CH3CN ALKALI COADSORPTION AT METAL-SURFACES - MODEL STUDIES FOR ALKALI INDUCED ADSORBATE GEOMETRY CHANGES/

From near edge X-ray absorption fine structure (NEXAFS) measurements o f the coadsorption of acetonitrile (CH3CN) and K on the Au(100) surfac e it has been concluded that the C-N bond length in adsorbed CH3CN is shortened by 0.13 bohr due to the interaction with the alkali coadsorb ate. This conclusion was based on a ''bond-length-with-a-ruler'' inter pretation of respective shifts of the sigma(CN) shape resonance. In o rder to understand the details of the coadsorptive interaction and its consequences for molecular geometries ab initio Hartree-Fock cluster calculations are performed on CH3CNK+ model systems. Full geometry opt imizations of the CH3CN molecule with and without the coadsorbate conf irm the decreased C-N bond length due to the coadsorbate interaction b ut the effect (0.04 bohr) is much smaller than suggested from experime nt. Calculations on selected CH3CN core-to-valence excitations (determ ining the NEXAFS absorption peaks) show that the presence of the K+ io n results in field induced shifts of the excitation energies which are larger than those due to geometry changes. This suggests for the CH3C N,K/Au(100) system that the observed NEXAFS peak shifts are determined by electrostatic field effects in the coadsorbate system and may not reflect actual adsorbate geometry changes.