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

The structure and the low-energy vibrational dynamics of CO and NO chemisor bed on a Rh(111) surface have been investigated by means of He-atom scatter ing (HAS). For a dilute phase of isolated CO molecules a characteristic fre quency of (h) over bar omega =5.7 meV is observed which shifts to 5.45 meV for the (root 3x root3)R30 degrees phase (theta =1/3). Based on isotope exc hange measurements this mode is assigned to the parallel frustrated transla tional mode (T-mode) of CO at on-top sites. For the (2x2)3CO saturation str ucture a further molecular vibrational mode with a frequency of (h) over ba r omega =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-ato m diffraction patterns indicate a successive appearance of various NO super structures upon increasing NO exposure including a low coverage c(4x2), an intermediate (3x3) and finally the (2x2)3NO saturation structure. The corre sponding inelastic HAS measurements reveal again two characteristic molecul ar vibrational modes at 7.5 and 11.5 meV which are identified as the T-mode s of NO adsorbed at on-top and hollow sites, respectively. Moreover, for th e NO saturation phase two additional phonon modes appear at energies below 14 meV. Based on a lattice dynamical analysis of their dispersion curves th ey are identified as a back-folded Rayleigh mode and a further perpendicula r polarized phonon mode caused by the reduced Brillouin zone of the NO adla yer. The different T-mode frequencies for CO or NO at on-top and hollow sit es suggest that this mode is a sensitive signature of adsorption sites rath er than the commonly used internal stretch mode which had led to wrong adso rption site assignments. (C) 2001 American Institute of Physics.