Ionization in low-keV-energy Ar+Ar collisions

Ionization in Ar+Ar collisions is studied at beam energies of 2.00, 2.50, and 3.00 keV over an angular range from 2.0° to 12.0°. The ionization probability ($P_1$) shows a threshold for $\tau \approx 5$ keV deg and a second rise for $\tau \approx 25$ keV deg. The scattered ion beam is energy analyzed at selected energies and angles. For $2<\mathrm{}\tau <12\mathrm{}$ keV deg the ion energy-loss spectra show a double-peaked structure, at an 18-eV and at a 29-eV energy loss. At larger $\tau$ there is predominately one peak in the spectra at 43 eV. The energy losses and $\tau$ dependence of $P_1$ suggest that the 29- and 43-eV processes involve excitation of autoionizing states. Additional results at beam energies of 0.50, 0.75, 1.00, and 1.50 keV show $P_1\left(\tau \right)$ to be velocity dependent at these lower energies.