STM and chemistry: A qualitative molecular orbital understanding of the image of CO oil a Pt surface

Theoretical calculations of STM images for isolated carbon monoxide on Pt(III) have been performed. These simulations are directly comparable to experimental data from Eigler's group. They also confirm a strong site dependence of the STM molecular contrast and assign the two experimental STM forms of CO with two distinct binding sites (the top and the bridge sites) which have already been a el-characterised by other surface techniques. Moreover, the approach used here allows a detailed understanding of the electronic origin of the STM molecular pattern. Hence, we demonstrate that the CO STM pattern results from the superposition, with interference effects, of two current contributions. The direct through space tunneling process between the tip and the metal surface is decreased by the presence of the adsorbate molecule, but this depression in the current is more or less compensated, depending on the binding site, by the tunnel current mediated by the molecular levels of the adsorbate. The frontier a orbitals of CO have a small contribution to the current and the sigma framework has the major influence with the 5 sigma lone pair (HOMO of CO) and also the low-lying 3 sigma. These two MO contributions, however, show a destructive interference effect, and this, together with the absence of a current, is responsible for the weak amplitude of the CO STM image.