Theoretical aspects of tunneling-current-induced bond excitation and breaking at surfaces

We have performed a density functional study of the electronic structure, images and vibrationally inelastic tunneling in the scanning tunneling microscope and vibrational damping by excitation of electron-hole pairs of CO chemisorbed on the (111) and (100) faces of Cu. We find that the 2 pi* molecular orbital of CO turns into a broad resonance with parameters that differ significantly from those suggested by inverse and two-photon photoemission measurements. The calculated vibrational damping rate for the internal stretch mode and relative changes in tunneling conductance across vibrational thresholds are in agreement with experiment. The non-adiabatic electron-vibration coupling is well described by the Newn-Anderson model for the 2 pi*-derived resonance whereas this model is not able to describe the non-adiabatic coupling between the tunneling electrons and the vibration. We believe that this model misses an important mechanism for vibrational excitation in tunneling that involves the change of tunneling amplitude by deformation of the tails of the one-electron wavefunctions with vibrational coordinate.