THE FLUORESCENCE AND ITS RELATIONSHIP TO PHOTOLYSIS IN HEXAFLUOROACETONE VAPOR

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Abstract
Hexafluoroacetone vapor, when excited at 25 °C by light of wavelength 3130 Å, gives rise to fluorescence. The fluorescence spectrum starts at about 3470 Å and extends to almost 6000 Å with a maximum at about 4200 Å, independent of pressure over the range 3 mm. to 170 mm. No fine structure is observed. Oxygen has almost no effect on the spectrum, contrary to the cases of acetone and biacetyl vapor, where oxygen causes a marked change. The absorption follows Beer's law between 5 mm. and 50 mm. of hexafluoroacetone pressure and the absorption coefficient has been measured.The fluorescence efficiency decreases slightly with the addition of less than 1 mm. of oxygen and there stays constant up to an oxygen pressure of 30 mm., indicating that the fluorescence may be principally from the short-lived singlet excited state. The fluorescence efficiency increases with pressure of hexafluoroacetone or with the addition of inert gases such as perfluorobutane, perfluoropropane, hexafluoroethane, or carbon dioxide. The present results may be compared with those of the photolysis, where it was found that the quantum yield of the decomposition decreases with increasing pressure and with added inert gases. A close relationship between the fluorescence and the photolysis suggests itself. A mechanism is proposed to explain the results, which assumes that the molecule is raised to the upper vibrational levels of a singlet excited state, and subsequently deactivated to the lowest vibrational levels of the same state, from which it fluoresces. On the basis of this mechanism it seems possible to correlate the photolysis with the fluorescence.

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