NO structures adsorbed on Rh(111): Theoretical approach to high-coverage STM images

Theoretical modeling of scanning tunneling microscopy (STM) measurements is used for the interpretation of images of nitrogen monoxide on Rh(111) surfaces in order to gain insight into the factors which control the contrast of an STM image, especially in the case of high coverage overlayers. Topographic images of NO/Rh(111) for different coverages and adsorption positions were calculated. These results were used to analyze the experimental images obtained for the p(2x2)-3NO and p(3x3)-7NO high coverage structures. The theoretical calculations confirm that not all NO molecules present on the surface can be observed experimentally, the image being dominated by the contribution of top NO molecules in the adlayer. In addition, the calculations reveal that destructive interference effects between molecular contributions in the tunnel current play a decisive role for the different contrast of the two high coverage structures. A general discussion of why and how the differences in the adsorbate surface configuration reflect the experimental STM images is given.