Collision Spectroscopy. III. Scattering in Low-Energy Charge-Transfer Collisions ofHe+and Ar

Differential charge-transfer cross sections $\sigma$ are measured for ${\mathrm{He}}^{+}$ in Ar at energies $E$ from 65 to 300 eV and angles $\theta$ from 1° to 30°. The relative measurements are calibrated by comparison with total charge-transfer measurements of Koopman. From reduced plots of $\rho =\theta sin\theta \sigma \left(\theta \right)$ versus $\tau =E\theta$, two, and perhaps three families of oscillations are identified, with reduced angular thresholds at $E\theta \cong 300,970, \mathrm{and} 1250$ eV deg and with spacing $\Delta \theta$ between peaks near threshold related to the wave number $k$ by the rule $\Delta b=\left(\frac{2\pi }{k\Delta \theta }\right)\cong 0.51,0.25, \mathrm{and} 0.09$ a. u., respectively. Using potential curves constructed from earlier information on elastic scattering, the first family is identified as arising from a crossing at 2.9 a. u. to a state dissociating to $\mathrm{He} \left(1\mathrm{}{s}^2\right)+{\mathrm{Ar}}^{+}\left(3s3\mathrm{}{p}^6\right)$, and the second from a crossing at 2.3 a. u. to a state producing $\mathrm{He} \mathrm{}\left(1s2s\right)+{\mathrm{Ar}}^{+}\left(3\mathrm{}{s}^{2}3p^5\right)$. Further crossings at distances between 2.25 and 1.95 a. u. leading to other excited-state channels are apparently responsible for more than half of all the charge transfer at these energies.