Some aspects of benzene vapor phase photochemistry

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Abstract
The fluorescence, triplet state, and isomerization yields from benzene vapor have been determined at several wavelengths but principally at 266.8 nm, where 1B2u molecules are formed in the zeroth vibrational level. The fluorescent yield at 266.8 nm varies little with pressures from 1 to 20 torr, but at higher pressures of either benzene or inert gases the yield decreases slowly to about 0.18, i.e., the same yield as obtained at 253 nm and pressures over about 15 torr at room temperature. Triplet yields were determined by the biacetyl method, and at low pressure the sum of the fluorescence and triplet yields at 266.8 nm was about 0.97. At shorter wavelengths the sum increasingly differs from unity and the difference is almost certainly due to isomerization. Primary isomerization yields are difficult if not impossible to measure quantitatively because of the photochemical and possibly the thermal instability of benzvalene, the predominant isomer formed at these wavelengths. Within experimental error there is no isomerization at 266.8 nm and the steady state concentration of benzvalene increases with decreasing wavelengths in general. The presently known facts about the photochemistry fit into a reasonably consistent picture, but more data along certain lines are badly needed even if they would be very difficult to obtain.