CO/Rh(111): Vibrational frequency shifts and lateral interactions in adsorbate layers

Citation
R. Linke et al., CO/Rh(111): Vibrational frequency shifts and lateral interactions in adsorbate layers, J CHEM PHYS, 115(17), 2001, pp. 8209-8216
Citations number
37
Categorie Soggetti
Physical Chemistry/Chemical Physics
Journal title
JOURNAL OF CHEMICAL PHYSICS
ISSN journal
00219606 → ACNP
Volume
115
Issue
17
Year of publication
2001
Pages
8209 - 8216
Database
ISI
SICI code
0021-9606(20011101)115:17<8209:CVFSAL>2.0.ZU;2-S
Abstract
High resolution electron energy loss spectroscopy (HREELS), low-energy elec tron diffraction (LEED), and thermal desorption spectroscopy (TDS) were use d to study lateral interactions in the adsorbate layer of the CO/Rh(111) sy stem. The vibrational spectra show that CO adsorbs exclusively on top at lo w coverage. At about half a monolayer a second adsorption site, the threefo ld hollow site, becomes occupied as well. A steady shift to higher frequenc ies of the internal C-O vibrations is observed over the whole coverage rang e. 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 adsorpti on site the M-CO vibrations experience a shift to lower frequencies. The po pulation of the second site is also accompanied by the development of a low temperature shoulder in the TD spectra, indicating an increasingly repulsi ve interaction in the adsorbed CO layer. Vibrational spectra of isotopic mi xtures of (CO)-C-12 and (CO)-C-13 were used to assess the origin of the obs erved frequency shifts. They confirm that frequency shifts of the C-O stret ching vibration at total CO coverage of 0.33 ML in the (root 3x root3)R30 d egrees structure arise purely from dipole-dipole coupling. Dilution of an i sotopic species effectively suppresses frequency shifts arising from dipole -dipole coupling. Therefore, experiments with a small amount of (CO)-C-13 a s a tracer to monitor the frequency shifts in the (CO)-C-12 adlayer were ca rried out over the entire coverage range of (CO)-C-12. The results demonstr ate 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 illust rate that HREELS in combination with isotopic dilution is a powerful tool i n the assessment of lateral interactions between adsorbed molecules. (C) 20 01 American Institute of Physics.