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 kilovolt L-shell x-ray emission spectra fkom atomic Xe cluster targets excited by intense sub-picosecond 248nrn ultraviolet radiation [A. McPherson et al., Nature 370, pp. 631-634 (1994)]. For incident ponderomotively-driven electrons photoionized by strong above threshold ionizatio~ the collisional ejection mechanism is shown to be highly l-state and signifka.ntly n-state (i.e. radially) selective for time periods shorter than the collisional dephasing time of the photoionized electronic wavefbnction. The resulting preference for the collisionai ejection of 2p electrons by an ionized 4p state produces the measured anomalous Xc(L) emission which contains direct evidence for (i) the generation of “ X&T+(2@3dO) and X@+(2pS3&) ions exhibiting inner-shell population inversion and (ii) a coherent correlated ekctron state collision responsible for the production of double 2p vacancies. For longer time periods, the selectivity of this coherent impact ionization mechanism is rapidly reduced by the combined effects of intrinsic quantum mechanical spreading and dephasing – in agreement with the experimentally observed and extremely strong -A4 pump-laser wavelength dependence of the efficiency of inner-shell (2p) vacancy production in Xe clu~ers excited in underdense plasmas ~. Kondo et al., J. Phys. B 30, pp. 2707-2716 (1997)]. +Present addr&s: Sandia National Laboratones, MS-1 188, P.O. Box 5800, Albuquerque, NM 87185-1188 Tel: (3 12) 996-2974 FAX: (3 12) 996-9016 E-mail: [email protected]