Interference between competing tunneling channels and chemical resolution of STM

In a recent scanning tunneling microscopy (STM) experiment [Appl. Phys. Lett. 71 (1997) 213], CO molecules have been distinguished from another adsorbate species on Cu(111) surface using a controlled transfer of a CO molecule to the microscope tip. With a clean tip, both adsorbates are seen as dark depressions in the STM image. Functionalizing the tip with a CO molecule inverts, the contrast of the adsorbed CO, but the images of the other species remain dark. In this Letter, we calculate STM images of CO for both the tip conditions, and reproduce the contrast inversion. Interference effects between different tunneling channels explain this behaviour: the contrast inversion takes place because the currents through both the adsorbate and the tip molecule are of the same order of magnitude as the current through vacuum, but they are in an opposite phase to through vacuum tunneling. (C) 2004 Elsevier B.V. All rights reserved.