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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 6765-6771 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Vibrational spectra of liquid oxygen, shock compressed to several high-pressure/high-temperature states, were obtained using single-pulse multiplex coherent anti-stokes Raman scattering (CARS). The experimental spectra were compared to synthetic spectra calculated using a semiclassical model for the CARS intensities and best fit vibrational frequencies, peak Raman susceptibilities, and Raman linewidths. Up to the maximum shock pressure of 9.6 GPa, the vibrational frequencies were found to increase monotonically with pressure. An empirical fit, which could be used as a pressure/temperature/frequency calibration standard, showed that the Raman frequency shifts could be accurately described by linear pressure and temperature dependences. Above ≈9 GPa, the liquid oxygen opacity at 632.8 nm increased rapidly, presumably because of proximity (collision)-induced absorption. Calculations showed that the induced absorption did not resonantly enhance the CARS spectra, but did attenuate the laser beams and the CARS signals. The measured linewidths suggest that the vibrational dephasing time decreased to approximately 1 ps at the highest pressures.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 1368-1376 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Vibrational spectra of liquid nitrogen shock compressed to several high pressure/high temperature states were recorded using single-pulse multiplex coherent anti-Stokes Raman scattering. Vibrational frequencies, third-order susceptibility ratios, and linewidths are presented for the fundamental and several excited-state transitions. Vibrational frequencies were found to increase monotonically up to (approximate)17.5 GPa single shock and (approximate)30 GPa double shock. Above these pressures, the vibrational frequencies were observed to decrease with further increases in pressure. The decrease in vibrational frequency occurs in a pressure regime where the shocked nitrogen is becoming optically opaque. The consequence of the decrease in vibrational frequency on the Grüneisen mode gamma and its effect on the N2 equation of state is discussed. The transition intensity and linewidth data suggest that thermal equilibrium of the vibrational levels is attained in less than 10 ns at these high pressures and temperatures. Finally, the measured linewidths exhibit an almost linear dependence on shock temperature, and also suggest that the vibrational dephasing time has decreased to less than 1 ps at the highest pressures.
    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 94 (1991), S. 7550-7553 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new Monte Carlo simulation method is introduced which gives the equilibrium chemical composition of a molecular fluid directly. The usual NPT ensemble (isothermal–isobaric)is implemented with N being the number of atoms instead of molecules. Changes in chemical composition are treated as correlated spatial moves of atoms.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 2312-2323 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The standard NVT ensemble Metropolis Monte Carlo method is modified to include uniformly weighted steps in ln β as well as weighted steps in U, the total potential energy. This simulation is used to sample a soft sphere (r−12 potential) fluid over a very wide range of energies. For each r−12 energy U12 , the corresponding r−6 energy U6 is also recorded. Information from the simulation is used to determine relative values of the configurational density of states γ12(U12 ) (i.e., the number of configurations with total potential energy between U12 and U12+dU12) for the soft sphere fluid over a large range in energy. Then using the distribution of U6 at each U12, the configurational density of states for arbitrary linear combinations of U12 and U6 is determined and the equation of state for the Lennard-Jones fluid is evaluated for a wide range of densities and temperatures in a very efficient manner. The generalization to simulations of other linear combinations of potentials is discussed.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 1271-1275 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We examine the fluid thermodynamics of a model homonuclear diatomic system with anisotropies characteristic of CO2. The density (CO2 densities) and temperature regime is 1.6 g/cm3(approximately-less-than)ρ(approximately-less-than)2.6 g/cm3 and 1000 K(approximately-less-than)T(approximately-less-than)7000 K. Extensive molecular dynamics data for the model equation of state are presented. Comparisons are then made to the thermodynamics from three effective spherical potentials; the potential median, the radial median, and an exponential-six with parameters adjusted to best fit the true thermodynamics. The two median potentials typically give 3% agreement for the higher temperature fluid with a 5%–10% comparison nearer the freezing line for both pressure and internal energy while the fit is good to 3% or better. Thus there exists an effective spherical potential that very accurately models the thermodynamics of dense fluid CO2, a system whose potential energy in the repulsive region varies by three to four orders of magnitude as a function of angles with fixed center of mass separation. The median averages give an excellent representation of this effective spherical potential.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 82 (1985), S. 4264-4275 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Sets of pressures and their corresponding specific volumes and internal energies are derived from measurements on steadily propagating, planar shock waves propelled by explosively driven metal assemblies into a 1:1 atomic mixture of the elements nitrogen and oxygen in each of two liquid initial states. One of these is the equimolar solution of O2 and N2, at T(approximately-equal-to)85 K, v0(approximately-equal-to)1.06 cm3/g; the other is the pure explosive compound NO, at T(approximately-equal-to)122 K, v0(approximately-equal-to)0.79 cm3/g. Results for this system are calculated with effective spherical potentials and presented graphically for comparison with the measurements. Single- and reflected-shock states are reported, as are incidental new results on pure liquid N2 at 85 K. The method of measurement is described, with reference to its previous applications to liquid O2 and Ar. First-shock pressures from both initial forms lie between 10 and 30 GPa, and the Hugoniots intersect at a common state, near 21 GPa, where a single reflected-shock Hugoniot is centered. Concordant measured state variables at this intersection provide novel confirmation of the expectation, inherently incorporated into theory, that unique equilibrium states are reached. Accounting for densities of these states by theory indicates a significant amount of oxidized nitrogen, in reversible equilibrium with major, but not exclusive, N2 and O2 components. This is treated as residual NO only, although the uncertainty in the potentials for other oxides does not assure their absence.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 82 (1985), S. 3765-3772 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular dynamics (MD) simulation data for rigid diatomic models of N2 and CO2 under conditions of extremely high density and temperature are analyzed for static correlation functions. The results show some significant qualitative differences from those for diatomic fluids at normal densities and temperatures (i.e., near the triple point). For a single thermodynamic state of N2, the radial distribution functions (RDFs) of the (spherical) RAM and median potentials are found, also by MD. Whereas the median gives good thermodynamic results and poor centers correlation functions, RAM produces just the opposite. Thus no explanation in terms of distribution functions is found for the success of the median for thermodynamics although an empirical correlation is found between the breakdown of median thermodynamics for CO2 and a distinctive feature of the molecular correlation functions.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 3488-3494 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Single-pulse multiplex coherent anti-Stokes Raman scattering (CARS) was used to obtain vibrational spectra of 20%/80% liquid nitrogen/argon mixtures, shock compressed to several high-pressure/high-temperature states. A semiclassical model for CARS spectra was used to extract best fit vibrational frequencies, peak Raman susceptibilities, and Raman linewidths from the data. Up to a maximum single shock pressure of 17.1 GPa, the N2 vibrational frequency was found to increase monotonically with pressure. The vibrational frequencies measured in both the singly and doubly shocked N2/Ar mixtures correspond within experimental error to those for pure nitrogen at equivalent pressures and temperatures, implying that the influence of the interaction potential on the N2 vibrational frequency for the N2/Ar collision is not significantly different from that of a N2/N2 collision at these conditions. The transition intensity and linewidth data suggest that thermal equilibrium of the vibrational levels is attained in less than 10 ns at these shock pressures. Vibrational temperatures obtained were used to improve the potential function of argon used to calculate equation-of-state pressures and temperatures. The measured linewidths suggest that the nitrogen vibrational dephasing time decreased to about 1 ps at the highest pressure shock state.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 84 (1986), S. 3479-3483 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: For a certain class of thermodynamic perturbation theories, a generalization of the Gibbs–Bogoliubov inequality holds through second order of perturbation theory and for a subset of terms the inequality is true to infinite order. Using this approximate variational principle, a perturbation theory is chosen for which the Helmholtz free energy of the reference system is minimized under the constraint that the first order term is identically zero. We apply these ideas to the determination of effective spherical potentials that accurately reproduce the thermodynamics of nonspherical molecular potentials. For a diatomic-Lennard-Jones (DLJ) potential with l/σ=0.793, the resulting spherical reference potential is identical to the median average over angles for the repulsive part of the potential, but differs in the attractive well. The variational effective spherical potential leads to more accurate thermodynamics than the median, however, particularly in the triple point region.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 325-336 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A two-stage light gas gun and single-pulse multiplex coherent anti-Stokes Raman scattering (CARS) have been used to obtain carbon monoxide, nitrogen, and oxygen vibrational spectra for several high-pressure/high-temperature, dense fluid, carbon monoxide–oxygen, and nitrogen–oxygen mixtures. The experimental spectra were compared to synthetic spectra calculated with a semiclassical model for CARS intensities and using best fit vibrational frequencies, peak Raman susceptibilities, and Raman linewidths for each mixture component. Up to a maximum shock pressure of 6.75 GPa for carbon monoxide–oxygen mixtures, the CO and O2 vibrational frequencies were found to increase monotonically with pressure and depended on the carbon monoxide–oxygen mixture ratio. For the nitrogen–oxygen mixtures, the N2 vibrational frequency increased monotonically with pressure to a maximum experimental pressure of 12.9 GPa, however the O2 vibrational frequency increased with pressure to about 11 GPa and then appeared to decrease slightly as the pressure increased to the experiment maximum of 12.9 GPa. Empirical fits of the measured Raman frequencies incorporating previously published neat nitrogen, carbon monoxide, and oxygen data and using a functional form dependent on pressure, temperature, and mixture ratio, accurately describe the N2 , CO, and O2 vibrational frequency shifts for both the carbon monoxide–oxygen and the nitrogen–oxygen mixtures. The transition intensity and linewidth data suggest that thermal equilibrium of the vibrational levels is attained in less than 10 ns at these shock pressures. The vibrational temperatures obtained for the nitrogen–oxygen mixtures were used to improve the oxygen potential function used to calculate equation-of-state pressures and temperatures. The measured linewidths for CO, N2 , and O2 were different for the different mixtures and did not appear to depended significantly on mixture ratios. The broadening of all spectral lines suggested that the vibrational dephasing time for each species decreased to a few ps at the highest pressure shock states. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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