Rotationless radiative lifetimes and transition probabilities of some low-lying quintet states of the NS+ cation

Abstract

Recently, the NS+ cation has been detected via spectroscopy as an interstellar species. To determine its transition properties, the potential energy curves of one singlet state and seven quintet states are investigated. For the eight states, the potential energy curves and transition dipole moments are calculated using the complete active space self-consistent field method followed by the valence internally contracted multireference configuration interaction approach. The spectroscopic parameters and transition dipole moments are compared with the available experimental and other theoretical results. The band origins, Franck–Condon factors, and Einstein coefficients of the spontaneous emissions are calculated for the transitions of 15 pairs of quintet states. It is found that the 15Π–15Σ+, 25Σ+ –15Σ+, 25Π–15Σ+, and 35Σ+–15Σ+ transitions can be detected in a spectroscopy experiment. The rotationless radiative lifetimes of the vibrational levels are approximately 10−5 to 10−6 s for the 25Σ+ state, 10−5 s for the 15Π state, and 10−4–10−5 s for the 35Σ+ and 25Π states. The radiative lifetimes of the vibrational levels are so long that it is rather difficult for the spontaneous emissions generated from the 15Δ and 25Δ states to occur, and the emissions from the two states are very difficult to observe via spectroscopy. The reasons may be that both the 15Δ and 25Δ states have long internuclear equilibrium separations, and the transition dipole moments originating from them are very small. The results reported in this work can be used to detect the NS+ cation in cold molecular clouds, prestellar cores, and shocks.