Intersystem crossing in isolated molecules. Magnetic field effects on the fluorescence decay behavior of 1B3u pyrazine with ‘‘single’’ rovibronic level excitation

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Abstract
Coherently prepared ‘‘single’’ rovibronic levels of electronically excited pyrazine typically exhibit a biexponential fluorescence decay whose preexponential factor ratio A+/A− is magnetic field dependent. Studies of this dependence for different J′ values show that complete spin decoupling from the molecular frame, leading to the intersystem crossing (ISC) selection rule ΔJ=0,±1, occurs at the anomalously low field of ∼100 G. The monoexponential decay of the J′=0 level is independent of the field. An analysis of these results suggests that Coriolis coupling plays an important role in the ISC process, a fact which explains the magnetic quenching behavior of many polyatomic molecules.