Satellite absorption lines and the temperature dependence of x-ray absorption features in high-temperature plasmas

Absorption spectroscopy can be used to diagnose compressed layers in spherical targets imploded by means of high-intensity laser irradiation [B. Yaakobi et al., Opt. Commun. 34, 213 (1980)]. In appropriately designed experiments, absorption lines are formed by 1s-2p absorption transitions in the heliumlike through fluorinelike species of certain ions in the shell that surrounds the strong continuum source in the compressed core of the target. These transitions form a distinct, broad linelike absorption band for each ion species. The areal density of each species can be inferred from the attenuation in the spectrum within its respective absorption band and from the known cross sections for each 1s-2p transition. Earlier analyses have assumed that all such transitions begin from states in the ground configuration of each species [A. Hauer et al., Phys. Rev. A 34, 411 (1986)], but, in fact, most of the absorption can be due to satellite absorption transitions originating from states in excited configurations which are, in total, less opaque than transitions originating from the ground configuration. As a result, the opacity becomes temperature dependent through the temperature dependence of the populations of excited configurations. This is illustrated for the specific case of chlorine, and approximate results, based on the screened hydrogenic approximation, are obtained for other elements. Estimates of areal density that include only the ground configuration can be too low by as much as 33%.