Pyrazine (1,4-diazabenzene) has played a central role in the development of the theory
of radiationless transitions. In this paper, we will describe studies of the decay behavior
and fluorescence quantum yield of the isolated molecule and several of its isotopically
labelled derivatives following pulsed dye laser excitation of “single” rotational levels
of the 1B3u (nπ*) state in a seeded supersonic jet, both in the presence and absence
of an external magnetic field. We shall demonstrate that pyrazine can be transformed
from a “small” molecule to a “large” molecule by very small perturbations; e.g., by
climbing up the rotational ladder of the first excited singlet state, by isotopic labeling,
and/or by applying dc magnetic fields of the order of 100 G. Thus, the experiments
provide important tests of the predictions of the theory for the first time. Specifically,
we will show that, under “intermediate-case” conditions, (1) the fast component of
the decay is indeed coherent in nature, (2) the coupled state is triplet in character
(and, thus, that the process we are monitoring is intersystem crossing (ISC)), (3) nuclear
spin is conserved in ISC, (4) rotations and small magnetic fields play an important role
because of the angular momentum selection rules for ISC, and (5) the levels mixed
by the magnetic field are most likely the fine-structure components of the different
angular momentum levels of the mixed, singlet–triplet state. The relationship of these
results to those of other workers in the field will also be discussed.
