MULTI-PHOTON DECOMPOSITION OF HEXAFLUOROACETONE - EFFECTS OF PRESSURE, FLUENCE, WAVELENGTH, AND TEMPERATURE ON THE DECOMPOSITION YIELD AND C-13 AND O-18 ISOTOPIC SELECTIVITY

The yield and isotopic selectivity of the infrared multiphoton induced decomposition of hexafluoroacetone have been studied in both parallel and focused beam geometries as a function of temperature, pressure, fluence, and photolysing wavelength. At low conversions, the chemical stoichiometry of the decomposition process is simple, CF3COCF3 →nhv CO + 2CF3. Several potential interferences are discussed but it is concluded that the yield of CO directly reflects the yield of the primary dissociation event. Pressure effects are complex and not amenable to quantitative description, however, varying parameters such as fluence and photolysing wavelength lead to predictions which give a clear qualitative picture. In particular, it is possible to distinguish parametric variations due to spontaneous dissociation and dissociation dependent upon energy pooling. Isotopic selectivity provides a powerful insight in this area. Preliminary data on the effect of temperature have been obtained at constant incident fluence and number density. Varying temperature probes the contribution of initially excited species (hot-bands). Reducing the temperature has no effect on the blue-edge of the absorption band but leads to a narrowing on the red-edge which is more marked for parallel beam geometries. The form of the wavelength dependence is in agreement with theoretical predictions. The observed carbon-13 isotopic selectivities of the dissociation are in basic agreement with those predicted from spectroscopy in that αCO is much higher than αC3F6. Very high values for this selectivity have been observed approaching αCO = 1OOO under extreme conditions. The equivalent selectivities for oxygen-18 are small, α≃2. The hexafluoroacetone molecule is demonstrated to be a good model for study of multiphoton events and the data presented offer a powerful basis set for theoretical treatment. © 1979 American Institute of Physics.