Autoionizing resonances in electron-impact ionization of O5+ ions - art. no. 062720

We report on a detailed experimental and theoretical study of electron-impa ct ionization of O5+ ions. A high-resolution scan measurement of the K-shel l excitation threshold region has been performed with statistical uncertain ties as low as 0.03%. At this level of precision a wealth of features in th e cross section arising from indirect ionization processes becomes visible, and even interference of direct ionization with resonant-excitation/auto-d ouble-ionization (READI) is clearly observed. The experimental results are compared with R-matrix calculations that include both direct and indirect p rocesses in a unified way. Radiative damping of autoionizing Li-like states is found to be about 10-15 %. The calculations almost perfectly reproduce most of the experimental resonance features found in the present measuremen t including READI. They also agree with the direct-ionization converged clo se-coupling results of I. Bray [J. Phys. B 28, L247 (1995)] and the absolut e total ionization cross section measurement of K. Rinn et al. [Phys. Rev. A 36, 595 (1987)].