An efficient, selective collisional ejection mechanism for inner-shell population inversion in laser-driven plasmas

A theoretical analysis of laser-driven collisional ejection of inner-shell electrons is presented to explain the previously observed anomalous kilovol t L-shell xray emission spectra from atomic Xe cluster targets excited by i ntense subpicosecond 248 nm ultraviolet radiation (McPherson A et al 1994 N ature 370 631-4). For incident ponderomotively-driven electrons photoionize d by strong above threshold ionization, the collisional ejection mechanism is shown to be highly I-state and significantly n-state (i.e. radially) sel ective for time periods shorter than the collisional dephasing time of the photoionized electronic wavefunction. The resulting preference for the coll isional ejection of 2p electrons by an ionized 4p state produces the measur ed anomalous Xe(L) emission which contains direct evidence for (i) the gene ration of Xe27+ (2p(5) 3d(10)) and Xe28+(2p(5)3d(9)) ions exhibiting inner- shell population inversion and (ii) a coherent correlated electron state co llision responsible for the production of double 2p vacancies. For longer t ime periods, the selectivity of this coherent impact ionization mechanism i s rapidly reduced by the combined effects of intrinsic quantum mechanical s preading and dephasing-in agreement with the experimentally observed and ex tremely strong similar to lambda (-6) pump-laser wavelength dependence of t he efficiency of inner-shell (2p) vacancy production in Xe clusters excited in underdense plasmas (Kondo K et al 1997 J. Phys. B: At. Mel. Opt. Phys. 30 2707-16).