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

The method of 0[degree] Auger spectroscopy was utilized to measure state-selective [ital K]-shell excitation in Li-like Ne[sup 7+] incident with 170-MeV on H[sub 2], He, CH[sub 4], Ne, and Ar. Interferences between first- and second-order mechanisms are searched for in the production of the doubly excited states 1[ital s]2[ital p][sup 2] [sup 2][ital D] and 1[ital s]2[ital p][sup 2] [sup 2][ital 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 1[ital s]2[ital p][sup 2] [sup 2][ital S] is predominantly produced by the single-electron transition 1[ital s][r arrow]2[ital s] followed by configuration interaction with 1[ital s]2[ital s][sup 2] [sup 2][ital S]. Discrepancies occur between theory and experiments as the state 1[ital s]2[ital p][sup 2] [sup 2][ital D] is interpreted in terms of the independent dipole transitions 1[ital s][r arrow]2[ital p] and 2[ital s][r arrow]2[ital p].