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  • 1
    Electronic Resource
    Electronic Resource
    Size-dependence of the electronic spectra of benzene⋅(N2)n clusters (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 6296-6302 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The evolution of the electronic spectral properties of benzene⋅(N2)n clusters to those of the corresponding bulk has been studied using a formalism that yields a direct correlation between cluster geometries and spectra. We show here that the calculated spectral shifts of the benzene chromophore as a function of cluster size are in good agreement with experimental values and that, as observed previously, the asymptotic limit of the shifts is considerably smaller in magnitude than the observed bulk values. An examination of the structures of these cryogenic clusters reveals a strong influence of quadrupolar interactions between dinitrogens, interactions that yield a degree of ordering not present in the ostensibly similar benzene⋅(Ar)n clusters. These observations suggest a resolution of the problem posed by the apparent lack of asymptotic convergence to bulk spectral shifts, that at the higher temperatures associated with the bulk measurements, higher local dinitrogen densities become possible as the thermal energy exceeds the quadrupolar orientational ordering energy. These higher "solvent" densities would be expected to produce the larger spectral shifts seen in experiments. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477271
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  • 2
    Electronic Resource
    Electronic Resource
    Sterols. XCI. Oxidation of the Diacetate of Cholestanediol-3,6 (1940)
    s.l. : American Chemical Society
    Journal of the American Chemical Society 62 (1940), S. 645-646 
    Details
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/ja01860a060
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  • 3
    Electronic Resource
    Electronic Resource
    Internal state dependence of HCl/Ar(111) desorption rates (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 4073-4078 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The rate of desorption of HCl from an Ar(111) surface as a function of the internal state of the physisorbed molecule is studied via a quasiclassical trajectory simulation of the system dynamics. Results from these calculations indicate that the desorption rate passes through a maximum at an initial rotational quantum number of 6, with the maximum rate being enhanced but the rate behavior not being altered qualitatively when a quantum of vibrational energy is added to the molecule. This observed rate dependence may be understood in terms of a transition of the system between distinct dynamical regimes that occurs as the rotational energy of the molecule is increased. Surface of section plots analogous to those used in the study of intramolecular energy transfer are found to provide a convenient means for identifying a transition of this sort.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.450878
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  • 4
    Electronic Resource
    Electronic Resource
    Semiclassical vibration–rotation spectra of gaseous and physisorbed molecules (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 84 (1986), S. 3589-3597 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A semiclassical spectral intensity method is applied to the calculation of vibration–rotation spectra both of isolated molecules and of molecules physically adsorbed on a solid surface. For the case of an isolated HCl molecule, we are able to generate discrete vibration–rotation spectral lines, the frequencies and integrated intensities of which agree well with the available literature values. Line shapes obtained for this case exhibit no evidence of broadening beyond the theoretical resolution of the calculation. Physisorption of the HCl on an Ar(111) surface leads, however, to a collapse of the R, P band structure at low rotational energies, with the free rotor dynamical limit being reached only at higher energies. The transition between dynamical regimes is associated with substantial line broadening and shifts in the line centers as well as with an increase in the desorption rate. A loss of rotational phase coherence appears to represent the principle line broadening mechanism in the present calculations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.450194
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  • 5
    Electronic Resource
    Electronic Resource
    Solvation by nonpolar solvents: Shifts of solute electronic spectra (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 775-788 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: It is only relatively recently that it has become possible to use spectroscopy to track the solvation of a molecule as one proceeds from small solute-plus-solvent clusters, through bulk liquids, and into cryogenic matrices. One of the more surprising findings of such studies is that, in a number of noteworthy instances—such as with benzene dissolved in Ar—the solvent shifts of spectral lines in even apparently sizable clusters seem not to go smoothly into the bulk results. In this and the following paper we consider just what level of theoretical treatment is necessary in order to be able to account for the solvent shift of electronic spectra consistently in environments ranging from clusters to the bulk. As we discuss in some detail, neither continuum dielectric approaches nor sums of pair potentials can adequately describe the solvation. What we propose here, instead, is that the effects of nonpolar solvents can be treated fully microscopically by a model incorporating both local repulsive effects and longer-ranged dielectric effects. The latter contribution, resulting from the solvent's polarizability, is formulated in terms of the so-called polarization modes of the solvent, which change with the detailed arrangement of the solute's environment. We illustrate the ideas by showing that one can understand the optical spectroscopy of benzene in liquid Ar more or less quantitatively by using this model, and we point out some connections with analogous time-dependent solvation studies. The application of this same approach to clusters is described in the succeeding paper.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466203
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  • 6
    Electronic Resource
    Electronic Resource
    Optical properties of a chromophore embedded in a rare-gas cluster: Cluster size dependence and the approach to bulk properties (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 789-799 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: One of the most intriguing aspects of the behavior of small clusters is the extent to which their physical and chemical properties depend sensitively on the size of the clusters. But for clusters that are "large enough,'' surely their properties must approach those of the corresponding bulk systems. The property of special interest in the present work is the shift in the electronic absorption of a chromophore (benzene) deriving from interaction with nonpolar solvent atoms (Ar), a shift that can be calculated using a microscopic formalism described in this and in the preceding paper which is equally well suited to the study of cluster and condensed phase environments. We are able to identify the evolution of the collective character of the dielectric response of the solvent atoms as being the key feature of the optical properties of these clusters that determines the degree to which their behavior is bulklike. Furthermore, we can associate specific spectral features with particular classes of cluster geometries, and in doing so we derive support for our previous speculations concerning the evidence for the contribution of metastable, nonwetting cluster structures to the experimental spectra.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465342
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  • 7
    Electronic Resource
    Electronic Resource
    Dynamics of a physisorbed dimer (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 522-528 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report an application of the semiclassical spectral intensity method to the calculation of the absorption spectrum of a simple molecular dimer, (HCl)2, for both the free and physisorbed species. The calculated spectra show a clear indication of the presence of two distinct kinds of hydrogen atoms in this system, one which is involved in a hydrogen bond and one which is unaffected by the dimerization. We also find direct evidence that vibrational energy transfer plays an important role in the dynamics of this system and markedly affects the observed structure of the absorption spectrum.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455496
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  • 8
    Electronic Resource
    Electronic Resource
    Dynamics of CO chemisorption on a metal cluster (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 1849-1859 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The understanding of the dynamics of collisions between molecules and small metal clusters is an important component of a general understanding of practical heterogeneous catalytic processes. We report here quasiclassical trajectory studies of CO colliding with a five-atom metal cluster, with our primary focus being on the modeling of collisions with a copper cluster. Since the forces in these systems are still largely unknown, we have considered a number of simple potential-energy parametrizations so as to determine the range of behavior which might be expected. In general, CO is found to add readily to the metal cluster, forming a complex which is stable on the time scale of the trajectories. The effects of variations in the cluster geometry and extensions to systems having higher molecule-cluster binding energies are also considered here, as is the question of whether or not there is a significant distortion of the metal cluster geometry upon addition of CO. This last topic is conveniently approached via a combination of trajectory and Monte Carlo simulated annealing techniques.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458067
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  • 9
    Electronic Resource
    Electronic Resource
    Extensions to the instantaneous normal mode analysis of cluster dynamics: Diffusion constants and the role of rotations in clusters (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 1632-1640 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: For the instantaneous normal mode analysis method to be generally useful in studying the dynamics of clusters of arbitrary size, it ought to yield values of atomic self-diffusion constants which agree with those derived directly from molecular dynamics calculations. The present study proposes that such agreement indeed can be obtained if a sufficiently sophisticated formalism for computing the diffusion constant is adopted, such as the one suggested by Madan, Keyes, and Seeley [J. Chem. Phys. 92, 7565 (1990)]. In order to implement this particular formalism, however, we have found it necessary to pay particular attention to the removal from the computed spectra of spurious rotational contributions. The utility of the formalism is demonstrated via a study of small argon clusters, for which numerous results generated using other approaches are available. We find the same temperature dependence of the Ar13 self-diffusion constant that Beck and Marchioro [J. Chem. Phys. 93, 1347 (1990)] do from their direct calculation of the velocity autocorrelation function: The diffusion constant rises quickly from zero to a liquid-like value as the cluster goes through (the finite-size equivalent of) the melting transition.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459091
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  • 10
    Electronic Resource
    Electronic Resource
    New insight into experimental probes of cluster melting (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 1358-1368 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Experiments are now appearing which attempt to probe melting in small clusters, a notable example of these being the recent studies of benzene–Arn clusters by Hahn and Whetten [Phys. Rev. Lett. 61, 1190 (1988)]. We report a study of the dynamics of these same benzene–Arn clusters which seeks to clarify further the nature of "phase transitions'' in small systems. The techniques used here, involving an instantaneous normal mode analysis based on the results of Monte Carlo calculations, have been shown previously to yield a picture of argon cluster melting which is more complete than the one which emerges from a mere enumeration of low-energy structures. Although the bare argon clusters are found to undergo dynamical transitions as the cluster temperature is increased, these transitions are inhibited by the presence of an embedded benzene molecule, which provides a template for ordering of the argon atoms. The calculations also suggest a possible explanation for the doubly peaked spectra observed in the experiments of Hahn and Whetten: the benzene may be found in two general types of bonding environments, either surrounded by or on surface of the Arn cluster. This occurrence is consistent with the finding of Bösiger, Knochenmuss, and Leutwyler [Phys. Rev. Lett. 62, 3058 (1989)] that the solute–solvent interaction in clusters can be either wetting or nonwetting. We discuss how the evidence necessary to resolve such issues can be derived from an analysis of cluster dynamics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459146
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