A model for the imaging of adsorbates on metal electrodes by the scanning tunneling microscope

We consider the interaction of the metal tip of a scanning tunneling microscope with a metal electrode and a simple adsorbate. The tunneling current has three contributions: direct electron exchange between the substrate and the tip, indirect exchange through the adsorbate, and an interference term. From a simple model Hamiltonian we obtain explicit expressions for these three terms, which indicate the dependence of the tunneling current on the electronic interactions and on the coupling to the solvent.