A high resolution electron energy loss spectroscopy study of the Fermi resonance of CO2 adsorbed on a Ag(110)/CO3 layer

The adsorption of CO2 on a Ag(110) surface precovered with a saturated CO3 layer has been studied by high resolution electron energy loss spectroscopy (HREELS). A weakly bound state of CO2 is stabilized by the presence of car bonate. In addition to the losses of the bend (658 cm(-1)) and the antisymm etric stretch (2343 cm(-1)) fundamental modes, the presence of two peaks in the frequency region of the symmetric stretch mode have been observed. Suc h a loss pair is characteristic for the existence of a Fermi resonance, bet ween the overtone of the bend and the symmetric stretch vibration, which ha d been studied in detail for the gas phase CO2 molecule. By use of three CO 2 isotopomers the existence of the Fermi resonance in the adsorbed state ha s been verified and the Fermi constant and the energy of the unperturbed vi brations have been determined from the isotopic shifts. The character of th e CO2 molecule is almost unaffected by the bonding and it adsorbs in a tilt ed geometry. (C) 1999 American Institute of Physics. [S0021-9606(99)71527-8 ].