Direct spectroscopic evidence for a new, low-lying electronic triplet state in SO2 at 4.2 K

Abstract

The phosphorescence emission and excitation spectra of crystalline SO2 at 4.2 K exhibit an anomalous energy gap of 83 cm−1 between their respective origins. The phosphorescence excitation, whose sharp origin occurs at 26 387 cm−1, involves a transition between the 1A1 ground state and the excited 3B1 state. However, our data indicate that the phosphorescence emission originates from a second triplet state, which lies 83 cm−1 lower in energy. A very weak absorption at 26 304 cm−1 is observed in the phosphorescence excitation spectrum, which exactly coincides with the observed phosphorescence emission origin. Site-selection experiments eliminate the possibility that the observed emission is due to defect sites in the crystal, and confirm the existence of the previously unreported low-lying triplet state. Phosphorescence lifetime data and the observed time-resolved emission spectra are consistent with this finding.