We performed ab initio plane-wave calculations for NO/Pt(111) using a slab model. The results show that at a low coverage of 0.25 ML, the fcc-hollow site is the most stable adsorption site. Our calculations at a higher coverage of 0.50 ML indicate that NO molecules prefer to be adsorbed at both fcc-hollow and on-top sites rather than only at hollow sites. This adsorption arrangement is consistent with a recent scanning tunneling microscopy experiment. We then calculated the peak intensities of the N-O stretching modes in vibrational spectra. It turned out that the peak corresponding to the fcc-hollow species becomes very small in the presence of the on-top species. This effect is not explained only in terms of the well-known intensity-transfer effect derived from vibrational couplings, but a short-range dipole-dipole interaction involving charge transfer between the substrate and adsorbates turns out to play a major role.
