Emission spectra and phosphorescence lifetime of benzene solutions at 77°K, as a function of concentration and excitation wavelength

The fluorescence and phosphorescence emission spectra and the phosphorescence lifetime of benzene in a rigid solution at 77°K have been measured as a function of concentration and excitation wavelength. The shape of the emission spectra is independent of concentration. The internal conversion efficiency βλ between higher excited states and the lowest excited singlet state is dependent on, excitation wavelength and concentration. The action spectra of fluorescence and phosphorescence are identical for all concentrations, except in pure benzene for which no phosphorescence was detected. The behaviour of βλ as a function of wavelength of excitation cannot be explained by an intersystem crossing process between higher singlet excited states and the triplet manifold. The decrease in phosphorescence with increase in concentration, which is accompanied by an increase in fluorescence, is attributed to a decrease not only of the phosphorescence lifetime, but also of the rate of intersystem crossing between the lowest excited singlet state and the triplet manifold. No delayed fluorescence was detectable.