Collisions between hydrogen atoms and helium ions

Using a new theory developed for collisions between atomic hydrogen and arbitrary ions or atoms, the author calculates the cross section for the direct excitation process ${\mathrm{He}}^{+}\mathrm{}\left(1s\right)+\mathrm{H}\mathrm{}\left(1s\right)\mathrm{}\to {\mathrm{He}}^{+}\mathrm{}\left(2s\right)+\mathrm{H}\left(\Sigma \right)$ in which the H atom is left in any discrete or continuum final state. The results are compared with other theories and with recent measurements for ${}_{}{}^3{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}$ energies between 22 and 346 keV. The theory predicts a maximum near 84 keV, in excellent agreement with the data. Above 200 keV all theories yield cross sections which do not decrease rapidly enough with increasing energy when compared with the data.