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 desorption (ESD) of CF 2 Cl 2 coadsorbed with polar molecules. We have measured the variations of the secondary-electron energy spectrum and the work function of a fractional-monolayer CF 2 Cl 2 precovered Ru(0001) coadsorbed with polar H 2 O/NH 3 molecules or nonpolar Xe atoms. The changes of Cl -, F -, and F + yields under bombardment by 250 eV electrons have also been measured for the coadsorption systems. It is observed that the anionic enhancement is much smaller for bombardment by 30 eV electrons. The results indicate that the variations in secondary-electron yield and energy distribution due to coadsorption have a negligible effect on the negative-ion enhancements observed. The work-function decrease caused by coadsorption may have some effect on the anionic enhancements, but it is not the dominant factor. The monotonic attenuation of F + with increasing coadsorbate coverage indicates that coadsorption does not induce exchange in the adsorption sites of CF 2 Cl 2 and coadsorbates. The observation of giant Cl - and F - enhancements only for coadsorption of CF 2 Cl 2 with polar CH 3 OH, but not for coadsorption with Kr, further supports the proposed mechanism that giant negative-ion enhancements are due to dissociation of CF 2 Cl 2 by capture of electrons self-trapped by polar molecules.