PROMOTION BY ALKALI-METALS - A THEORETICAL-ANALYSIS OF THE VIBRATIONAL SHIFT OF CO COADSORBED WITH K ON CU(100)

By means of ab initio cluster model wavefunctions we have analyzed the electronic and electrostatic mechanisms which determine the very large negative shift of the CO vibrational frequency, omega(e), when CO is coadsorbed with alkali metal atoms on metal surfaces. The clusters considered, CU32/K2/CO and CU12/K2/CO, model K and CO coadsorption on Cu(100) at various K-CO distances. In order to explain the observed large vibrational red-shifts, of the order of 600 cm-1 and more, short K-CO distances, < 3 angstrom, must be considered. For larger K-CO distances the omega shift is small, < 200 cm-1, and almost entirely due to electrostatic effects. In fact, on a metal surface the adsorbed K atoms become positively charged, a mechanism which is reinforced when the CO molecules are coadsorbed. The interaction between the resulting electric field and the CO dipole lowers the CO omega(e). However, the large shift found for short K-CO distances has a dominantly electronic origin. We show unambiguously that the chemical mechanism which determines the large negative shift is not the direct charge transfer from the K 4s orbital to the empty levels of CO, but rather the increased back donation from the Cu conduction band electrons. These latter are strongly polarized toward CO because of the presence of the K ions on the surface and can overlap more efficiently with the CO accepting orbitals.