Optical Spectroscopy, Modeling Techniques Featured at 1997 Shock Compression Meeting

The latest results in molecular modeling, spectroscopic techniques, and mechanical properties of shock-compressed materials were presented during the 10th biennial International Conference of the APS Topical Group on Shock Compression of Condensed Matter (SCCM), held July 27 through August 1, 1997, at the University of Massachusetts in Amherst. The conference emphasis was on the physics of materials at elevated stresses or pressures. In addition to standard invited and contributed papers, the conference featured a lecture on molecular and planetary fluids at high shock pressures, focusing on the achievement of metallic hydrogen, by the 1997 APS Shock Compression Award winner, W.J. Nellis of Lawrence Livermore National Laboratory.

Time-Resolved Optical Spectroscopy

Despite many experimental developments, a detailed understanding of shock induced chemical decomposition in explosives at the molecular level remains an outstanding problem, according to Y.A. Gruzdkov of Washington State University, who has been using time resolved optical spectroscopic techniques in his laboratory to address this problem. Gruzdkov reported the detection of a transient intermediate in sensitized nitromethane with one of these techniques, subsequently identified as a radical anion of nitromethane, and the base catalysis by amines is favored as the most plausible mechanism.

In a Tuesday afternoon session, T.P. Russell of the Naval Research Laboratory described a new approach that permits real time (nanosecond to microsecond) investigation of condensed phase chemical reactions under extreme conditions of high pressure and high temperature. Time- resolved optical techniques are used in conjunction with a high pressure gem anvil cell to monitor reaction time, reaction sequences, and products in chemical changes induced by pulsed laser heating in statically compressed samples.

Deformation of Ceramics

Determining the material strength and understanding the inelastic deformation mechanisms of ceramics under plane shock wave compression is important for characterizing their response to rapid impulsive loading. In recent experiments, R. Feng of Washington State University used two independent methods to determine the material strength of ceramics in the shocked state: longitudinal and lateral stress gauge measurements, and combined compression and shear wave experiments. Feng reported that the data collected, along with related one- and two-dimensional computations, have provided a complete characterization of the stress state in shocked silicon carbide.

Molecular Modeling

In a Thursday afternoon session, Brad Lee Holian of Los Alamos National Laboratory described the history and recent developments of nonequilibrium molecular-dynamics (MD) simulations. His research group carried out the first such simulations of shock waves in single crystals in 1979. Nearly a decade later, the LANL team performed similar calculations in systems of up to 10,000 atoms. Today, with the advent of massively parallel computers, Holian and his colleagues have studied systems with approximately 270,000 atoms, and are attempting simulations with even larger cross-sectional areas, and with pre-existing defects embedded in the sample, which could nucleate plastic flow at lower shock strengths than those characteristic of pure single crystals.