Radiative and non-radiative processes in p-benzoquinone vapour

Emission and excitation spectra of p-benzoquinone vapour have been measured at the pressure range from 0·2 to 3 Torr as well as at high total pressure (∼ 100 Torr) in the presence of an added foreign gas. It is shown that p-benzoquinone vapour excited into 1 B 3g (π, π*) exhibits fluorescence from high vibrational levels in the 1 B 1g (n, π*) state in addition to the phosphorescence, 3 Au (n, π*) → 1 g , at low pressure. The quantum yield of the fluorescence increases as the pressure is lowered, while that of the phosphorescence decreases to approach zero. The phosphorescence quantum yield varies markedly according to the nature of the singlet state to which the molecule is initially excited. On the basis of the pressure and excitation-energy dependence of the emission quantum yields, the occurrence of the fluorescence is interpreted in terms of a mechanism involving reversible intersystem crossing. It is shown also that the fluorescence at high total pressure exhibits a character of thermally activated d...