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Abstract

The photochemistry of chemisorbed CH₃Br on Pt{111} has been investigated using high resolution electron energy loss spectroscopy (HREELS) and thermal desorption. The primary photon‐induced reaction involves the cleavage of the C–Br bond, giving rise to chemisorbed CH₃ and Br, both of which can be identified in HREELS. From the angular dependence of the loss peaks, the symmetry of the CH₃ surface complex is shown to be C_3v. HBr can also be identified in subsequent thermal desorption. Experiments performed directly with HBr on Pt{111} indicate that molecular HBr adsorbs dissociatively on this surface. This result, in combination with observations of the C–H vibrational mode as a function of temperature, shows that the production of HBr arises from a secondary surface reaction between Br and CH_x fragments. Based on the wavelength dependence of the fragmentation cross section and the photoemission spectrum of adsorbed CH₃Br the primary photon‐induced reaction to a charge transfer excitation is ascribed.