Plasmons at shock fronts

In dense solids, plasmons are the collective analogues of the individual polarization oscillations that occur in molecules. This type of excitation can occur in all solids. It consists of motion of the valence electrons relative to the background of positively charged ions. The oscillation frequency, omega(p) is given by (4 pi nq(2)/m)(1/2), where n = electron density, q = electron charge, and m = electron mass. Experimental plasma frequencies (from EELS) are also given by c(1/2)B, where B = bulk modulus and c is a constant. In addition, for the simple metals, omega(p) = mu S-4/5, where S is the specific surface energy, and mu is a constant. In semiconductors, the plasmon energy is related to, but much larger than, the band gap. Plasmas are transparent to optical wavelengths shorter than the plasmon wavelength (1550 Angstrom for Li, for example), so plasmon diagnosis might be useful for studying in situ shock propagation. Since a shock front is itself a collective excitation with a large chemical potential gradient, it can readily excite plasmons. For example, a 200 kbar impact can excite the 15 keV plasmon in Al. The damping in the electron gas is very small, so resonant excitation can occur readily. In metals, shock fronts are interfaces between plasmas of differing densities. From the plasmon energies, estimates of the interfacial energies which relate to the structures of shock fronts can be made.