Systematics in Rydberg-state excitations for ion-atom collisions

Rydberg-state excitations in ${\mathrm{Ne}}^{+}$, ${\mathrm{Mg}}^{+}$ -He collisions have been studied in the projectile energy range 10-75 keV by means of optical spectrometry in a search for systematic trends. The relative excitation cross sections for levels of a Rydberg-term series are found to follow a general ${\left(n^{*}\right)}^p$ behavior ($n^{*}$ being the effective quantum number) with $p\lesssim -3\mathrm{}$ varying with collision energy and particles, regardless of whether the excited-state population results from direct excitation, single-electron transfer, or double-electron transfer. At higher collision energies $p\simeq -3\mathrm{}$ as predicted by theory. Polarization of the emitted line radiation indicates that there is no general rule for the relative excitation of the different magnetic substates of the same level. A statistical distribution of excitation is found for levels within the same term when the fine-structure splitting is small.