The electronic relaxation of biacetyl in the vapor phase

Abstract

The emissions of biacetyl after pulsed dye-laser excitation were studied at pressures down to 0.05 mtorr. At all energies the time-resolved fluorescence was composed of a nanosecond and a microsecond component. At “zero” pressure the long lived phosphorescence was absent while the “hot” phosphorescence has the same time characteristics as the slow fluorescence. By increasing the pressure the slow fluorescence was quenched while the milisecond phosphorescence was induced. We determined the low-pressure emission characteristics and the pressure effects as a function of excitation energy. From our data we obtained the parameters describing the intermediate type singlet-triplet coupling, the radiative and non-radiative relaxation rates from the singlet and triplet levels and the cross sections for the slow fluorescence quenching, all as a function of energy. Strong evidence is obtained for the participation of rotational states in the intra-molecular relaxation. The important difference between the situation where the singlet levels are isolated (low energy) and where the singlet level widths overlap (at higher energies) is demonstrated. In the former situation very large fluorescence quenching cross sectios were found. It is further shown that for high energies at least two effective collisions are needed to obtain a thermalized triplet; the mean energy removed per effective collision is 2200 cm −1.