Investigation of simultaneous inner- and outer-shell ionization and Auger-electron emission in slow Ar++Ar collisions at intermediate impact parameters.

Abstract

The impact-parameter dependence of electron emission following slow ${\mathrm{Ar}}^{+}$+Ar collisions has been investigated experimentally. Coincidence measurements have been performed for incident energies of 150, 200, and 300 keV at electron emission angles of 40\ifmmode^\circ\else\textdegree\fi{}--100\ifmmode^\circ\else\textdegree\fi{} and projectile scattering angles of 0.3\ifmmode^\circ\else\textdegree\fi{}--3.0\ifmmode^\circ\else\textdegree\fi{}. An improved method of determining absolute doubly differential ionization yields from electron time-of-flight spectra has been applied. The experimental results are interpreted in terms of the molecular-orbital model of atomic collisions. It is shown that the well-known 4f\ensuremath{\sigma} promotion mechanism leading to L-shell vacancy production results in the emission of electrons to the continuum. A detailed analysis of the experimental data indicates that both promoted electrons are directly ionized out of the 4f\ensuremath{\sigma} level for small impact parameters. This is in good agreement with preliminary results of coupled-channel calculations and consistent with earlier inelastic energy loss measurements. From the total number of emitted electrons it was further possible to determine the number of highly excited and directly ionized M-shell electrons.