Photochemistry of adsorbed molecules. XI. Charge-transfer photodissociation and photoreaction in chloromethanes on Ag(111)

Abstract

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 laserirradiation of the chloromethane covered metal surface gave direct evidence for CTphotodissociation RCl/Ag(111)+hν→(RCl−)‡/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 (σ g DA). 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, σS/PRXN CT 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 σ g tot. 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)+hν→(RCl−)‡→R/Cl–Ag(111). This CT photoreaction was shown to occur with a photon energy ∼1 eV below the work function. Hot electrons, rather than free electrons, were the major agent for inducing photoreaction at all wavelengths.