Low-energy electron-induced chemistry of CF2Cl2:: Implications for the ozone hole?

We report on the first direct investigation of the low-energy electron-induced production of neutral species from the chlorofluorocarbon CF2Cl2, commonly known as Freon-12 or CFC-12. Our experiments were motivated by a newly proposed hypothesis, which suggests that low-energy electrons produced by cosmic rays, in addition to UV-vis photons from the sun, interact with chlorofluorocarbons to produce chlorine atoms that subsequently destroy ozone in the Antarctic. Our experimental procedure involves low-energy (5-100 eV) electron irradiation of nanoscale thin films (similar to10 Angstrom thickness) of CF2Cl2 grown at 100 K on a molybdenum single crystal in an ultrahigh vacuum chamber (p similar to 1 x 10(-10) Tort). Post-irradiation temperature-programmed desorption experiments were used to identify C2F4Cl2, C2F3Cl3, C2F2Cl4, C2F3Cl, C2F2Cl2, and C2F4 as electron-induced radiolysis products of CF2Cl2. In contrast to previous studies of photon-induced dissociation, our studies of electron-induced dissociation demonstrate facile C-F bond cleavage in CF2Cl2. This finding may have implications for understanding the partitioning of Cl and F among source, sink, and reservoir gases in the stratosphere.