Dissociative electron attachment to form Cl^- and F^- can be an important process for the destruction of ozone-depleting chlorofluorocarbons (CFCs) in the upper atmosphere, because of their extremely large electron attachment cross sections. We have observed giant Cl^- and F^- enhancements by several orders of magnitude in electron-stimulated desorption of a fractional monolayer of CF₂Cl₂ coadsorbed with water ice and ammonia ice on a Ru(0001) surface at ~25 K, respectively. The yields of negative ions are measured by an electron-stimulated desorption ion angular distribution (ESDIAD) detector with time-of-flight capability. The enhancement of Cl^- is much stronger than that of F^-, and the enhancements for both ion species by NH₃ coadsorbate are stronger than by H₂O. Moreover, all magnitudes of enhancement increase strongly with decreasing CF₂Cl₂ concentration; for 0.3 ML CF₂Cl₂, the largest Cl^- enhancements are ~3x10⁴ for NH₃ and ~10² for H₂O. In contrast, the enhancements are much smaller for CF₂Cl₂ coadsorption with rare-gas atoms or nonpolar molecules. Whereas the primary electron beam energy is 250 eV, the giant negative-ion enhancements are attributed to dissociation of CF₂Cl₂ by capture of low-energy secondary electrons self-trapped (solvated) in polar water or ammonia clusters. This process may be an important sink for chlorofluorocarbons (CFCs) in the atmosphere, where low-energy electrons created by cosmic ray ionization can be trapped in clouds. Cl^- ions produced may be directly or indirectly converted to Cl atoms, which then destroy ozone.

References:

1.	Q.-B.Lu, T.E.Madey	Mechanism for giant Cl- and F- enhancements in electron-induced dissociation of CF2Cl2 coadsorbed with water or ammonia ices	Surf. Sci. 2000, V.451, P.238-243
2.	Q.-B.Lu, T.E.Madey	Factors influencing Cl- and F- enhancements in electron-stimulated desorption of CF2Cl2 coadsorbed with other gases	J. Phys.Chem.B., 2001, V.105, No.14, P.2779-2784