Spectroscopy of AlS: Rotational Analysis of the A′2Πi-X2Σ+ Band System in the 2-μm Region

Abstract

An infrared band system consisting of a number of vibrational bands, all belonging to a new electronic transition A′2Πi-X2Σ+ Of the AlS radical, has been recorded with Fourier transform techniques. The bands are analogous to the bands of the A2Πi-X2Σ+ transition of AlO at 2 μm. In total, 28 vibrational bands consisting of 23 000 rotational lines have been included in the rotational analysis. As in the case of AlO, the spin splittings of the X2Σ+ state show clearly noticeable nuclear hyperfine-induced effects. Numerous local perturbations affect the A′2Πi state, all shown to be caused by vibrational levels of the X2Σ+ state. A fit of the X2Σ+ state has been performed both with and without hyperfine parameters. Attempts to perform a global deperturbation involving the X2Σ+(v = 0-13)and A′2Πi(v = 0-8) levels have been made. The structure of the X2Σ+ state has been discussed in the light of results from the A2Πi-X2Σ+ system of AlO, and a model has been proposed suggesting that the spin splittings of the X2Σ+(v = 0) levels in both molecules are mainly due to nuclear hyperfine interaction and second-order spin-orbit effects due to the high-lying regular 2Π state (C2Π in AlO and B2Π in AlS), while the influence of the low-lying inverted 2Π state increases dramatically upon increasing v″. These counteracting trends result in a seemingly pure (bβS) coupling for X2Σ+(v = 1) in AlO and for the N < 60 region of X2Σ+(v = 2) in AlS. Support for this interpretation has been obtained by comparing synthetically generated hyperfine spectra with the observed line profiles in transitions to the X2Σ+(v = 0, 1, 2) levels of AlS and AlO.