Theoretical study of low-lying electronic states and emission spectra of the excimer ions NaHe + and NaNe +

Abstract

Ab initio CASSCF calculations using the Gaussian basis set aug_cc_pVTZ developed by Dunning et al. specifically for calculations at the post-HF level was carried out for excimer NaHe + and NaNe + ions in order to determine spectroscopic characteristics and radiation lifetimes of their low-lying excited electronic states. The computed emission spectra of these were shown to be in good agreement with the experimental ones observed recently by Hammer et al. (Hyperfine Interactions 88 (1994) 151). Contrary to the earlier popular opinion, all excited electronic states of these ions, correlating with the limit Rg ++Na, and not just the radiating singlet states turn out to be bound. It was shown that under the conditions of an experiment similar to the one performed by Hammer et al., the excited triplet NaHe +(1 3Σ +) states can decay as a result of their interactions with a high-energy Ar + beam. This decay has to be accompanied with the formation of the ions He +, which in turn can interact with sodium atoms to yield the radiating NaHe +(2 1Σ +) states again and, thus to maintain the emission observed in the experiment.