The Quantum Yield of Diacetyl Fluorescence

Abstract

The quantum yield of fluorescence of diacetyl, excited by radiation of λ4047 and λ4358, has been measured and found to be 0.145±0.03. It is approximately independent of pressure. On the other hand, the yield with excitation of λ3650, as first observed by Henriques and Noyes, increases apparently from zero at zero pressure to nearly the value obtained for the longer wave‐length excitations at a pressure of 5 cm Hg. This marked difference in pressure effect for different energies of excitation is explained by assuming that predissociation occurs at a level between those reached upon λ3650 and λ4047 excitations. By removing vibrational energy collisions can save for fluorescence a molecule excited by λ3650, but they do not play an important role in the case of molecules with excitation energy below the predissociation limit. The observed phenomena of diacetyl fluorescence can be explained in terms of a model which involves a metastable state just below the state reached on absorption. This model is compared with McMurry's predictions of states for molecules of the type of diacetyl.