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The primary photochemical process in hexafluoroacetone vapour Bowers, Peter George

Abstract

The photochemical dissociation of hexafluoroacetone vapour (HFA) has been studied at room temperature using five exciting wavelengths between 2500 and 3500 Å, and at -78°, using 3130 Å excitation. The effect of the presence of biacetyl during the photolysis has been investigated. Complementary experiments on the visible emission from HFA were performed, paying particular attention to the quenching action of oxygen and biacetyl. Plots of reciprocal quantum yield of carbon monoxide versus HFA pressure have either a positive and constant, or positive and decreasing slope, down to 1 mm pressures there is virtually no evidence that a weak (multistage) collision mechanism is operative in deactivation of the excited molecules. A simple extension of classical unimolecular theory is examined, in order to account, for observed photo-dissociation rate constants, k₂(λ, T). The model shows that Arrhenius plots of k₂(T) at constant wavelength will not, in general, be linear, but that upper and lower limits for the critical (activation) energy, may still be obtained. For HFA, this energy is placed between 5.7 and 8.6 kcals/mole. Agreement in order-of-magnitude is found between calculated and observed values of k₂(T), only if it is assumed that a fraction (rather more than half) of the normal modes participate in intramolecular energy exchange. This fraction has to be increased with decreasing wavelength to explain the observed k₂(λ) values. The inadequacy of a classical treatment of the system is recognized. The emission spectrum of HFA consists of both fluorescence and phosphorescence. Small concentrations of biacetyl quench phosphorescence and triplet dissociation. The phosphorescence to fluorescence ratio is 2.77 at 25º and 9.95 at -78°, while the total emission yield increases by a factory of 16.7 on decreasing the temperature over this range. From a study of biacetyl emission, direct and sensitized by HFA, it is shown that nearly 50% of HFA molecules excited with 3130 Å, eventually reach the triplet state at 25º. The phosphorescence yield of HFA at -78°was estimated to be 0.51. At relatively high pressures, dissociation yields are apparently greater when less energetic excitation is used. Various modifications to the intersystem crossing mechanism are discussed to account for this, but no definite conclusion can be reached.

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