CH 3CN⧸alkali coadsorption at metal surfaces: model studies for alkali induced adsorbate geometry changes

Abstract

From near edge X-ray absorption fine structure (NEXAFS) measurements of the coadsorption of acetonitrile (CH 3CN) and K on the Au(100) surface it has been concluded that the C-N bond length in adsorbed CH 3CN is shortened by 0.13 bohr due to the interaction with the alkali coadsorbate. This conclusion was based on a “bond-length-with-a-ruler” interpretation of respective shifts of the σ CN ∗ shape resonance. In order to understand the details of the coadsorptive interaction and its consequences for molecular geometries ab initio Hartree-Fock cluster calculations are performed on CH 3CNK + model systems. Full geometry optimizations of the CH 3CN molecule with and without the coadsorbate confirm the decreased C-N bond length due to the coadsorbate interaction but the effect (0.04 bohr) is much smaller than suggested from experiment. Calculations on selected CH 3CN core-to-valence excitations (determining the NEXAFS absorption peaks) show that the presence of the K+ ion results in field induced shifts of the excitation energies which are larger than those due to geometry changes. This suggests for the CH 3CN , 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.