The wavelength- and temperature-dependence of the fluorescence efficiency and of the primary photochemical yield in hexafluoroacetone vapour

Abstract

The measurement of the fluorescence and photochemical yields of hexafluoroacetone previously made at 3130 Å and at room temperature is extended to various other wavelengths and temperatures. Relative fluorescence yields at room temperature increase with decrease of excitation energy. This wavelength dependence is smaller at higher concentrations. The yields increase with concentration eventually converging almost to a single value. On the other hand, the photochemical yields measured show the opposite trends with respect to variations in concentration and excitation energy. As concentration increases they also converge to a value almost identical with that at 3130 Å. The temperature effect on the fluorescence yield is also opposite to the one on the photolysis at 3130 Å. The fluorescence decreases with increase in temperature, the variation being much bigger at higher concentrations. The intensity distribution of the fluorescence band is independent of temperature. It seems difficult to interpret the observed temperature dependence for fluorescence quenching and for dissociation at infinite concentration on the basis of a single excited state. Two modes of dissociation are postulated arising respectively from low and from high vibrational levels of different electronic states.