As previously noted, 2-photon spectra was taken at both θ = ¼ coverage 12 and θ = ½ coverage 11 for the LUMO, where the peak shifts from EF +3.6 eV to EF +3.35 eV. This provides another opportunity to benchmark the theory, which indeed predicts a similar coverage dependence (compare Figure to Figure ). Rather unexpectedly the peaks become sharper for all three geometries. Taking experiment and theory together we can conclude the following: (1) at low coverage, CO predominantly resides on the atomic site;