Single-Electron Capture byC4+in Helium, Neon, and Argon below 40 keV

Total cross sections for single-electron capture by ${\mathrm{C}}^{4+}$ from the target gases He, Ne, and Ar were measured at laboratory energies between 400 eV and 40 keV. The fast-collision products that scattered less than 1° were charge analyzed after single collision in a gas cell. Experimental results are interpreted in terms of pseudocrossings of the adiabatic potential curves for the initial and final states. The cross sections for ${\mathrm{C}}^{4+}$ + He and ${\mathrm{C}}^{4+}$ + Ne are consistent with the second maximum predicted for distant pseudocrossings by a numerical solution of the two-state semiclassical approximation by Bates, Johnson, and Stewart. Electron capture with an Auger-type ionization of the target is exothermic for ${\mathrm{C}}^{4+}$ + Ne and ${\mathrm{C}}^{4+}$ + Ar and is considered as the explanation of the large (50 ${\mathrm{\AA }}^2$) and slowly varying cross section measured for ${\mathrm{C}}^{4+}$ + Ar.