Ethylene adsorption on ZnO: CARS spectroscopy with optical waveguides

Abstract

Surface CARS spectroscopy using the planar optical waveguide geometry has been employed to study the adsorption of ethylene on a nonsingle crystal 0001 ZnO waveguide surface. The required CARS sensitivity was achieved by establishing an interference condition within a ZnO waveguide to reduce the contribution from the nonresonant susceptibility of the ZnO and to enhance the ratio of the adsorbate signal to the background signal. Under high vacuum conditions, spectra in the region of the C=C stretch provided evidence that the hydroxylated surface supported four sites for the absorption of ethylene. Physisorbed ethylene had a vibrational frequency of 1615 cm−1 and was readily desorbed. Chemisorbed species exhibited frequencies of 1602, 1590, and 1580 cm−1, and the sites are likely to be Zn2+ ions.