Spin polarization in SO photochemically generated from SO2

Abstract

The photochemical processes induced in sulfur dioxide by 193 nm excimer laser excitation have been investigated by examining vibration-rotation transitions of the reaction product SO with the use of tunable infrared diode-laser spectroscopy. Only X 3Σ− ground-state SO molecules were observed. About 70% of the nascent SO molecules were found to be in the v=2 state, about 20% in v=1, and some in v=5. The rotational distribution in each vibrational state differed significantly from the thermal distribution, and was shifted towards higher rotational levels in lower vibrational states. The electron spin was observed to be polarized in the v=1 and 2 states in a direction either parallel or antiparallel to the rotational angular momentum, i.e., the F1 and F3 spin sublevels are more populated than the F2 level in these states. This selective population was not observed for v=5. The origin of the spin polarization is discussed in terms of spin-orbit mixing between the C̃ state and a repulsive triplet state of SO2. The results obtained here emphasize the usefulness of high resolution, high sensitivity infrared laser spectroscopy in the study of primary processes in photochemistry.