Time-ordering effects in K-shell excitation of 170-MeV Ne7+ colliding with gas atoms: Double excitation.

Abstract

The method of 0\ifmmode^\circ\else\textdegree\fi{} Auger spectroscopy was utilized to measure state-selective K-shell excitation in Li-like ${\mathrm{Ne}}^{7+}$ incident with 170-MeV on ${\mathrm{H}}_{2}$, He, ${\mathrm{CH}}_{4}$, Ne, and Ar. Interferences between first- and second-order mechanisms are searched for in the production of the doubly excited states 1s2${\mathit{p}}^{2}$ $^{2}$D and 1s2${\mathit{p}}^{2}$ $^{2}$S. The semiclassical approximation is applied to study time ordering of the double-excitation process. It is shown that time ordering is lost for the second-order amplitude so that it is unable to interfere with the first-order amplitude. The doubly excited state 1s2${\mathit{p}}^{2}$ $^{2}$S is predominantly produced by the single-electron transition 1s\ensuremath{\rightarrow}2s followed by configuration interaction with 1s2${\mathit{s}}^{2}$ $^{2}$S. Discrepancies occur between theory and experiments as the state 1s2${\mathit{p}}^{2}$ $^{2}$D is interpreted in terms of the independent dipole transitions 1s\ensuremath{\rightarrow}2p and 2s\ensuremath{\rightarrow}2p.