Optical radiation from shock-compressed materials and interfaces

Recent observations of shock-induced radiation from oxides, silicates and metals of geophysical interest constrain the shock-compressed temperature of these materials. In these experiments, a projectile impacts a target consisting of a metal driver plate, a metal film or foil layer, and a transparent window. We investigate the relationship between the temperature inferred from the observed radiation, and the temperatures of different high-pressures states (e.g. shocked state) of the shock-compressed film (or foil) and window. Deviations of the temperature in each target component away from that of their respective shock-compressed states occur because of (1) shock-impedance mismatch between target components, (2) thermal mismatch between target components, (3) surface roughness at target interfaces, and (4) conduction within and between target components. In particular, conduction may affect the temperature of the film/foil material at the film/foil-window interface, a major thermal radiation source in the target, on the time scale of the experiments. To be observed, radiation from sources at the film/foil-window interface or in the shocked window material must propagate through (1) the shocked window material, (2) the shock front, (3) the unshocked window material, and (4) the unshocked window free surface. Consequently, the observed intensity of target radiation sources is affected by the optical properties of each region. In particular, the source radiation intensity may be greatly reduced due to absorption in the shocked and/or unshocked window material, and/or only partial transmission through the film/foil-window interface, shock front and/or unshocked window free surface. To illustrate various aspects of the model, we apply it to radiation data from targets composed of an Fe driver plate, and Fe film or foil layer, and either an Al₂O₃ or LiF window layer.