Excitation of vibrational modes of adsorbates with the scanning tunneling microscope: many-orbital theory

We derive theoretical expressions for taking into account inelastic tunneling effects in the current of a scanning tunneling microscope (STM), and relate them to the event rate of current-induced processes like desorption, rotation or dissociation of adsorbed molecules. In a realistic case, the current involves tunneling through many hybridized channels, and also several different modes can be excited simultaneously. Our scheme provides a tool to deal with such a general case. The theory presented allows us to predict vibrational-mode excitation of adsorbates, and calculate event rates. A model calculation is performed illustrating the capabilities of the calculational scheme, to obtain event rates as a function of the STM tip position ('inelastic image' of the adsorbate) and the bias. New features associated with the many-orbital character of the coupling are reported. (C) 1999 Elsevier Science B.V. All rights reserved.