Low-energy dynamics of CO and NO chemisorbed on Rh(111)

The structure and the low-energy vibrational dynamics of CO and NO chemisorbed on a Rh(111) surface have been investigated by means of He-atom scattering (HAS). For a dilute phase of isolated CO molecules a characteristic frequency of ℏω=5.7 meV is observed which shifts to 5.45 meV for the $(3×3)R30°$ phase (θ=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 ℏω=11.7 meV is 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 superstructures 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 meV 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 meV. 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 mode 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.