Luminescent Properties of Hexafluoroacetone. III. Vibrational Relaxation and Radiationless Processes in the Gas Phase

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Abstract
Flourescence decay of hexafluoroacetone (HFA) vapor, excited at 2977, 3160, 3370, and 3577 Å, was examined over a pressure range 0.1–700 torr. The single photon technique, which possesses very high sensitivity, was employed. Under isolated conditions, HFA fluorescence followed a single exponential decay, and the lifetime gradually decreased with increasing excitation energy. The radiative lifetime was found to be constant over this wavelength range. From a comparison of the energy dependence of the nonradiative rate constant with that of the dissociation rate predicted from RRKM theory, it is suggested that the radiationless process depleting the excited singlet state at the above wavelengths is intersystem crossing. At intermediate pressures, fluorescence decay was nonexponential, showing the effects of vibrational relaxation. The analysis of these decay curves resulted in the rejection of the hard collision mechanism. By comparing these time‐dependent studies with recent steady‐state fluorescence data, a four‐level model is proposed which combines an intermediate vibronic level with the hard collision mechanism. Approximately one collision in three was calculated to be effective in removing excess vibrational energy. Some discrepancies between these kinetic data and the steady‐state results are discussed. The temperature dependence of the fluorescence lifetime was also examined, and an equivalence between optical and thermal excitation was found. HFA was calculated to be in the statistical limit of radiationless transitions.

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