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 DELTA-phi. For monolayer (ML) coverage, the electron induced dissociation (EID) cross section has an upper limit of (6 +/- 1) x 10(-16) cm2. For coverages up to 1 ML, C2H5 radicals and C2H6 and C2H4 molecules, but no H-2 or Cl-containing species, desorb during electron exposure. At the surface, H and Cl atoms, C2Hx (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 C2Hx (x = 3-5) radicals recombine to produce H-2, C2H4, 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 AgCl (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, H-2, C2H4, C2H5, C2H6, C4H8, C4H10, Cl, HCl and C2H5Cl desorb. The post-EID TPD products below 350 K are desorption-limited HCl, H-2, C2H4, C4H6, C4H8 and C4H10, and reaction-limited C2H6 and C4H6. At higher temperatures, HCl and H-2, 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(x)H(y)Cl(z) (x > 2) species.