Theory of inelastic tunneling induced motion of adsorbates on metal surfaces

We describe a new way of how inelastic tunneling can induce motions of adsorbates on metal surfaces. The theory shows how the excitation of high frequency internal vibrational modes can induce rotational or translational motion of adsorbates. The probability for adsorbate rearrangements following the vibrational excitation of the high-frequency mode is calculated using a model which includes vibrational relaxation via electron-hole pair excitation. and the anharmonic coupling to the relevant reaction coordinate. The adsorbate rearrangement probability depends on the anharmonic coupling strength. the height of the potential barrier along the reaction coordinate, and the energy of the low-frequency mode. The theory explains why the excitation of the C-O stretch vibration can induce hopping of CO on Pd(110), but not on Cu(100). (C) 2002 Elsevier Science B.V. All rights reserved.