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 hav e been investigated with helium atom scattering (HAS). At low coverages (Th eta=0.03) NO forms a dilute phase of isolated molecules which have a charac teristic frequency of (h) over bar omega=8.0 meV. Based on isotope exchange measurements this mode is assigned to the parallel frustrated translationa l 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 mode s 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 s hown by the asymmetric helium diffraction peak intensities. A lattice dynam ical analysis identified the lower frequency mode as a back-folded substrat e surface phonon, emphasizing that the surface phonon Umklapp processes mus t be properly identified in order to assign the low energy adsorbate vibrat ions correctly. (C) 2000 American Institute of Physics. [S0021-9606(00)7011 7-6].