Coherent anti-Stokes Raman scattering of molecular oxygen adsorbed on a zinc oxide optical waveguide

Abstract

The vibrational spectrum of physically adsorbed molecular oxygen has never been observed on oxide surfaces. Using the planar optical waveguide geometry has made it possible to observe the CARS spectrum of this species on ZnO at an O/sub 2/ pressure of 1 atm. The required CARS sensitivity was achieved by establishing an interference condition within a ZnO waveguide to eliminate the contribution from the nonresonant susceptibility of the ZnO and by taking advantage of the short penetration depth of the guided waves into the region above the surface. The observed vibrational frequency as 1549 cm/sup -1/. The nature of the adsorption site is discussed in terms of interactions of O/sub 2/ with the ZnO, surface hydroxyl groups, and adsorbed water.