Factors influencing Cl- and F- enhancements in electron-stimulated desorption of CF2Cl2 coadsorbed with other gases

We have investigated the possible influence of various factors on the giant enhancement of Cl- and F- yields observed for the electron stimulated deso rption (ESD) of CF2Cl2 coadsorbed with polar molecules. We have measured th e variations of the secondary-electron energy spectrum and the work functio n of a fractional-monolayer CF2Cl2 precovered Ru(0001) coadsorbed with pola r H2O/NH3 molecules or nonpolar Xe atoms. The changes of Cl-, F-, and F+ yi elds under bombardment by 250 eV electrons have also been measured for the coadsorption systems, It is observed that the anionic enhancement is much s maller for bombardment by 30 eV electrons. The results indicate that the va riations in secondary-electron yield and energy distribution due to coadsor ption have a negligible effect on the negative-ion enhancements observed. T he work-function decrease caused by coadsorption may have some effect on th e anionic enhancements, but it is not the dominant factor. The monotonic at tenuation of F+ with increasing coadsorbate coverage indicates that coadsor ption does not induce exchange in the adsorption sites of CF2Cl2 and coadso rbates. The observation of giant Cl- and F- enhancements only for coadsorpt ion of CF2Cl2 with polar CH3OH, but not for coadsorption with Kr, further s upports the proposed mechanism that giant negative-ion enhancements are due to dissociation of CF2Cl2 by capture of electrons self-trapped by polar mo lecules.