THE WAVELENGTH DEPENDENCE OF THE PHOTODISSOCIATION CROSS-SECTION OF CH3BR CHEMISORBED ON CU(111)

The photodissociation of methylbromide adsorbed on Cu{111} has been investigated as a function of the photon energy between 2.6 and 6.0 eV using a Xe arc lamp. The photodissociation products remaining on the surface, identified using HREELS, are CH3 (in a C3v configuration) and Br. In subsequent thermal desorption CH4, C2H4, H-2 and CuBr are detected. The cross-section curve as a function of the wavelength shows an onset at approximately 2.6 eV and a maximum of 1.2 X 10(-20) cm2 at 3.5 eV. The results are compared with those obtained on a Pt{111} surface and both sets of data are interpreted within the framework of a one-step charge transfer model involving electron excitation from the substrate d-band into the sigma * C-Br antibonding orbital.