DETERMINATION OF MOLECULAR CHEMISORPTION GEOMETRIES USING REFLECTION-ABSORPTION INFRARED-SPECTROSCOPY - ALKYL-HALIDES ON PT(111)

Reflection-absorption infrared spectra (RAIRS) have been obtained for methyl chloride, methyl iodide, ethyl bromide and ethyl iodide chemisorbed on Pt(111) at 100 K as a function of initial exposures. Coverages as low as 5% of a monolayer were detected, in spite of the inherent infrared absorption low cross sections for hydrocarbon molecules. Adsorption geometries, determined by using the surface selection rule obeyed by RAIRS, indicate that a sudden molecular rearrangement occurs in all cases around half saturation: at low coverages the molecules tend to chemisorb with the molecular axis close to parallel to the surface, but after exposures above 3 L they rearrange collectively to stand in a perpendicular geometry.