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

We have performed a density functional study of the electronic structure, i mages and vibrationally inelastic tunneling in the scanning tunneling micro scope and vibrational damping by excitation of electron-hole pairs of CO ch emisorbed on the (111) and (100) faces of Cu. We find that the 2 pi* molecu lar 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 stre tch mode and relative changes in tunneling conductance across vibrational t hresholds are in agreement with experiment. The non-adiabatic electron-vibr ation coupling is well described by the Newn-Anderson model for the 2 pi*-d erived resonance whereas this model is not able to describe the non-adiabat ic coupling between the tunneling electrons and the vibration. We believe t hat 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.