X-Ray absorption and photoionization of laser excited molecules

This study presents theory and simulations of X-ray absorption and X-ray photoionization spectra of optically excited molecules. The simulations are carried out within a two-step excitation model in which the laser radiation prepares an optically excited state which subsequently is core-electron excited by X-rays. The model makes use of a newly derived time-dependent theory based on the Nozieres-De Dominicis equations. It describes the valence electron relaxation of the optically excited state in the field of the core hole and the implication of this relaxation on the formation of the X-ray spectra. It is found that an optical excitation results in a significant general enhancement of two-electron transitions giving rise to shake-up and shake-down type spectral lines in the X-ray spectra. The excitonic character of the optically excited states is found to considerably influence the formation of the X-ray spectral profile, something that is strongly size- and site-dependent. (C) 2003 Elsevier Science B.V. All rights reserved.