Dynamic equation-of-state data for D2 and H2 were measured in the pressure range 2–76 GPa (20–760 kbar) using a two-state light-gas gun. Liquid specimens were shocked from initial states near the saturation curve at 20 K. Maximum compression was sixfold over initial liquid density at a calculated temperature of 7000 K for D2. The data is discussed in terms of the theory of Ross et al., which includes an effective intermolecular pair potential, molecular vibration, free molecular rotation, and molecular dissociation.

Dynamic equation-of-state data for liquid CO and CH4 were measured in the shock pressure range 5–92 GPa (50–920 kbar) using a two-stage light-gas gun. The liquids were shocked from initial states near their saturation curves at 77 and 111 K for CO and CH4, respectively. The experimental technique used to double-shock CH4 is described. The CO data were examined by using three theoretical models: (1) a chemically nonreactive model, (2) a quasi-chemical-equilibrium model that allows CO to dissociate into gaseous species and graphite, and (3) a chemical-equilibrium model that also includes a dense carbon phase which exists at higher pressures and temperatures than graphite. This dense phase is assumed to be diamond. Our analysis shows that at low pressure chemical equilibrium takes much longer than a typical shock passage time. As a consequence, the experimental data initially follow the nonreactive Hugoniot to pressures well beyond the chemical dissociation limit. Both the experimental data and the Hugoniot computed with case (3) agree satisfactorily at high pressure. Further consequences of these observations to high-explosive studies are discussed. The theoretical analysis for the CH4 data was presented in an earlier paper.

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Dynamic equation-of-state data are presented for liquid N 2 single-and double-shocked to pressures of 100 GPa (1 Mbar), compressions to fourfold over initial liquid density, and internal energies to 1 MJ/mole. Three double-shock points lie above the principal Hugoniot in pressure-volume space, the first such observation in condensed matter. The data are interpreted in terms of a continuous phase transition, identified as molecular dissociation by comparison of the shock compression curves of liquid N 2 and isoelectronic Nitrogen is an ideal system for study of the insulator-to-metal transition. N 2 is a small, simple diatomic molecule. Extensive dynamic 1 " 3 and static 4 high-pressure equation-of-state data have been measured. An effective spherical intermolecular pair potential has been derived from the dynamic data. 5 The N 2 -N 2 repulsive pair potential has been calculated ab initio. 6 The increase in the shockwave compressibility at 30 GPa (300 kbar) 3 and 2.5-fold compression over initial liquid density has been attributed to molecular dissociation to the monatomic state. 5,7 Since nitrogen retains its diatomic molecular structure to 40 GPa at 300 K, 8 dissociation, if it occurs in the dynamic experiments, must be driven by the high shock temperatures which are calculated to be greater than 6000 K at 30 GPa. 9 Liquid N 2 can be shock compressed fourfold, which corresponds to a final density comparable to that at which monatomic metallic nitrogen may exist. 10 Thus, experimental and theoretical results suggest that N 2 undergoes a transition to the monatomic state at the densities and temperatures achieved in dynamic experiments at threefold to fourfold compressions.By comparison, H 2 requires a greater compression of about fourteenfold to achieve the transition to the monatomic metallic state at 0 K. 11 The highest densities achieved in D 2 shock-wave experiments 12, 13 are a factor of 2 smaller than that predicted 11 for the metallic transition and the dynamic data do not indicate dissociation of H 2 . 14 Raman spectroscopic measurements show that H 2 retains its molecular nature to 60 GPa at 300 K. 15 Nitrogen is believed to retain its diatomic molecular structure up to shock pressures of 30 GPa on the principal Hugoniot. This statement is a result of excellent agreement between Hugoniot data and the theoretical P-V curve calculated with use of an intermolecular pair potential derived from an Ar potential by corresponding-states scaling. 5 Because the estimated shock temperature above 30 GPa is ~~ 1 eV, 7 and approaching the dissociation energy of an isolated N 2 molecule, 9.8 eV, 16 the softening above 30 GPa has been attributed to molecular dissociation. 5,7 However, the amount of experimental data was insufficient to constrain a physical model because several other mechanisms could possibly account for the softening in the P-V data-for example, electronic excitation as in Ar, 17 or a transition from hindered to free molecular rotation as the shock temperature becomes comparable to the large rotationa...

The dissociative phase transition of fluid nitrogen at pressures in the range 30-110 GPa (0.3-1.1 Mbar) , temperatures in the range 4000-14 000 K, densities up to 3.5 gicm 3 , and internal energies up to 1 MJ/mol was investigated by shock compression. Equation-of-state, shocktemperature, and electrical-conductivity experimental data are presented and analyzed in detail.