ADSORBATE PHOTOCHEMISTRY ON A COLLOID SURFACE - PHTHALAZINE ON SILVER

Phthalazine adsorbed on colloidal silver surfaces is found to convert photochemically to a product in which the N=N bond of the molecule lik ely breaks to form an adsorbed species resembling an ortho-substituted benzene. The photochemical kinetics was studied using a simple flow c ell. The photochemical rate constant was found to be large in the visi ble region of the spectrum, increasing toward the blue. We show, incid entally, that SERS spectra of phthalazine reported previously by us an d by others were heavily contaminated by the spectral features of the photoproduct. Hence previous explanations of the unusual excitation wa velength and coverage dependence are incorrect. The photochemical reac tion is found to be a one-photon process; hence, the large absorption cross section in the visible is likely due to a metal to molecule char ge transfer transition. (Solution-phase phthalazine is transparent in the visible.) It is likely that a significant number of published SERS spectra of other molecules contain spectral features due to photoprod ucts. By using dynamic methods such as that described, one can avoid t hese complications.