Photon- and electron-induced chemistry of chlorobenzene on Ag(111)

Abstract

The surface chemistry, induced by UV photons and low energy electrons, of chlorobenzene (C6H5Cl) on Ag(111) has been studied. C6H5Cl adsorbs molecularly on Ag(111) at 100 K and desorbs molecularly at 170 K (physisorbed) and 230–240 K (chemisorbed) with no thermal decomposition. The adsorption is accompanied by a surface work function change (ΔΦ) of −0.65 eV at one monolayer and −0.9 eV at multilayer coverages. Both UV photons and low energy electrons induce the decomposition of adsorbed C6H5Cl. The energy threshold for the decomposition is 3.5–3.8 eV for photons and ∼5 eV for electrons. For photons and <12 eV electrons, the decomposition involves only C–Cl bond cleavage, producing surface phenyl groups and Cl atoms. Phenyl groups recombine and desorb as biphenyl at 390–400 K during post-irradiation temperature programmed desorption (TPD). They do not dehydrogenate. However, for electron energies higher than ∼12 eV, the decomposition involves cleavage of C–H, C–Cl, and, probably, C–C bonds. In this case, post-irradiation TPD shows the desorptions of H2, HCl, AgCl, benzene, biphenyl, and C6H5Cl with C left on the surface. The cross section for the photodissociation is 3×1021 cm2 at 254 nm and decreases with increasing wavelength. The cross section for the electron-induced decomposition (EID) is 8×1017 cm2 (upper limit) at 50 eV and decreases with electron energy. The photodissociation on the surface is ascribed to direct photon induced n→σ* transition and/or photoexcited electron attachment processes. Compared to the gas phase, the photodissociation threshold is red shifted. This is qualitatively explained by direct excitation process assuming an effective stabilization of the excited C6H5Cl by Ag(111) (strong adsorbate–substrate interactions) and/or ‘‘hot’’ electrons. EID by <12 eV electrons probably involves dissociative electron attachment and electronic excitation processes; EID by >12 eV electrons also involves ionization processes.