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-mu-J cm-2 of laser radiation with photon energies in excess of a threshold h-nu-0 almost-equal-to 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) + h-nu --> CCl4-/Ag(111) --> Cl-(g) + CCl3/Ag(111). In part 2 the cross-section, sigma(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, DELTA-h-nu-0, 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.