Low-Energy Electron-Induced Chemistry of CF2Cl2: Implications for the Ozone Hole?

Abstract

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 (∼10 Å thickness) of CF2Cl2 grown at 100 K on a molybdenum single crystal in an ultrahigh vacuum chamber (p ∼ 1 × 10-10 Torr). 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.