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

Abstract

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~.

Xe(L) X-ray emission from laser–cluster interaction

“…Our results contradict previous results [4,7,8] both on line assignments and wavelength dependency. An extensive theoretical work is needed before interpretation of the observed spectral shapes, intensity threshold and possible selection rule on vacancy production can be made.…”

“…Kondo et al [4] have assign the two group of lines to 3d →2p transitions in multicharged (25 ≤ q ≤ 35) Xe ions with one or two 2p vacancies. The production of such multiple inner-shell vacancies has been interpreted [7,8] as due to a "coherent, ordered many-electron motion" acting as a single heavy, highly charged, particle. From our calculations, we find however that the energy difference between transitions involving one or two 2p vacancies amounts to ~ 120 eV (0.08 Å) on the average, a value much too small to account for the observed energy difference between the two groups of transitions.…”

Dynamical ionization ignition of clusters in intense short laser pulses

“…Several mechanisms for the increased ionization (as compared to single atoms in the same laser field) have been proposed. Boyer et al [9] suggested that collective electron motion (CEM) inside the cluster is responsible for inner shell vacancies the corresponding radiation of which was observed in experiments [10]. Rose-Petruck et al [11] introduced the socalled "ionization ignition" (IONIG) where the combined field of the laser and the ions inside the cluster leads to increased inner ionization, i.e., electrons are more easily removed from their parent ion as if there was the laser field alone.…”

Intense ultrashort laser–Xe cluster interaction

“…Krainov and Smirnov investigated a broad range of issues involving the laser cluster interaction and coupling problem extending from cluster formation to X-ray generation [12]. Schroeder et al introduced the so-called "collective oscillation model" for small clusters (10e30 Å) [13,14]. In its coherent motion the outer electrons behave like a giant quasi-particle, hitting the cluster with energy of tens of keV causing inner-shell ionization.…”