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

Excited-state formation in collisions of H⁺, He²⁺, He⁺, H₂⁺, and He₂⁺ 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 He⁺-Li collisions. The absence of the corresponding HeI singlet is attributed to nearly inaccessible crossings of the He⁺(1²S)-Li(2s²S) potential-energy curve with other singlet [HeLi]⁺ curves. The results for other collision systems are also discussed in terms of curve crossings.