Photochemistry of adsorbed molecules. XII. Photoinduced ion–molecule reactions at a metal surface for CH3X/RCl/Ag(111) (X=Br, I)

A photoinduced ion–molecule reaction is reported between superimposed molecular layers of alkyl halides on a metal substrate CH3X/RCl/Ag(111) (where X=Br or I and R=CCl3, CHCl2, or CH2Cl) to form CH3Cl(ad) (wavelengths 193, 248, and 350 nm). The reaction is mediated by charge-transfer (CT) photodissociation, in which photoelectrons from the metal surface transfer to the lower layer of adsorbate RCl to form RCl−. These negative ions then react with the upper layer CH3X in an ion–molecule reaction to form CH3Cl+X−. The yield of product CH3Cl is found to be enhanced at ∼1 ML of adsorbed CH3X (upper layer) due to a decrease in the local potential in the region of the adsorbate–adsorbate interface that enhances the probability of CT to the lower layer. In addition to lowering the local potential at the interface, the adsorbed CH3X also lowers the surface work function; as a result changes in the microscopic local potential correlate (via the CT reaction rate) with changes in the observed macroscopic work function. The yield of CH3Cl decreases at still higher CH3X coverage in the upper layer as the work function increases. The ion–molecule reaction gives evidence of being a concerted process in which the Cl− reacts as it separates from RCl− rather than following separation. The reagent RCl−, as in the surface reaction discussed in the previous paper, is formed by CT from ‘‘hot’’ electrons rather than free photoelectrons.