Total-capture cross sections for very slowAr4+-Ar andAr6+-Ar collisions

We present experimental and calculated total single-electron capture cross sections for ${\mathrm{Ar}}^{4+\mathrm{-}}$Ar collisions in the laboratory energy ranges 100–800 and 10–800 eV, respectively. The experimental results for post-collisional energy and angular distributions of ${\mathrm{Ar}}^{3+}$ extend down to 19 eV; below 100 eV a part of the angular distribution falls outside the ±9° angular acceptance. The absolute calibration of our ${\mathrm{Ar}}^{4+\mathrm{-}}$Ar results is achieved by means of normalization of relative ${\mathrm{Ar}}^{6+\mathrm{-}}$Ar single-capture cross sections to absolute total cross sections from the literature. Calculated single-electron capture cross sections for ${\mathrm{Ar}}^{4+\mathrm{-}}$Ar are obtained from a multichannel Landau-Zener model with semiempirical coupling strengths that reproduce the experimental state-selective angular distributions. When these theoretical angular distributions are integrated over the experimental angular acceptance, the agreement between experimental and calculated cross sections is excellent down to 19 eV. We also present total experimental cross sections for removing two electrons from Ar for collisions with ${\mathrm{Ar}}^{6+}$ at 800, 600, and 450 eV, and for 800-eV collisions with ${\mathrm{Ar}}^{4+}$.