Interaction of pyridine with photoelectrons emitted at the silver/aqueous solution interface

Abstract

Pyridine added to a solution containing scavengers of photoelectrons emitted from silver (N 2O, CO 2) causes a strong decrease in the measured photocurrent or even its complete suppression for concentrations of pyridine exceeding 0.1 M. Such behaviour, which remains in apparent contradiction to the effect of deposited pyridine on photoemission from silver in vacuum, is due to the fact that pyridine itself acts as an acceptor of the emitted photoelectrons. As the anion radical formed undergoes reoxidation (back electron transfer) at the silver surface, this process leads to a decrease in the net photocurrent which is dependent on the concentration ratio of the two competing acceptors (i.e. pyridine and N 2O). The observation that pyridine (adsorbed on silver and in solution) sustains a reversible electron transfer, along with the previous observation that photoemission from a roughened Ag electrode into an aqueous solution extends to at least 650 nm, provides a possible interpretation of the surface-enhanced Raman spectroscopy effect for pyridine, which involves both electromagnetic and charge-transfer mechanisms.