Structural and temperature effects on Cl- yields in electron-induced dissociation of CF2Cl2 adsorbed on water ice

The temperature (T) dependence of the Cl- yield in the electron-stimulated desorption (ESD) of a submonolayer of CF2Cl2 adsorbed on amorphous H2O ice grown at 25 K exhibits a peak between 50 and 70 K. This peak and a sharp dr op in electron-trapping coefficient of ice near 50 K show strong evidence f or a phase transition. The peak disappears for CF2Cl2 on crystalline ice; i nstead, a non-Arrhenius Cl- dependence on Tis observed with a critical temp erature of similar to 224 K. The peak is also not pronounced in the T depen dence of the ESD H- yield of amorphous ice, which involves no transfer of t rapped electrons. (C) 2001 Elsevier Science B.V. All rights reserved.