A helium atom scattering study of the adsorption of NO on Pt(111)

The low frequency vibrational modes of NO adsorbed on a Pt(111) surface have been investigated with helium atom scattering (HAS). At low coverages (Theta=0.03) NO forms a dilute phase of isolated molecules which have a characteristic frequency of (h) over bar omega=8.0 meV. Based on isotope exchange measurements this mode is assigned to the parallel frustrated translational mode (T-mode) of NO at on top sites. At saturation a well ordered p(2x2) structure (Theta=0.25) appears, for which two different dispersionless modes with frequencies of (h) over bar omega=9.5 meV and 12.3 meV were observed. Isotope substitution revealed that the higher frequency mode corresponds to a vibration of the NO molecules in specific threefold hollow sites, as shown by the asymmetric helium diffraction peak intensities. A lattice dynamical analysis identified the lower frequency mode as a back-folded substrate surface phonon, emphasizing that the surface phonon Umklapp processes must be properly identified in order to assign the low energy adsorbate vibrations correctly. (C) 2000 American Institute of Physics. [S0021-9606(00)70117-6].