SINGLET-TRIPLET PERTURBATIONS IN PYRIMIDINE. MAGNETIC FIELD EFFECTS ON COLLISION-INDUCED INTERSYSTEM $CROSSING^{\ast}$

$^{\ast}$ Work supported by NSF.Author Institution: Department of Chemistry, University of PittsburghPyrimidine (1,3-diazabenzene) has been expanded in a seeded supersonic jet of helium and its laser-induced fluorescence spectrum has been recorded using a pulsed dye laser. Under these conditions, the K structure of the parallel-type $0^{0}_{0}$ band is partially resolved. Exciting specific (J,K) states, we have measured the fluorescence lifetime as a function of an external magnetic field (0-250 G) and the distance from the nozzle (x/D = 25-50). Typically, the lifetime is increased by the field, owing to an increase in the triplet character of the prepared state. However, plots of $\tau$ vs. H reveal a resonant structure which suggests the presence of isolated level anticrossings between nearly pure singlet and triplet rovibronic levels. A similar structure appears in plots of the collision cross section, deduced from the x/D dependence of the lifetime, vs. the magnetic field. This similarity shows that, as suggested by theory, the cross section for collision-induced intersystem crossing increases linearly with the average triplet character of the prepared state