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.