Pyrazine and triazine are found to photofragment on rough silver surfaces to form graphitic carbon as a major product. The ratehyphen;determining step is found to be biphotonic. The enhanced photochemistry is shown to take place most efficiently directly at the metal surface rather than a short distance above the surface as would be expected on the basis of a competition between nonradiative relaxation of the excited molecule and excitation by the surfacehyphen;enhanced electric field. Moreover, it was found possible to increase the photolysis rate further by overcoating the adsorbed molecules with an appropriate number of layers either of the photoreagent itself or of another molecule, tuning thereby the localized surface plasmon of the rough surface into resonance with the exciting laser. We suggest that in a previous report lsqb;Goncher, Parsons, and Harris, J. Phys. Chem.88, 4200 (1984)rsqb; of enhanced photochemistry at rough surfaces the effect of displacing the photoreagent from the surface using spacer layers was in fact to tune the photochemical rate by the same mechanism. The graphitic photoproduct is speculated to arise from a multiphoton process occuring subsequent to the initial twohyphen;photon step, and the apparent absence of nonradiative decay is ascribed to intersystem crossing to a triplet manifold from which the nonradiative decay rate is substantially reduced. These last proposals are presented tentatively and other possibilities are discussed.
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