The Fluorescence of Diacetyl

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Abstract
Diacetyl, in the vapor phase or in water solution, exhibits a bright green fluorescence when excited with light within its absorption band in the violet and near ultraviolet. A detailed measurement of the structure shows that the spectrum is the same as is shown in the fluorescence of acetone vapor when excited with λ3130. The fluorescence spectrum is insensitive to changes in diacetyl pressure (0.1 mm to 50 mm), temperature (10°C to 100°C), exciting wave‐length (3650A to 4358A) or amount of added oxygen, which quenches the fluorescence strongly. The intensity of fluorescence, measured with photo‐cell and amplifier, is constant in time, with weak excitation. The ratio of intensity of fluorescence to light absorbed is independent of the pressure and the intensity of the fluorescence is linearly proportional to the exciting intensity, at constant pressure. The fluorescence is quenched by oxygen, the intensity falling to one‐half at 0.013 mm oxygen. The Stern‐Volmer law is followed over a range of oxygen pressures in which the fluorescence drops to 20 percent of the intensity without oxygen; at higher pressures the intensity falls more rapidly than expected. The oxygen is consumed, presumably in the quenching process, and the fluorescence returns to full intensity. From the quenching curve and from an experiment on the diffusion of the excited molecules into shadow the lifetime of the fluorescing molecule is estimated to be about 10—5 sec. These results can be understood on the assumption that the diacetyl molecule emits the fluorescence after excitation and redistribution (spontaneous and through collisions) of the vibrational energy. In the case of acetone the fluorescence builds up gradually from practically zero intensity, as expected if diacetyl is accumulating; it is excited, not by light, but probably by collisions with excited acetone or with combining radicals. The diffusion experiments can scarcely be reconciled with the assumption that the observed emission in pure diacetyl is a recombination spectrum.
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