M. S. Shaw

The thermodynamics of dense fluid nitrogen by molecular dynamics

We present an extensive set of molecular dynamics results for the thermodynamics of dense fluid N2. The density and temperature regime is 1.3 g/cm3≲ρ≲2.3 g/cm3 and 500 K≲T≲12 000 K. These data are then analyzed to study the effects of internal degrees of freedom on the N2 equation of state. Most importantly, we demonstrate the existence of an effective spherical potential that models very accurately (to 1.5% or better in pressure and energy) the true equation of state for the anisotropic N2 potential. We discuss the relation of this effective potential to the median average over angles and other averaging methods, including the arithmetic mean.

Coherent anti‐Stokes Raman spectroscopy of shock‐compressed liquid oxygen

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 ≈17.5 GPa single shock and ≈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 Gruneisen 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 me...

Shocked states from initially liquid oxygen-nitrogen systems

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≂85 K, v0≂1.06 cm3/g; the other is the pure explosive compound NO, at T≂122 K, v0≂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 var...

Monte Carlo simulation of equilibrium chemical composition of molecular fluid mixtures in the Natoms PT ensemble

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.

Thermodynamics using effective spherical potentials for CO2 anisotropies

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≲ρ≲2.6 g/cm3 and 1000 K≲T≲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 aver...

An electron–gas plus damped‐dispersion calculation of the N2–N2 interaction

We present the results of a calculation of the N2–N2 intermolecular potential using the modified Gordon–Kim (electron–gas) model with damped‐dispersion terms (MGKD potential). The calculated potential agrees well in the well region with other proposed potentials. An analytical form that fits the potential with an average error of 0.2% from the well region to 30 kK on the repulsive wall is given. Solid‐state properties, such as the 0 K phase diagram and the pressure–volume curve, are calculated and are in good agreement with experiment. As a test of the repulsive region, the shock Hugoniot calculated with this potential is compared with experiment and also shows good agreement.

Angular correlations in dense hot diatomic fluids

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.

Coherent anti‐Stokes Raman spectroscopy of shock‐compressed liquid carbon monoxide

Vibrational spectra of liquid carbon monoxide shock compressed to several high pressure/high temperature states were recorded using single‐pulse multiplex coherent anti‐Stokes Raman scattering. Vibrational frequencies, third‐order suceptibility ratios, and linewidths are reported for the fundamental and first excited‐state transition. The observed vibrational frequency shift with shock pressure was substantially less than that observed previously in nitrogen, implying a significant difference in the details of their inter‐ and intramolecular potentials. 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, and the vibrational temperatures obtained are comparable to calculated equation‐of‐state temperatures. The measured linewidths suggest that the vibrational dephasing time decreased to ∼2 ps at our highest pressure shock state.

Coherent anti‐Stokes Raman spectroscopy of shock‐compressed liquid nitrogen/carbon monoxide mixtures

Vibrational spectra of liquid nitrogen/carbon monoxide mixtures, 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 with a semiclassical model for CARS intensities and using best fit vibrational frequencies, peak Raman susceptibilities, and Raman linewidths. Up to a maximum shock pressure of 9.3 GPa, both the N2 and CO vibrational frequencies were found to increase monotonically with pressure but depended strongly on the nitrogen/carbon monoxide mixture ratio. An empirical fit of the Raman frequency shifts incorporating previously published neat nitrogen and carbon monoxide data, using a functional form dependent on pressure, temperature, and mixture ratio, accurately describes both the N2 and CO shifts. The transition intensity and linewidth data suggest that thermal equilibrium of the vibrational levels is attained in less than 10...

Coherent anti-Stokes Raman spectroscopy of shock-compressed liquid nitrogen/argon mixtures

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...