HUNDS CASE (A)-CASE (B) TRANSITION IN THE SINGLET-TRIPLET ABSORPTION-SPECTRUM OF PYRAZINE IN A SUPERSONIC JET

An analytic expression is derived for calculating the intensities of i ndividual spin-rovibronic lines in the fully resolved gas phase electr onic spectrum of a polyatomic molecule, in which one of the zero-order electronic states is a triplet state. The expression is employed to c alculate the effect of fine structure splitting on the singlet-triplet absorption spectrum of pyrazine using the parameters available from e xperiment. A transition from Hund's coupling Case (a) to Case (b) on g oing from low J to high J rotational levels is predicted to occur at a moderate resolution of a few hundred MHz. The effect is more pronounc ed in pyrazine-d(4) and the pyrazine-argon van der Waals complex owing to their larger mass. (C) 1998 American Institute of Physics.