Autoionization widths for Ne*(3s)-Ar and Ne**(3p)-Ar collisions.

Abstract

Following Jones and Dahler [Phys. Rev. A 37, 2916 (1988)] we study the theory of the process of ionization within the framework of the Feshbach projection-operator formalism, with the discrete fine-structure states (before ionization) and the continuum states (after ionization) as the two separate subspaces of Hilbert space. For the Ne*(3s)-Ar and Ne**(3p)-Ar systems we have calculated ab initio autoionization widths. In the approximation in which only two electrons play an active role, the coupling matrix element between the two subspaces is a linear combination of two-center two-electron integrals for the exchange and the radiative mechanisms. In general, our calculations support the semiempirical results of Morgner [J. Phys. B 18, 251 (1985)] for the coupling matrix elements as derived from experimental data. The autoionization widths \ensuremath{\Gamma}J\ensuremath{'},J\ensuremath{\Omega} are presented for initial states |J,\ensuremath{\Omega}〉 of the Ne*(*) atom and final states |J\ensuremath{'}〉 of the Ar+ ion. They show a pronounced \ensuremath{\Omega} dependence and a strong correlation of initial and final states, e.g., |0,0〉\ensuremath{\rightarrow}|3/2〉 and |2,2〉\ensuremath{\rightarrow}|3/2〉. For the Ne*(3s) states the two-center two-electron calculation is in good agreement with the two-state basis of Driessen et al. [Phys. Rev. A 42, 4058 (1990)], based on a one-electron overlap approximation. In a semiclassical model the polarized-atom ionization cross sections are calculated. For the Ne**(3p)-Ar system the energy dependence of both the polarization effect and the average cross-section value is reproduced very well, in contrast to the one-electron approximation. For the Ne*(3s)-Ar system we have to conclude that the ab initio results cannot explain all experimental observed features, such as the large cross-section ratio Q(P30)/Q(P32).