PHOTOCHEMISTRY OF ADSORBED MOLECULES .11. CHARGE-TRANSFER PHOTODISSOCIATION AND PHOTOREACTION IN CHLOROMETHANES ON AG(111)

Photoinduced charge-transfer (CT) dissociation of adsorbates is reported in this paper for a series of chloromethanes (RCl) adsorbed on Ag (111). The chloromethanes were CCl4, CHCl3, CH2Cl2, CH3Cl, and CCl3Br. The observation that Cl- ions were emitted following UV laser irradiation of the chloromethane covered metal surface gave direct evidence for CT photodissociation RCl/Ag(111) + hv --> (RCl-)double dagger/Ag(111) --> Cl- + R/Ag, in which a photoexcited substrate electron caused dissociation of RCl. The yield of Cl- emission for varying R decreased in a manner similar to the gas-phase dissociative attachment cross sections at low electron energy ( < 1 eV) for RCl (sigma(DA)g). The cross section for Cl- emission as a function of RCl coverage showed a sharp maximum at the completion of the first molecular layer of CCl4, CHCl3, CH2Cl2, and CCl3Br. This enhanced cross section was ascribed to exothermicity of a photoreaction of R with the metal surface which imparted translational energy to Cl- permitting it to escape from its image potential. The cross section, sigma(S)CT/PRXN for photoinduced surface reaction yielding Cl-Ag (111) at the RCl covered surface was measured for CCl4, CHCl3, CH2Cl2, and CH3Cl. The yield of Cl-Ag (111) for varying R was found to change in a manner similar to the total low energy electron capture cross section in the gas phase sigma(tot)g. The yield decreased only slightly with increasing laser wavelength in striking contrast to the direct photolysis cross section. A CT mechanism was proposed in which a temporary negative ion formed by photoinduced CT reacted with the metal surface RCl/Ag (111) + hv --> (RCl-)double dagger --> R/Cl-Ag(111). This CT photoreaction was shown to occur with a photon energy approximately 1 eV below the work function. Hot electrons, rather than free electrons, were the major agent for inducing photoreaction at all wavelengths.