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

Abstract

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.