Photochemical desorption of 4-vinylbenzoic acid adsorbed on silver colloid surfaces

Temporal changes in the surface-enhanced Raman scattering (SERS) of 4-vinylbenzoic acid (4vba) adsorbed on silver colloid surfaces were studied using a method in which the solution of the adsorbate-covered colloid solution flowed through a capillary at differing rates. In this way the photochemical effect of the illuminating laser was studied quantitatively. 4vba appears to exist on the colloid surface in two forms: one that photodesorbs under the influence of the laser and one that does not. An analysis of the SERS spectra suggests that the former adopts a surface geometry in which the benzene ring and carboxylate group are adsorbed flat on surface but where the vinyl group is not bonded directly to the surface. In the latter, all three groups are adsorbed on surface simultaneously: the benzene and vinyl groups are pi-bonded, while the carboxylate group is adsorbed through a nonbonding electron pair of one of its oxygens. The photodesorption rate constant was found to be proportional to laser power. Its value was found to be large in the visible region of the spectrum, increasing toward the blue; hence, the photodesorption mechanism is likely initiated by a metal to adsorbate charge-transfer absorption.