Site preferences, energetics and orientation of CH3Br on Pt(111): investigation by RAIRS, TPD and photofragment angular distributions

Reflection absorption infrared and thermal programmed desorption spectroscopies were used to examine the thermal chemistry of CH3Br on Pt(lll). Abrupt changes in the RAIRS and TPD spectra provided evidence for the population of different adsorption phases at coverages below saturation of the first monolayer. At low coverages, coadsorption experiments with CO and O atoms indicate that CH3Br occupies only top sites. As the coverage is increased past 0.10 ML, less stable multi-coordinate sites begin to populate. Assuming an initial sticking coefficient of unity, saturation coverage of CH3Br is 0.25+/-0.02 ML. The adsorption energy of CH3Br decreased from 0.62 to 0.33 eV as the coverage increased to saturation. Changes in the adsorbate orientation as a Function of coverage were investigated using RAIRS and the angular distributions of CH3 photofragments from dissociative electron attachment of CH3Br induced by irradiation at 193 nm. Orientational information derived from these two methods were contrasted. Photofragment angular distributions were consistent with CH3Br undergoing large amplitude librational motions, with a preferred orientation close to the surface plane at low coverages. With increasing coverage the molecules gradually tip upwards to point closer to the surface normal. (C) 1997 Elsevier Science B.V.