Near-Infrared Transitions from the Singlet Excited States to the Ground Triplet State of the S2 Molecule

Abstract

Intensity of transitions from the b1∑g+  and a1Δg states to the ground state X3∑g−  in the near IR emission spectrum of the S2 molecule has been calculated by the multireference configuration interaction method taking into account spin-orbit coupling (SOC). The intensity of the b1∑g+ −  X3∑g,Ms=±1− transition is largely determined by the spin interaction with the electromagnetic wave, which comes from the zero-field splitting of the ground X multiplet and the SOC-induced mixing between b and X3∑g,0− states. The Einstein coefficients for the experimentally detected 0−0, 0−1, 1−1 bands of the b1∑g+−X3∑g,Ms=±1− emission system are calculated in good agreement with observations. The Einstein coefficient of the a1∆g−X3∑g,Ms=±1− magnetic dipole transition is very low, being equal to 0.0014 s−1. Nonetheless, the weakest of all experimentally observed bands (the 0−0 band of the a-XMs=±1 transition) qualitatively corresponds to this calculation. Most importantly, we provide many other IR bands for magnetic dipole b1∑g+ −  X3∑g,Ms=±1− and a1∆g−X3∑g,Ms=±1− transitions, which could be experimentally observable in the S2 transparency windows from a theoretical point of view. We hope that these results will contribute to the further experimental exploration of the magnetic infrared bands in the S2 dimer.