PHOTOCHEMICAL DECOMPOSITION AT COLLOID SURFACES

The kinetics of the photodecomposition of 2-pyrazinecarboxylic acid adsorbed on colloidal silver is investigated using SERS as a probe. Analysis of the rate of formation of surface carbon as a function of laser fluence and wavelength indicates that the rate-determining step in the photoprocess involves two-photon excitation of the arene to a pi* excited state. At high laser fluences, both the SERS signal and the apparent photodecomposition rate appear to decrease with time. This is ascribed to thermal fragmentation of the colloidal aggregates responsible for SERS. The energy barrier to aggregate fragmentation is found to be approximately 15 kcal mol-1. Photodecomposition is observed only with those substituted benzenes that bind with the ring perpendicular to the silver surface. This is explained in terms of the direction of the transition dipole describing the two-photon process and the relative magnitudes of electromagnetic field components tangential and normal to the metal surface.