Direct observation of the symmetric bending vibrations (ν2) of NH3 physisorbed on a partially hydroxylated planar (0001) surface of ZnO

Article abstract of DOI:10.1016/0039-6028(93)90150-I

For the first time the symmetric bending vibrational frequencies (ν₂) of NH₃ physisorbed on a partially hydroxylated metal oxide surface have been obtained with the coherent anti-Stokes Raman spectroscopy (CARS) using the planar optical waveguide geometry. The CARS signal from the adsorbed NH₃ was enhanced by establishing an interference condition within the waveguide to reduce the CARS contribution from the oxide surface. The surface CARS spectra, in the investigated ν₂ spectral region, provided evidence that the partially hydroxylated ZnO surface supported several sites for the physisorption of NH₃. The physisorbed species had ν₂ frequencies around 950, 963, 975 and 1012 cm^-1 and were very quickly desorbed as soon as evacuation of the vacuum chamber began. In addition, the presence of NH₃ which is coordinated to Zn²⁺ sites on the surface was detected.