Mechanism for giant Cl- and F- enhancements in electron-induced dissociation of CF2Cl2 coadsorbed with water or ammonia ices

We have observed giant Cl- and F- enhancements by factors of 10(2)-10(4) in electron-induced dissociation of a fractional monolayer of CF2Cl2 when CF2 Cl2 is coadsorbed with water ice or ammonia ice on a surface. The enhanceme nt of Cl- is much stronger than that of F-, and the enhancements for both i on species by NH3 coadsorbate are stronger than those by H2O. Moreover, the magnitudes of enhancement increase strongly with decreasing CF2Cl2 concent ration. Only small negative-ion enhancements are observed for coadsorption with nonpolar CH4 molecules. It is proposed that giant negative-ion enhance ments are due to dissociation of CF2Cl2 by capture of electrons self-trappe d in polar water or ammonia molecules. This process may have far-reaching i mplications for the ozone-hole creation in the polar stratosphere. (C) 2000 Elsevier Science B.V. All rights reserved.