An efficient, selective collisional ejection mechanismfor inner-shell population inversionin laser-driven plasmas

Abstract

A theoretical analysis of laser-driven collisionalejection of inner-shell electrons is presented to explain thepreviously observed anomalous kilovolt L-shell x-ray emissionspectra from atomic Xe cluster targets excited by intensesub-picosecond 248 nm ultraviolet radiation (McPherson Aet al 1994 Nature 370 631-4). Forincident ponderomotively-driven electrons photoionized bystrong above threshold ionization, the collisionalejection mechanism is shown to be highly l-state andsignificantly n-state (i.e. radially) selective fortime periods shorter than the collisional dephasing time of thephotoionized electronic wavefunction. The resulting preferencefor the collisional ejection of 2p electrons by anionized 4p state produces the measured anomalous Xe(L)emission which contains direct evidence for(i) the generation of Xe27+(2p53d10)and Xe28+(2p53d9) ions exhibiting inner-shellpopulation inversion and (ii) acoherent correlated electron state collision responsiblefor the production of double 2p vacancies. For longertime periods, the selectivity of this coherent impact ionizationmechanism is rapidly reduced by the combined effects ofintrinsic quantum mechanical spreading and dephasing - inagreement with the experimentally observed and extremely strong~λ-6 pump-laser wavelength dependence of theefficiency of inner-shell (2p) vacancy production in Xeclusters excited in underdense plasmas (Kondo Ket al 1997 J. Phys. B: At. Mol. Opt. Phys. 30 2707-16).