K-shell ionization of ${\mathrm{O}}^{4+}$ and ${\mathrm{C}}^{2+}$ ions in fast collisions with ${\mathrm{H}}_2$ and He gas targets

Cross sections for 1s ionization of the 1${\mathit{s}}^2$2${\mathit{s}}^2$ and 1${\mathit{s}}^2$2s2${\mathit{p}}^3$P (metastable) states of ${\mathrm{C}}^{2+}$ and ${\mathrm{O}}^{4+}$ ions in 0.5–1.8 MeV/u collisions with ${\mathrm{H}}_2$ and He targets were measured using projectile Auger electron spectroscopy at 0°. Calculations of the K-shell ionization cross section ${\mathrm{\sigma }}_{\mathit{K}}$ were performed including contributions from projectile-electron–target-nucleus interactions (${\mathrm{\sigma }}_{\mathit{e}\mathit{n}\mathit{I}}$) within a plane-wave Born approximation (PWBA), and contributions from projectile-electron–target-electron interactions (${\mathrm{\sigma }}_{\mathit{e}\mathit{e}\mathit{I}}$) within an impulse approximation (IA). The theoretical total cross section ${\mathrm{\sigma }}_{\mathit{K}}$=${\mathrm{\sigma }}_{\mathit{e}\mathit{n}\mathit{I}}$+${\mathrm{\sigma }}_{\mathit{e}\mathit{e}\mathit{I}}$ was found to be in overall agreement with the K-shell ionization measurements indicating a contribution of up to ∼30% due to electron-electron interactions. Separate screening-antiscreening calculations of ${\mathrm{\sigma }}_{\mathit{K}}$ were also performed and found to be in overall agreement with our data. Additionally, our PWBA-IA ionization calculation was also tested for H-like projectiles and found to be in agreement with recently published data. These results suggest that the IA calculation of ${\mathrm{\sigma }}_{\mathit{e}\mathit{e}\mathit{I}}$ when combined with a PWBA calculation of ${\mathrm{\sigma }}_{\mathit{e}\mathit{n}\mathit{I}}$ is quite adequate for describing K-shell ionization.