Fluorobenzene, fluorescence yields

Thus it can be concluded that fluorobenzene gives triplet-state molecules to the extent that it does not fluoresce but there is a relatively large uncertainty in this statement. The fluorescence yield decreases and the crossover yield apparently increases as the frequency of the incident light increases. 

The absolute fluorescence yields for QHj derive with one exception from excitation at 2536 A. Two of these are based on calibration against the phosphorescence yield of biacetyl,which has been a gas phase standard for many years. The first reports a yield of about 0.23 0.05 at 11 torr, but this was subsequently remeasured by Poole as 0.17 in further evolution of that technique in the same laboratory. A later measurement independent of the biacetyl standard is reported by Noyes, Mulac, and Harter. They observe (in their Method B) that f = 0.18 0.04 at 11.5 torr. Further support for this value comes from its use in calibration of singlet relaxation quantum yields in fluorobenzene and toluene in which the sum of observed radiative and nonradiative yields is unity within experimental error. A higher benzene fluorescence yield would push that sum paradoxically above unity. 

When irradiated at 2420 A or above, the sum of the fluorescent and triplet-state yields of fluorobenzene is unity or slightly greater than unity. Use of the correct branching ratio is impossible because it has not been determined for fluorobenzene.30,31 This has been substantiated recently, using the Cundall technique, to measure triplet-state yields rather than the biacetyl sensitization technique. The... 

A method for measuring fluorescence quantum yields and cascade free lifetimes for open shell cations has been reported. The lifetimes are calculated from the coincidence between undispersed fluorescence photons and energy selected photo-electrons. A similar system has been used to evaluate the fluorescence lifetimes of and and fluorobenzene cations." ... 

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