Thermal chemistry of CF3I on Ag(111): A TPD and RAIRS study

Temperature programmed desorption and reflection absorption infrared spectroscopy have been used to study the thermal chemistry of CF3I on Ag(111). At coverages less than or equal to 0.03 CF3I/Ag, adsorption occurs with I-CF3 bond dissociation at T=86 K, while at higher coverages CF3I adsorbs molecularly. The C-I bond direction of the adsorbed molecules depends on the coverage. When molecular adsorption occurs at coverages less than or equal to 0.15 CF3I/Ag the C-I bond direction is parallel to the surface. Above this coverage the adsorbed molecules reorder to a form in which the molecular axis is tilted away from the surface plane. The first layer saturates at 0.3 CF3I/Ag coverage. Multilayers form readily at 86 K. When the CF3I/Ag(111) system is heated, the multilayers desorb at similar to 100 K, and some of the CF3I thermally dissociates into CF3 and I at similar to 110 K. The fate of the products depends on the molecular orientation prior to the fragmentation. When the coverage is high and the C-I axis is tilted from the surface, the CF3 radicals desorb during dissociation at similar to 110 K, leaving only iodine atoms on the surface. At low coverage, when the C-I axis is parallel to the surface, both the CF3 and the iodine remain bound. The non-dissociated CF3I desorbs molecularly at T less than or equal to 150 K. The CF3 that remains after C-I bond scission desorbs as radicals at similar to 300 K, and the adsorbed iodine desorbs atomically at similar to 800 K. The relationship of molecular orientation to previous results on CF3I/Ag(111) thermal and photochemistry is discussed.