Lifetime of Fluorescence in Diacetyl and Acetone

The mean lifetime of the fluorescence of diacetyl vapor has been determined by direct measurement with a phosphoroscope to be 1.65 × 10—3 sec. Quantitative measurement of the diffusion of the excited molecules from a beam of exciting illumination, at different pressures, has also been made and the results are compatible with this lifetime. Integration of the absorption coefficient over the band associated with the fluorescence leads, on the other hand, to a lifetime of the excited state of 10—5 sec. To explain this discrepancy and other facts known about the fluorescence various mechanisms are considered, of which the most satisfactory seems to be this: Following light absorption, X→A, the diacetyl molecule goes without radiation into a long-lived state M, lying near A. Fluorescence occurs only upon return to A. M may correspond to a tautomeric rearrangement of the molecule. Acetone, radiated with λ3130, shows fluorescence identical with that of diacetyl radiated with λ4358, but the fluorescence grows with time. It can be produced immediately with high intensity by adding diacetyl. The growth curve has been determined and is of form, It = I0 (1 — e—kt). Diffusion experiments show the lifetime of the fluorescence in acetone to be equal to that in diacetyl and that the lifetime, or rate of decay, is independent of exciting intensity. The conclusion is that the same molecule, presumably diacetyl, is responsible for the fluorescence in both cases. Possible mechanisms for the excitation of diacetyl in acetone are discussed.