Shock-wave profiles of highly ordered pyrolytic graphite shocked normal to the basal plane of the graphite crystal structure have been measured. For graphite with sufficient orientational order a martensitic transformation to a diamond-like phase is observed with a transition onset pressure 19.6±0.7 GPa, the stability limit of the graphite structure under shock compression. The minimum overpressure required for the transformation is not more than 6 GPa and the two-wave structure of the transition is overdriven to a single wave above 40 GPa.

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ABSTRACT.A theory of operation of a wideband interferometric Doppler spectroscopy technique, called externally dispersed interferometry (EDI), is presented. The first EDI prototype was tested on sunlight and detected the 12 m s Ϫ1 amplitude lunar signature in Earth's motion. The hybrid instrument is an undispersed Michelson interferometer having a fixed delay of about 1 cm, in series with an external spectrograph of about 20,000 resolution. The Michelson provides the Doppler shift discrimination, while the external spectrograph boosts net white-light fringe visibility by reducing cross talk from adjacent continuum channels. A moiré effect between the sinusoidal interferometer transmission and the input spectrum heterodynes high spectral details to broad moiré patterns, which carry the Doppler information in its phase. These broad patterns survive the blurring of the spectrograph, which can have several times lower resolution than grating-only spectrographs typically used now for the Doppler planet search. This enables the net instrument to be dramatically smaller in size (∼1 m) and cost. The EDI behavior is compared and contrasted to the conventional grating-only technique.

Electrical conductivities were measured for liquid D2 and H2 shock compressed to pressures of 10-20 GPa (100-200 kbar), molar volumes near 8 cm 3 /mol, and calculated temperatures of 2900-4600 K. The semiconducting energy gap derived from the conductivities is 12 eV, in good agreement with recent quasiparticle calculations and with oscillator frequencies measured in diamond-anvil cells. PACS numbers: 62.50.+p, 72.20.-i The density dependence of the electronic energy band gap of hydrogen is of great current interest with respect to the insulator-metal (IM) transition. Since the valence-conduction band gap of solid hydrogen is about 15 eV at zero pressure, very high pressures are required to close the gap and achieve metallization. Low-temperature phase transitions [1,2] and increases in infrared reflectivity and absorption [3,4] have been reported near and below 150 GPa in diamond-cell experiments. It has been suggested that the 150-GPa transition is caused by closure of the indirect band gap in the molecular solid [3], Measurements of the frequency dispersion of the refractive index n((o) of solid hydrogen in a diamond cell yield effective oscillator frequencies co\ which correlate closely with the energies of direct valence-conduction band transitions [5-7]. Values of (0\ in the range 4-12 eV were obtained by fitting nico) for photon energies below 3-4 eV, the absorption edge of diamond. Electronic structure calculations for some structures indicate that orientationally ordered states of H2 favor smaller band gaps and metallization pressures [8], However, consideration of other structures indicates that the commonly assumed structure [8] is energetically unfavorable and that lower-energy structures have wider band gaps to higher densities [9]. Quasiparticle calculations of the H2 band gap place the IM transition at 150 and 300 GPa, respectively, for c axis aligned and orientationally disordered H2 in the hep phase at 0 K [10]. The quasiparticle method is the most effective in calculating experimental band gaps of semiconductors. Intermolecular potentials derived from Raman vibron data [11] indicate that the fully dissociated metallic transition occurs at about 300 GPa [121.The purpose of this Letter is to report measurements of electrical conductivities of hydrogen at high pressures. Because shock compression is used, the associated high temperatures activate electron carriers and enable determination of the semiconducting energy gap E g . Also, the high shock temperatures cause nearly constant-volume states in D2 over the pressure range of these measurements. Thus, shock pressure is changed primarily to vary temperature and carrier concentration. Electrical con-

We report a theoretical equation of state (EOS) table for boron across a wide range of temperatures (5.1×10^{4}-5.2×10^{8} K) and densities (0.25-49 g/cm^{3}) and experimental shock Hugoniot data at unprecedented high pressures (5608±118 GPa). The calculations are performed with first-principles methods combining path-integral Monte Carlo (PIMC) at high temperatures and density-functional-theory molecular-dynamics (DFT-MD) methods at lower temperatures. PIMC and DFT-MD cross-validate each other by providing coherent EOS (difference <1.5 Hartree/boron in energy and <5% in pressure) at 5.1×10^{5} K. The Hugoniot measurement is conducted at the National Ignition Facility using a planar shock platform. The pressure-density relation found in our shock experiment is on top of the shock Hugoniot profile predicted with our first-principles EOS and a semiempirical EOS table (LEOS 50). We investigate the self-diffusivity and the effect of thermal and pressure-driven ionization on the EOS and shock compression behavior in high-pressure and -temperature conditions. We also study the sensitivity of a polar direct-drive exploding pusher platform to pressure variations based on applying pressure multipliers to LEOS 50 and by utilizing a new EOS model based on our ab initio simulations via one-dimensional radiation-hydrodynamic calculations. The results are valuable for future theoretical and experimental studies and engineering design in high-energy density research.