AVS 46th International Symposium
    Surface Science Division Monday Sessions
       Session SS3-MoM

Paper SS3-MoM8
Giant Cl@super-@ and F@super-@ Enhancements in Electron-Stimulated Desorption of CF@sub2@Cl@sub2@ Coadsorbed with Water or Ammonia Ices: Implications for Atmospheric Ozone Depletion

Monday, October 25, 1999, 10:40 am, Room 612

Session: Water-Surface Interactions
Presenter: T.E. Madey, Rutgers, The State University of New Jersey
Authors: T.E. Madey, Rutgers, The State University of New Jersey
Q.-B. Lu, Rutgers, The State University of New Jersey
Correspondent: Click to Email
Dissociative electron attachment to form Cl@super-@ and F@super-@ 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@super-@ and F@super-@ enhancements by several orders of magnitude in electron-stimulated desorption of a fractional monolayer of CF@sub2@Cl@sub2@ 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@super-@ is much stronger than that of F@super-@, and the enhancements for both ion species by NH@sub3@ coadsorbate are stronger than by H@sub2@O. Moreover, all magnitudes of enhancement increase strongly with decreasing CF@sub2@Cl@sub2@ concentration; for 0.3 ML CF@sub2@Cl@sub2@, the largest Cl@super-@ enhancements are ~3x10@super4@ for NH@sub3@ and ~10@super2@ for H@sub2@O. In contrast, the enhancements are much smaller for CF@sub2@Cl@sub2@ 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 CF@sub2@Cl@sub2@ 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@super-@ ions produced may be directly or indirectly converted to Cl atoms, which then destroy ozone.