Chemical Reactions in CF2Cl2/Water (Ice) Films Induced by X-ray Radiation

Abstract

The chemical reactions initiated by high-energy radiation (Mg or Al K α X-rays) in amorphous CF2Cl2/H 2O(ice) films have been studied using a combination of reflection absorption infrared spectroscopy (RAIRS), X-ray photoelectron spectroscopy (XPS), and temperature programmed desorption (TPD). Following deposition, the structure of the CF2Cl2/H2O(ice) film resembles an amorphous ice phase having CF2Cl2 molecules caged within the film, and a smaller number of CF2Cl2 molecules adsorbed on the ice surface. X-ray irradiation produces a broad distribution of low-energy secondary electrons whose interactions with CF 2Cl2/H2O(ice) films are associated with the production of H3O+, CO2, and COF2 (carbonyl fluoride) as detected by RAIRS. COF2 is identified as an intermediate species whose electron-stimulated decomposition leads to CO 2 production. The product partitioning is dependent on the film's initial composition; in water rich films, CO2 and COF2 production is favored, whereas a more thermally stable, partially halogenated polymeric CFxCly film is detected by XPS in CF 2Cl2 rich films. Chloride and fluoride anions are also produced and solvated (trapped) within the ice film. During the early stages of X-ray irradiation, the dominance of Cl- anions formed in the film by reaction with low-energy secondary electrons is consistent with the suggestion that C-Cl bond cleavage of CF2Cl2 via dissociative electron attachment (CF2Cl2 + e- → ·CF2Cl + Cl-) is the dominant initial process.