Sublevel phosphorescence spectra of pyrazine. Interpretation of ‘‘anomalous’’ behavior of the radiative rate constant ratios for totally symmetric vibrations and the reinvestigation of the triplet state geometry

The phosphorescence spectra from the three individual triplet spin sublevels of pyrazine in a single crystal of benzene are obtained. The radiative rate constants are determined for individual vibronic bands of the individual sublevel phosphorescence spectra. The nonradiative decay rate constants are also determined for individual sublevels. The mechanisms of the radiative and nonradiative transitions from the sublevels are discussed, and are satisfactorily interpreted within a conventional perturbation theory. For vibronic bands involving totally symmetric skeletal vibrations ν1 and ν8a, the ratios of the radiative rate constants are markedly different from the ratio observed at the 0–0 band. This is interpreted as due to the importance of vibronic coupling due to these totally symmetric vibrations and the smallness of the displacement of the potential energy surfaces between the triplet and the ground states. This interpretation, which is achieved within a framework of the Born–Oppenheimer and Herzberg–Teller approximations, substantially differs from the interpretation given for an analogous observation by Breiland and Harris. The vibrational structures of the sublevel phosphorescence substantiate our earlier conclusion that the triplet state of pyrazine is distorted along the ν4(b2g) coordinate leading to the point group of C2h[C2(y)].