ADSORPTION OF O, HCO2H AND C6H5CO2H ON CU(110) STUDIED USING REFLECTANCE ANISOTROPY - CHEMICAL AND STRUCTURAL INFLUENCES ON AN OPTICALLY-ACTIVE SURFACE RESONANCE

Real-time reflectance anisotropy (RA) transients have been recorded fr om a Cu(110) surface during exposure to O-2, O-2-HCO2H and C6H5CO2H, u sing a simple HeNe laser-based system operating at 1.96 eV polarised p arallel to first the [1 ($) over bar 10] and then the [100] directions on the crystal surface. The marked changes in the RA response observe d during the formation of the reconstructed overlayers is attributed t o the quenching of an allowed optical transition of the surface, as id entified using electron spectroscopy, by the process of adsorption and restructuring. However, the influence of new adsorbate-induced states on the RA response cannot be ruled out at present. The magnitude of t he observed changes in RA are comparable for the formation of the p(2 x 1)O and (3 x 1)O/HCO2H structures and are tentatively assigned to a combination of the rapid quenching of an allowed surface transition an d the possible modification of new adsorbate-induced states. For the f ormation of the c(8 x 2) benzoate overlayer the RA change is considera bly larger, which is believed to be due to a more thorough quenching o f the allowed surface transition than that which occurs for the other systems studied. For the formation of the initial stages of the (2 x 1 )O overlayer, in particular, the RA response would be consistent with simple Langmuir adsorption kinetics providing that a model in which th e RA response was directly proportional to coverage was applicable.