Kinetics of electron-induced decomposition of CF 2Cl 2 coadsorbed with water (ice): A comparison with CCl 4

Abstract

The kinetics of decomposition and subsequent chemistry of adsorbed CF 2Cl 2, activated by low-energy electron irradiation, have been examined and compared with CCl 4. These molecules have been adsorbed alone and coadsorbed with water ice films of different thicknesses on metal surfaces (Ru; Au) at low temperatures (25 K; 100 K). The studies have been performed with temperature programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS), and x-ray photoelectron spectroscopy (XPS). TPD data reveal the efficient decomposition of both halocarbon molecules under electron bombardment, which proceeds via dissociative electron attachment (DEA) of low-energy secondary electrons. The rates of CF 2Cl 2 and CCl 4 dissociation increase in an H 2O (D 2O) environment (2-3X), but the increase is smaller than that reported in recent literature. The highest initial cross sections for halocarbon decomposition coadsorbed with H 2O, using 180 eV incident electrons, are measured (using TPD) to be 1.0±0.2×10 -15 cm 2 for CF 2Cl 2 and 2.5±0.2×10 -15 cm 2 for CCl 4. RAIRS and XPS studies confirm the decomposition of halocarbon molecules codeposited with water molecules, and provide insights into the irradiation products. Electron-induced generation of Cl - and F - anions in the halocarbon/water films and production of H3O +, CO 2, and intermediate compounds COF 2 (for CF 2Cl 2) and COCl 2, C 2Cl 4 (for CCl 4) under electron irradiation have been detected using XPS, TPD, and RAIRS. The products and the decomposition kinetics are similar to those observed in our recent experiments involving x-ray photons as the source of ionizing irradiation. © 2004 American Institute of Physics.