-allowed channel dominates above this threshold. By determining the location of this threshold, we obtain a new ΔHf0 for NCO of 30.5±1 kcal/mol, several kcal/mol lower than the previously accepted value.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.463989
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  • 9
    Digitale Medien
    Digitale Medien
    Evolution of the core and unoccupied orbitals of biphenyl and bithiophene on Cs doping (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 4489-4495 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: X-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS) have been used to study the core and unoccupied orbitals of condensed multilayers of biphenyl and bithiophene on Cs dosing. The CK XPS shifts, observed on dosing are understandable in terms of repositioning of the Fermi level due to creation of states in the band gap, and at higher Cs concentrations to the high polarizability of the charge transfer complexes formed. The behavior of the CK XPS satellites on increasing Cs exposure are related to changes observed in the electron energy loss spectra (EELS). The CK NEXAFS of biphenyl and the SL23 NEXAFS of bithiophene indicate that an unoccupied molecular orbital is filled by charge transfer from Cs. In the case of bithiophene, both the SL23 XPS and NEXAFS features shift by 2 eV to lower binding and adsorption energy, respectively. This strong chemical shift suggests significant localization of the transferred charge on the S sites. Comparison of the SL23 NEXAFS of doped and undoped bithiophene allows the identification of the unoccupied orbital with S character and leads to a reassignment for the NEXAFS features of thiophene and its oligomers.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.463892
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  • 10
    Digitale Medien
    Digitale Medien
    Torsion–rotation interactions in a two-top molecule: High resolution S1←S0 electronic spectrum of 2,3-dimethylnaphthalene (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 7862-7871 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A rotationally resolved fluorescence excitation spectrum of the O00 band in the S1←S0 transition of 2,3-dimethylnaphthalene has been obtained using a cw laser/molecular beam spectrometer. More than 3000 lines were observed, each exhibiting a width of about 3 MHz. Despite the proximity of the two methyl groups, the observed rotational structure can be interpreted satisfactorily using an uncoupled rotor model. However, extensive torsion–rotation interactions are observed. Accurate measurements of these perturbations are used to determine the effective threefold hindering potentials in both electronic states, V3(S0)=652 cm−1 and V3(S1)=391 cm−1, respectively. Rotor–rotor couplings do influence these barriers as well as the patterns of lines observed in the rotationally resolved spectra of higher vibronic bands.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.461315
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