Zeeman quantum beat spectroscopy applied to the à 1A2–X̃ 1A1 transition of SO2

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Abstract
Zeeman quantum beats were observed in the fluorescence from a number of rovibronic levels of SO2 in the à 1A2 state. Rotationally cooled SO2 was prepared by supersonic expansion of its seeded mixture in Ar and was excited to the first electronically allowed 1A2 state by a frequency‐doubled dye laser in the wavelength range 260–320 nm. In the time‐resolved fluorescence measurements, a number of rovibronic lines were found to exhibit beating fluorescence decays in the presence of a weak magnetic field. The beating phenomena were analyzed on the basis of Zeeman quantum beat theory; of particular concern are the oscillation amplitude of the beating and the g value. It was found that almost all rotational levels of upper vibronic states of various bands (D, F, L, and N in Clements’ notation) possess sizable magnetic moments. On the other hand, only a limited number of rovibronic lines of several less perturbed bands (E and G) showed beating fluorescence decays. The derived g values and fluorescence lifetimes distribute rather irregularly over a wide range, but an almost linear relationship between the g value and the lifetime was found for the rR0(0) lines of various vibronic transitions. The intramolecular coupling mechanism needed to explain the anomalous behaviors of these excited rovibronic states are discussed. It is suggested that the dominant mechanism is spin‐orbit coupling.