AN ANALYSIS OF VIBRONIC COUPLING IN THE TRIPLET ABSORPTION-SPECTRUM OF PYRAZINE

New measurements of the singlet-triplet (aB-3(3u) <-- X1A(g)) absorption spectra of pyrazine and pyrazine-d4 at longer pathlengths and higher vapour pressures have been recorded. Particular attention was paid to the hot band region to the red of the electronic origin. The activities of the out-of-plane vibrations, nu4 and nu10a, were identified in sequence bands, and their triplet state vibrational frequencies were determined, 295 (276, -d4) and 254 (194, -d4) cm-1, respectively. Model calculations were undertaken to match the observed singlet and triplet absorption and emission spectra for vibronic coupling induced by (i) nu10a(b1g), and (ii) nu4 and nu5(b2g). The calculations allowed the ordering and the relative energies of the higher lying triplet states to be determined. The B-3(2u) and B-3(1u) electronic states are above the ground electronic state by about 3.9 and 4.4 eV, respectively. The ordering is in analogy with the corresponding singlet states but opposite to that predicted by most theoretical calculations.