K-SHELL IONIZATION CROSS-SECTIONS OF AL, SI, S, CA, AND ZN FOR OXYGEN IONS IN THE ENERGY-RANGE 1.1-8 MEV

K-shell ionization cross sections induced by 1.1-8-MeV oxygen ions in Al, Si, S, Ca, and Zn were measured using different target thicknesses. The cross sections for vanishingly thin and for charge-equilibrium targets were obtained by extrapolation. The experimental results are compared to the perturbed stationary-state approximation with energy-loss, Coulomb, and relativistic corrections (ECPSSR) cross sections [Brandt and Lapicki, Phys. Rev. A 23, 1717 (1981)], to the modification of the ECPSSR theory (MECPSSR) [Benka, Geretschlager, and Paul, J Phys. (Paris) Collq. Suppl. 12, C9-251 (1987)], to the theory for direct Coulomb ionization of the 1s-sigma molecular orbital [Montenegro and Sigaud, J. Phys. B 18 299 (1985)], and to several semiclassical approximation codes using either the united atom binding procedure or the variational approach of Andersen et al. [Nucl. Instrum. Methods 192, 79 (1982)]. The cross sections were also compared to the statistical molecular-orbital theory of inner-shell ionization for (nearly) symmetric atomic collisions [Mittleman and Wilets, Phys. Rev. 154, 12 (1967)]. For fast collisions (xi approximately 1), the ionization cross sections are well reproduced by theories for direct Coulomb ionization. For slower collisions (xi < 1), the experimental cross sections are systematically higher than the direct-ionization values, but they agree satisfactorily with the summed cross sections for direct Coulomb ionization and for molecular-orbital ionization. Best agreement (within a factor of 2) was found for the sums of MECPSSR and statistical cross sections.