ON THE COADSORPTION OF CO AND ALKALI ATOMS AT TRANSITION-METAL SURFACES - A LCGTO-LDF CLUSTER STUDY

The coadsorption system consisting of carbon monoxide and alkali atoms on transition metal surfaces has been studied theoretically by first principles electronic structure calculations. Special attention has be en paid to the reduction of the CO stretching frequency in the presenc e of coadsorbed alkali atoms, an effect that indicates a weakening of the CO bond. To investigate CO coadsorbed with alkali atoms, a hierarc hy of models has been constructed based on the clusters CO, Ni2CO and Ni(n)(CO)K2 (n = 8, 14). These models permit the separate and joint st udy of several interaction mechanisms and the evaluation of their rela tive contributions. In the smaller clusters, the electric field of the surface dipole layer is modeled by point charges. The electronic stru cture calculations have been carried out using the self-consistent lin ear combination of Gaussian-type orbitals local density functional (LC GTO-LDF) method. The calculated values for the reduction of the CO str etching frequency and the shifts of core and valence levels of CO and alkali atoms are in good agreement with experimental data. A comprehen sive model for the CO/alkali coadsorption on transition metal surfaces emerges which allows the explanation of a variety of experimental fin dings. This model is corroborated by a detailed analysis of the electr onic structure of the coadsorption system. In quantitative agreement w ith experimental data, about half of the reduction of the CO vibration al frequency has to be attributed to substrate-induced backbonding int o the antibonding 2pi orbital of CO as first suggested by Blyholder. The alkali-induced additional frequency shift is dominated by the elec trostatic interaction between CO and the surface dipole layer which is modified by alkali coadsorbates. To a smaller extent, this frequency shift is also affected by enhanced backbonding due to a raised Fermi e nergy of the substrate. Direct orbital interactions between the coadso rbates were found to be negligible. Ionic models ot rehybridization mo dels are not supported by the present study.