THE PHOTON-INDUCED REACTIONS OF CHEMISORBED CH3BR ON PT(111)

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 C3-upsilon. 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.