Knowledge Bank

We report theoritical calculations of the $T1-S0$ excitation spectrum of pyrazine, pyrazine-$d4$, and the pyrazine-Arvan der Waals complex. Towards this end, a closed analytical formula for the intensities of individual rovibronic transitions has been derived. The triplet state Hamiltonian includes, in addition to the usual rigid rotor terms, both intramanifold spin-spin coupling and intermanifold spin-orbit coupling. Taking reasonable values for these parametres leads to several experimental predictions, including a transition from Hund#### Case (a) to Case (b) with increasing J and the apperance of fine structure splittings at even moderate spectral resolution. Detailed examples will be discussed.