Adsorption of O, HCO2H and C6H5CO2H on Cu(110) studied using reflectance anisotropy. Chemical and structural influences on an optically active surface resonance

Abstract

Real-time reflectance anisotropy (RA) transients have been recorded from a cu(110) surface during exposure to O₂, O₂–HCO₂H and C₆H₅CO₂H, using a simple HeNe laser-based system operating at 1.96 eV polarised parallel to first the [110] and then the [100] directions on the crystal surface. The marked changes in the RA response observed during the formation of the reconstructed overlayers is attributed to the quenching of an allowed optical transition of the surface, as identified 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 the observed changes in RA are comparable for the formation of the p(2 × 1)O and (3 × 1)O/HCO₂H structures and are tentatively assigned to a combination of the rapid quenching of an allowed surface transition and the possible modification of new adsorbate-induced states. For the formation of the c(8 × 2) benzoate overlayer the RA change is considerably larger, which is believed to be due to a more thorough quenching of the allowed surface transition than that which occurs for the other systems studied. For the formation of the initial stages of the (2 × 1)O overlayer, in particular, the RA response would be consistent with simple Langmuir adsorption kinetics providing that a model in which the RA response was directly proportional to coverage was applicable.