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  • 1
    Electronic Resource
    Electronic Resource
    Phase space bifurcation structure and the generalized local-to-normal transition in resonantly coupled vibrations (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 2251-2268 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The generalization of the local-to-normal transition seen in symmetric triatomics is considered for nonsymmetric molecules and 2:1 Fermi resonance systems. A straightforward generalization based on a division of phase space into local and normal regions is not possible. Instead, classification of the phase space bifurcation structure is presented as the complete generalization of the local–normal concept for all spectroscopically relevant systems of two vibrations interacting via a single nonlinear resonance. The polyad phase sphere (PPS) is shown to be the natural arena to analyze the bifurcation structure for resonances of arbitrary order. For 1:1 and 2:1 resonances, the bifurcation problem is reduced to one or two great circles on the phase sphere. All bifurcations are shown to be examples of elementary bifurcations of vector fields in one dimension. The classification of the bifurcation structure is therefore governed and greatly simplified by the theory of the universal unfolding and codimension of elementary bifurcations. The implications for large-scale bifurcation structure and transport in molecules with chaotic motion are briefly discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458018
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  • 2
    Electronic Resource
    Electronic Resource
    Electronic excitation dynamics of Li(H2)2: Dissociation mechanisms, lifetimes, and the validity of a hybrid quantum/classical approach (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 4056-4062 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dissociation dynamics of the cluster Li(H2)2, following the 2s→2p excitation of the Li atom, is studied in the framework of a collinear model. The process was investigated by exact quantum wave packet calculations, and the results were used to test a hybrid quantum/classical method, in which the highly quantum mechanical initial state of the cluster is described by a wave function, and the latter is used to sample initial positions and momenta for a classical treatment of the excited state dynamics. We found that the dynamics was dominated by two predissociation processes, Li*(H2)2→Li*–H2+H2 and Li*(H2)2→Li*+(H2)2, with the latter process having a higher yield. A relatively long dissociation lifetime, ∼10 ps, was found for the excited cluster. The slow vibrational predissociation rate was interpreted as due to the very low density of state involved. The hybrid quantum/classical approach was found to give product vibrational energy and velocity distributions in good accord with the distribution from exact calculation. However, the lifetimes from the hybrid approach were found to be much shorter than those from the exact quantum calculations. The hybrid approach is thus applicable even to photoexcitation of quantum clusters for studying certain selected properties. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468533
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  • 3
    Electronic Resource
    Electronic Resource
    Spectral patterns and dynamical bifurcation analysis of highly excited vibrational spectra (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 4340-4354 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Spectral patterns associated with recently proposed assignments of Fermi resonance systems are investigated with specific application to the 2:1 Fermi resonance fitting Hamiltonian. It is shown that the spectrum of a pair of resonant modes has characteristic patterns associated with the structure of the corresponding classical phase space. In particular, when a spectral fitting Hamiltonian has a separatrix structure in its classical phase space, the quantum Hamiltonian has an emblematic spectral pattern, a "dip'' in the level spacings. This basic pattern is the starting point for an investigation of level patterns corresponding to the bifurcation and catastrophe map classification and associated dynamically based assignments of Fermi resonance Hamiltonians. The 2:1 Fermi resonance Hamiltonian is investigated in detail as a typical system. There are distinctive patterns for polyads from different zones of the catastrophe map classification of the 2:1 system. Conversely, when these patterns occur in an experimental spectrum, then in order to reproduce them in a reasonably behaved spectral fit, it is necessary and sufficient to invoke a resonant coupling term in the fitting Hamiltonian. Spectral fitting therefore gives reliable information about the phase space structure of a molecule. These considerations are used to address the interpretation of recent experimental and theoretical investigations of H2CO and benzophenone vibrational spectra. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469483
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  • 4
    Electronic Resource
    Electronic Resource
    A theoretical study of solid hydrogens doped with atomic oxygen (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 942-953 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Structure and reaction dynamics in solid H2/D2 doped with O(3P, 1D, 1S) is investigated through simulations based on accurate ab initio potential energy surfaces. The ab initio calculations are performed at MCSCF level, with neglect of spin–orbit interactions. The dynamical simulations rely on nonadditive effective potentials, taking into account the anisotropy of the open shell atom by using diabatic representations for the globally fitted potential energy surfaces of O–H2. The ground state of the doped solid is well described as O(3P) isolated in para-H2(J=0) since the atom–molecule interaction anisotropy is not sufficient to orient H2. O(3P) atoms radially localize the nearest-neighbor shell, and lead to a linear increase in the density of the solid as a function of impurity concentration. The doped solid is stable at cryogenic temperatures, with a free energy barrier for recombination of next nearest-neighbor O(3P) atoms of 120 K. The solid state O(1D)+H2 reaction is considered in some depth. While in high symmetry sites the reaction is forbidden, even at 4 K, thermal fluctuations are sufficient to promote the insertion reaction. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473174
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  • 5
    Electronic Resource
    Electronic Resource
    Erratum: Spectral patterns and dynamical bifurcation analysis of highly excited vibrational spectra [J. Chem. Phys. 102, 4340 (1995)] (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 3870-3870 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470735
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  • 6
    Electronic Resource
    Electronic Resource
    Simulation of ultrafast dynamics and pump–probe spectroscopy using classical trajectories (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 6919-6929 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper, we develop a method for accurately modeling ultrafast molecular dynamics and pump–probe spectroscopy using classical trajectory simulations. The approach is based on a semiclassical limit of the Liouville formulation of quantum mechanics. Expressions for the nonstationary classical phase space probability density created by an ultrashort laser pulse on an excited electronic state, and the observable fluorescence signal resulting from a pump–probe experiment, are derived in the weak-field limit using perturbation theory. By introducing additional approximations, these expressions are cast in a form that can be directly implemented using classical trajectory integration and ensemble averaging. The method is tested against multisurface time-dependent quantum mechanical wave packet calculations for a one-dimensional model system representing I2 photodissociation-recombination in a static Ar lattice. Nearly quantitative agreement between the exact calculations and the trajectory-based method is obtained. Although demonstrated for a one-dimensional system, the method is easily incorporated in conventional molecular dynamics programs, allowing efficient treatment of many-body ultrafast dynamics and spectroscopy. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471407
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  • 7
    Electronic Resource
    Electronic Resource
    Nonlinear resonance and correlated binary collisions in the vibrational predissociation dynamics of I2(B,v)–Ar13 clusters (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 8514-8524 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The vibrational predissociation dynamics of the van der Waals cluster I2(B,v)–Ar13 are studied theoretically from a classical mechanical perspective. The focus is on the ejection of the first Ar atom from the excited cluster. This process is found to occur by two competing mechanisms (1) evaporation of a cluster atom induced by the slow vibrational relaxation of the I2 impurity, and (2) direct ejection of a geometrically favored Ar atom by two or more impulsive collisions with the vibrating I2. In contrast with the picture of independent binary collisions, the multiple argon–iodine interactions leading to successful direct dissociation are correlated in time. The relative propensity of the direct channel is a highly structured function of v, the initial vibrational level of the I2. This behavior results from the energy-dependent frequencies of motion of the diatomic and the localized Ar cluster mode, and is a novel example of a nonlinear resonance between "system'' and "bath'' degrees of freedom.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.464510
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  • 8
    Electronic Resource
    Electronic Resource
    Nonexponential behavior in the vibrational predissociation dynamics of I2(B,ν)–Ar13 (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 7234-7241 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The vibrational predissociation dynamics of an Ar13 cluster containing a vibrationally excited I2 molecule in its B electronic state are studied by classical trajectory simulation. The kinetics of the loss of the first Ar atom from the cluster induced by vibrational relaxation of the diatomic exhibits unusual nonexponential behavior, characterized by an instantaneous rate that increases with time. The simulation results are modeled by a hybrid statistical model, which explicitly takes into account the slow relaxation of the impurity while assuming rapid redistribution of energy among the low frequency cluster modes. Minor deviations between the simulation and theory are observed, and indicate nonstatistical effects in the "solvent'' degrees of freedom.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463549
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  • 9
    Electronic Resource
    Electronic Resource
    Treatment of zero-point motions in cluster dynamics: Semiclassical time-dependent self-consistent-field simulation of (Ne)N (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 8637-8643 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A semiclassical time-dependent self-consistent-field (TDSCF) method is developed for dealing with the difficulties caused by the nonpreservation of zero-point energy in classical molecular dynamics simulation. The method is applied to a collinear model of a (Ne)12 cluster at very low temperatures. Classically, this system dissociates rapidly due to its zero-point energy. We show that the system remains stable when treated by the new method. The normal mode dynamics of the anharmonic cluster are calculated and discussed. Interesting results are obtained on the lifetimes of single-mode states and energies due to the coupling between the modes. Some of the single-mode states have subpicosecond lifetimes, while others are stable for at least 60 ps. The results illustrate the power of semiclassical TDSCF as a tool for studying the vibrational dynamics of anharmonic cluster at very low temperatures.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465587
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  • 10
    Electronic Resource
    Electronic Resource
    U. S. Householder survey of functional gastrointestinal disorders (1993)
    Springer
    Digestive diseases and sciences 38 (1993), S. 1569-1580 
    Details
    ISSN: 1573-2568
    Keywords: functional bowel disorders ; epidemiology ; diagnosis ; demography ; health care use ; work absenteeism
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract Our objective was to obtain national data of the estimated prevalence, sociodemographic relationships, and health impact of persons with functional gastrointestinal disorders. We surveyed a stratified probability random sample of U.S householders selected from a data base of a national market firm (National Family Opinion, Inc.). Questions were asked about bowel symptoms, sociodemographic associations, work absenteeism, and physician visits. The sampling frame was constructed to be demographically similar to the U.S. householder population based on geographic region, age of householder, population density, household income and household size. Of 8250 mailings, 5430 were returned suitable for analysis (66% response). The survey assessed the prevalence of 20 functional gastrointestinal syndromes based on fulfillment of multinational diagnostic (Rome) criteria. Additional variables studied included: demographic status, work absenteeism, health care use, employment status, family income, geographic area of residence, population density, and number of persons in household. For this sample, 69% reported having at least one of 20 functional gastrointestinal syndromes in the previous three months. The symptoms were attributed to four major anatomic regions: esophageal (42%), gastroduodenal (26%), bowel (44%), and anorectal (26%), with considerable overlap. Females reported greater frequencies of globus, functional dysphagia, irritable bowel syndrome, functional constipation, functional abdominal pain, functional biliary pain and dyschezia; males reported greater frequencies of aerophagia and functional bloating. Symptom reporting, except for incontinence, declines with age, and low income is associated with greater symptom reporting. The rate of work/school absenteeism and physician visits is increased for those having a functional gastrointestinal disorder. Furthermore, the greatest rates are associated with those having gross fecal incontinence and certain more painful functional gastrointestinal disorders such as chronic abdominal pain, biliary pain, functional dyspepsia and IBS. Preliminary information on the prevalence, socio-demographic features and health impact is provided for persons who fulfill diagnostic criteria for functional gastrointestinal disorders.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01303162
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