Low energy electron induced chemistry: C₂H₅Cl on Ag(111)

The chemistry, induced by 50 eV electrons, of C₂H₅Cl on clean, Cl- and D-covered Ag(111) at 100 K has been studied using TPD, XPS, UPS and δφ. For monolayer (ML) coverage, the electron induced dissociation (EID) cross section has an upper limit of (6±1)×10^-16 cm². For coverages up to 1 ML, C₂H₅ radicals and C₂H₆ and C₂H₄ molecules, but no H₂ or Cl- containing species, desorb during electron exposure. At the surface, H and Cl atoms, C₂H_x (x=3-5) radicals, and C₂H₄, C₄H₆, C₄H₈ and C₄H₁₀ molecules are synthesized and retained. Their relative concentrations depend on the electron exposure. There is no evidence for electron-driven C-C bond breaking, except for very high electron doses. In post-EID TPD, synthesized molecules, C₂H₄, C₄H₆, C₄H₈ and C₄H₁₀, desorb, whereas H atoms and C₂H_x (x=3-5) radicals recombine to produce H₂, C₂H₆, 1,3-butadiene, butene and n-butane, all below 400 K. There are no C₁'s or C₃'s. The only other TPD product is AfCl (750 K) and unless the electron dose is very high, TPD to 950 K leaves a clean surface. In the presence of coadsorbed Cl, the TPD production of saturated hydrocarbons is suppressed and the desorption of olefins, C₂H₄, C₄H₆ and C₄H₈, shifts to a lower temperature. When atomic D is preadsorbed, C-D bonds form both during irradiation and during TPD, producing reaction-limited C₂H₂D₂, C₂H₄D₂ and C₂H₅D, and desorption-limited C₄H₈D₂ and C₄H₉D. Multilayers are different. AgCl desorption is sharply suppressed above 2 ML and not detected above 4 ML initial C₂H₅Cl coverage. During electron irradiation, H₂, C₂H₄, C₂H₅, C₂H₆, C₄H₈, C₄H₁₀, Cl, HCl and C₂H₅Cl desorb. The post-EID TPD products below 350 K are desorption-limited HCl, H₂ C₂H₄, C₄H₆, C₄H₈ and C₄H₁₀, and reaction-limited C₂H₆ and C₄H₆. At higher temperatures, HCl and H₂ not AgCl, desorb and surface carbon is retained after TPD to 950 K. These are ascribed to decomposition of a hydrogen-deficient strongly bound C_xH_yCl_z(x > 2) species.