Vibrations of pyrazine and its ion as studied by threshold ionization spectroscopy.

The pump–probe threshold ionization photoelectron spectra of pyrazine have been recorded using nine different vibrations of S1 as intermediate resonances. The extensive vibrational structure in these spectra of the ionic ground state have enabled the measurement of 12 of its vibrational frequencies and their assignments. Three new vibrational assignments of S1 are also made. MP2/6-31G* calculations of the vibrational frequencies of the neutral ground, S1, and the ionic ground state are compared with the experimental values, finding that certain vibrations of S1 and the ion which engage in extensive vibronic coupling are not properly determined by the calculated force field. Most vibrational frequencies are accurately reproduced, however. Variations in the complexity of the threshold ionization spectra with the level of S1 excitation indicate that internal vibrational relaxation is taking place at a very low energy in that state, possibly involving vibronic interactions and mixing with the triplet manifold. [ABSTRACT FROM AUTHOR]