Adlayer geometry and structural effects in the CO/Ni(110) system

The formation of ordered adlayers of CO on Ni(110) and the correlation between structure and adsorption energy, sticking coefficient and adsorbate induced change of work function was investigated. LEED, TDS and work function measurements served to monitor adsorption and desorption. Models are presented for the structures formed at intermediate coverages (0.5 < θ < 0.85) - identified as a c(8×2) and a c(4×2) structure - and the (2×1) formed close to saturation: The CO molecules are adsorbed on the Ni rows in the [110] direction, their separation is dominated by short range COCO repulsions rather than by the NiCO interaction. The repulsions in the [001] direction lead only to the formation of structures with staggered configurations. In the first two structures formed only below room temperature the CO stands upright and the repulsion is weak, leading to considerable disorder (antiphase domains) and a streaky LEED pattern. In the (2×1) structure which does not thermally disorder in the experimental temperature range, the high density of the adlayer results in a lateral tilt of the CO, and subsequently also to good correlation in the [001] direction. The repulsions become evident in TDS as a low temperature shoulder at the main peak (c(8×2) and c(4×2) structure) or as a distinct extra peak at 330 K ((2×1) structure). The adsorption kinetics can be modelled by a first order precursor model (K = 0.95). The work function almost linearly increases with coverage to 1500 mV at saturation. Both quantities are not noticeably affected by the degree of order in the adlayer.