Photochemistry of adsorbed molecules. Part 10.—Harpooning a fixed target: charge transfer from Ag or K substrates to halide adsorbates

In part 1 of this paper we report direct evidence for the process of charge-transfer photodissociation (CT/PDIS), namely the formation of negatively charged photofragments from a halide adsorbed on a metal substrate. The system was CCl4/Ag(111). The observation was that 1 µJ cm–2 of laser radiation with photon energies in excess of a threshold hν0≈ 5 eV, approximately equal to the work function of the substrate, gave rise to a readily detectable flux of Cl– leaving the surface, attributable to a harpooning process CCl4/Ag(111)+hν→ CCl–4/Ag(111)→ Cl–(g)+ CCl3/Ag(111). In part 2 the cross-section, σ(CT/PDIS), is reported over the range 193–700 nm incident wavelength for charge-transfer photodissociation of HI and HBr adsorbed on 2–3 monolayers of K deposited on Ag(110) or LiF(001). The observed shift in threshold photon energy, Δhν0, to higher energy in going from HI(ads) to HBr(ads) was in the direction expected for dissociative electron attachment (DA). There was evidence of a significant tunnelling regime for CT/PDIS of HI/Ag but not for HBr/Ag, a finding that can be understood in terms of DA.