Excited-state formation in collisions between simple ions and Li atoms

Excited-state formation in collisions of ${\mathrm{H}}^{+}$, ${\mathrm{He}}^{2+}$, ${\mathrm{He}}^{+}$, ${\mathrm{H}}_2^{+}$, and ${\mathrm{He}}_2^{+}$ ions with Li atoms has been studied in the kinetic energy range 5-1500 eV. Data were obtained by spectral analysis of near-uv-visible near-ir radiation resulting from spontaneous emission of excited species formed in the collisions. The dominant feature of all collision-produced spectra at all kinetic energies was the 670.8-nm LiI resonance line. Absolute emission cross sections for this line and a number of other spectral features as functions of ion kinetic energy are reported. Although there are no spin restrictions on the final states in the observed charge-transfer processes, only triplet HeI radiation was observed from ${\mathrm{He}}^{+}$-Li collisions. The absence of the corresponding HeI singlet is attributed to nearly inaccessible crossings of the ${\mathrm{He}}^{+}\left(1\mathrm{}{}_{}{}^2{}_{}{}^{}S\right)-\mathrm{Li}\left(2s{}_{}{}^2{}_{}{}^{}S\right)$ potential-energy curve with other singlet ${\mathrm{}\left[\mathrm{HeLi}\right]\mathrm{}}^{+}$ curves. The results for other collision systems are also discussed in terms of curve crossings.