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

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.