ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
High resolution electron energy loss spectroscopy (HREELS), low-energy electron diffraction (LEED), and thermal desorption spectroscopy (TDS) were used to study lateral interactions in the adsorbate layer of the CO/Rh(111) system. The vibrational spectra show that CO adsorbs exclusively on top at low coverage. At about half a monolayer a second adsorption site, the threefold hollow site, becomes occupied as well. A steady shift to higher frequencies of the internal C–O vibrations is observed over the whole coverage range. The frequency of the metal CO (M–CO) vibration in the on-top mode hardly shifts at low coverage. However, upon the emergence of the second adsorption site the M–CO vibrations experience a shift to lower frequencies. The population of the second site is also accompanied by the development of a low temperature shoulder in the TD spectra, indicating an increasingly repulsive interaction in the adsorbed CO layer. Vibrational spectra of isotopic mixtures of 12CO and 13CO were used to assess the origin of the observed frequency shifts. They confirm that frequency shifts of the C–O stretching vibration at total CO coverage of 0.33 ML in the ((square root of)3×(square root of)3)R30° structure arise purely from dipole–dipole coupling. Dilution of an isotopic species effectively suppresses frequency shifts arising from dipole–dipole coupling. Therefore, experiments with a small amount of 13CO as a tracer to monitor the frequency shifts in the 12CO adlayer were carried out over the entire coverage range of 12CO. The results demonstrate that dipole–dipole coupling causes the frequency shifts at low coverage (〈0.5 ML), whereas chemical effects set in at higher coverage (0.5–0.75 ML), connected with the population of the threefold sites. The results illustrate that HREELS in combination with isotopic dilution is a powerful tool in the assessment of lateral interactions between adsorbed molecules. © 2001 American Institute of Physics.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.1355767
