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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 10152-10161 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Results from nonequilibrium molecular-dynamics simulations of collisional energy transfer from vibrationally highly excited azulene in compressed CO2 are compared with experimental results from our laboratory obtained under comparable physical conditions. As observed in the experiment, the cooling rates show a purely monoexponential decay of the excess energy. The influence of the microscopic solvent shell structure on these processes is investigated using the full three-dimensional anisotropic CO2 structure around azulene obtained from the simulation. The analysis shows that local heating effects of any kind do not play a role in our model system. Predictions of the pressure dependence of the energy transfer rates by the isolated binary collision model are compared with results from the simulations using two different definitions of the collision frequency in dense fluids. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The expressions for vibrational energy relaxation (VER) rates of polyatomic molecules in terms of equilibrium capacity time correlation functions (TCFs) derived in the first paper of this series [J. Chem. Phys. 110, 5273 (1999)] are used for the investigation of VER of azulene in carbon dioxide at low (3.2 MPa) and high (270 MPa) pressure. It is shown that for both cases the VER times evaluated on the basis of the same potential model via solute–solvent interaction capacity TCFs by means of equilibrium molecular dynamics (EMD) simulations satisfactorily agree with the nonequilibrium (NEMD) molecular dynamics [J. Chem. Phys. 110, 5286 (1999)] and experimental [J. Chem. Phys. 105, 3121 (1996)] results as well. Thus it follows that these methods can complement each other in characterizing VER from different points of view. Although more computational power and refined methods of dealing with simulated data are required for EMD simulations, they allow the use of powerful tools of equilibrium statistical mechanics for investigating the relaxation process. To this end, an analysis of VER mechanisms on the basis of normal mode and atomic representations is carried out. The influence of temperature and CO2 pressure on azulene normal mode spectra and solvent assisted intermode coupling in connection with the eigenvector structure is investigated in great detail. The normal mode capacity cross-correlation matrix reveals the significance of intermode coupling, which significantly contributes to intramolecular vibrational energy redistribution (IVR). As a new concept, partial normal mode relaxation rates are introduced. It is shown that these rates demonstrate similar properties as the energy exchange rates through particular normal modes in nonequilibrium simulations. Atomic spectra and friction coefficients are characterized by a complicated frequency dependence due to contributions from many normal modes. Atomic capacity TCFs and partial relaxation rates are analyzed and reveal a similar picture to that obtained from NEMD simulations. These results show that VER and IVR cannot be separated from each other and have to be considered as mutually connected processes. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 7749-7755 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular dynamic (MD) simulations of liquid CO2 at 250 MPa pressure and room temperature have been performed using a flexible model potential. A detailed analysis of the data reveal the full three-dimensional local structure of coordination shells that exhibit significant deviations from spherical symmetry with strong angular correlations among the molecules which form the inner coordination shell of the local liquid structure. Structures resembling T-shaped and offset-parallel CO2 dimers similar to those found in molecular beam and low temperature experiments have been identified, the T-shaped dimer having higher probability to be formed than the offset-parallel configuration. Local motion on short time scale is found to be different along three principal directions of a local coordinate frame. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Journal of statistical physics 52 (1988), S. 1005-1029 
    ISSN: 1572-9613
    Keywords: Noise in dynamic systems ; bifurcations ; Fokker-Planck equations ; degenerating parabolic-type equations
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract By an example of a two-dimensional hydrodynamic system, second-order Langevin equations with two correlated noise sources are investigated. It is shown that the asymptotic expression (t→∞) for the stationary distribution functionP depends on the order in which the limiting transitions;t→∞ andN 22→0 (N 22 is the power of one of the noises) are made. Using the method of local expansions in trigonometric form, approximate expressions are written for the distribution functionP at small but finiteN 22 tending atN 22→0 to the known exact solution.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    The European physical journal 82 (1991), S. 441-451 
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A method of adiabatic elimination is proposed based on the use of the Furutsu-Novikov formula. A case of two nonlinear Langevin equations and a spatially distributed problem typical for the nonlinear wave propagation in random media have been considered. The method not only permits adiabatic elimination of the fast-decaying variable from the equation for the slow-decaying one but also allows for the return effect of the slow-decaying subsystem on the fast-decaying one.
    Type of Medium: Electronic Resource
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  • 6
    ISSN: 1572-9613
    Keywords: White noise ; bifurcation ; dynamical systems ; hydrodynamic system ; Gaussian approximation ; functional-rational approximation ; stationary distribution function
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The influence is considered of two additive correlated noise effects on a two-dimensional quadratic-nonlinear system describing the behavior of two hydrodynamic modes. Using the method of Gaussian approximation, local characteristics of the distribution function are calculated, which are used to construct the global distribution function with the aid of the method of fraction-rational approximations. It is shown that for a system at whose bifurcation point the asymptotic stability is lost, in an expanded space of parameters (bifurcation parameter in the absence of noise plus noise parameters) there appears an instability zone within which the stationary distribution function does not exist. The effect of noise correlation on the stationary characteristics of the system is studied.
    Type of Medium: Electronic Resource
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