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

The chemical reactions initiated by high-energy radiation (Mg or Al K α X-rays) in amorphous CF 2 Cl 2 /H 2 O(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 CF 2 Cl 2 /H 2 O(ice) film resembles an amorphous ice phase having CF 2 Cl 2 molecules caged within the film, and a smaller number of CF 2 Cl 2 molecules adsorbed on the ice surface. X-ray irradiation produces a broad distribution of low-energy secondary electrons whose interactions with CF 2 Cl 2 /H 2 O(ice) films are associated with the production of H 3 O +, CO 2, and COF 2 (carbonyl fluoride) as detected by RAIRS. COF 2 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, CO 2 and COF 2 production is favored, whereas a more thermally stable, partially halogenated polymeric CF x Cl y film is detected by XPS in CF 2 Cl 2 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 CF 2 Cl 2 via dissociative electron attachment (CF 2 Cl 2 + e - → ·CF 2 Cl + Cl - ) is the dominant initial process.