The structure and the low-energy vibrational dynamics of CO ~d NO chemisorbed on a Rh(UI) surface have been investigated by means of He-atom scatt~ring (HAS). For a dilute phase of isolated CO molecules a characteristic frequency of h(J)=5.7 meV is observed which shifts to 5.45 meV for the (2/3 *2/3 )R30° phase (0= 1/3). Based on isotope exchange measurements this mode is assigned to the parallel frustrated translational mode (T-mode) of CO at on-top sites. For the (2 * 2)3CO saturation structure a further molecular vibrational mode with a frequency of h(J)= 11.7 me Vis obtained and is assigned to the T -mode of CO adsorbed at hollow sites which are predominantly populated in this phase. The He-atom diffraction patterns indicate a successive appearance of various NO superstruc;tures upon increasing NO exposure including a low coverage c(4 * 2), an intermediate (3 * 3) and finally the (2 * 2)3NO saturation structure. The corresponding inelastic HAS measurements reveal again two characteristic molecular vibrational modes at 7.5 and 11.5 me V which are identified as the T -modes of NO adsorbed at on-top and hollow sites, respectively, Moreover, for the NO saturation phase two additional phonon modes appear at energies below 14 me V.. Based on a lattice dynamical analysis of their dispersion curves they are identified as a back-folded Rayleigh mode and a further perpendicular polarized phonon mQde caused by the reduced Brillouin zone of the NO adlayer. The different T -mode frequencies for CO or NO at on-top and hollow sites suggest that this mode is a sensitive signature of adsorption sites rather than the commonly used internal stretch mode which had led to wrong adsorption site assignments.
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