High resolution lifetime measurements of the perturbed J′=0 levels of the 1B3u state of pyrazine

The lowest excited singlet 1B3u state of pyrazine is known to be coupled to a number of triplet 3B3u states. Using a strongly collimated molecular beam and a single frequency laser it is shown that the J′=0 of the 000 transition contains at least 36 states. We have individually excited eight of these states and studied its decay. The lifetimes found (typically 450 ns) do not scale with the intensities of the excitation spectrum. This deviation is caused by a nonradiative decay of the zero order 3B3u states. With a simple model it was possible to reconstruct the absorption spectrum, the energies of the zero order states and its coupling strengths. The zero order decay rates of the singlet and triplet states have been determined. The value obtained for the zero order singlet state is 5 MHz; the values for the triplet states range from 0.6 to about 5 MHz.