High resolution electron energy loss vibrational studies of CO coordination to the (1010) surface of ZnO

High Resolution Electron Energy Loss Spectroscopy (HREELS) has been used to study the interaction of CO with the (1010) ZnO surface. The results presented represent the first successful effort to use HREELS to measure the vibrations of CO on any single crystal metal oxide surface and serve as a complement to our earlier photoelectron spectroscopic studies of the CO/ZnO system. Observation of the intraligand C-O stretching mode (273 meV, 2202 cm/sup -1/) and its first overtone (539 meV, 4348 cm/sup -1/) verify that the CO molecule adsorbed at low coverages under ultrahigh-vacuum (UHV) conditions on (1010) is indeed the same '' high frequency'' CO observed on ZnO powders. In addition, observation of the Zn-C metal-ligand stretch (31 meV, 250 cm/sup -1/) enables application of a normal mode calculation which indicates that the majority of the increased CO stretching frequency is due to an increase in the CO force constant and not just due to mechanical coupling to the surface. These results are discussed in light of the mechanism of methanol synthesis on ZnO.