Theory of the local tunneling spectrum of a vibrating adsorbate

We present a model describing the essentials of tunneling between the tip of a scanning tunneling microscope (STM) and an electronic resonance on an isolated adsorbed molecule. Conditions for observing enhanced inelastic tunneling and its experimental manifestations are identified in terms of the energy and width of the resonance and of its coupling to a particular molecular vibration. For a broad resonance, this occurs if it is partially filled; we then predict a conductance decrease as large as 10% at the inelastic threshold for typical chemisorption systems. For a resonance weakly hybridized with the underlying electrode, strong multiphonon excitation is possible, although as a rule it will not be readily apparent. Relevant systems of interest for local spectroscopy with future STM's are discussed. © 1988, American Vacuum Society. All rights reserved.