LOW-ENERGY ELECTRON-INDUCED CHEMISTRY OF ETHYLENE ON CLEAN AND CL-COVERED AG(111)

The chemistry, induced by 50-eV electrons, of ethylene (C2H4) on clean and Cl- and D-covered Ag(111) has been studied. In the absence of electron irradiation, C2H4 is weakly pi-bonded and desorbs, with no thermal decomposition, below 170 K. Consistent with pi-donation and pi-polarization, one monolayer of ethylene lowers the work function by 0.38 eV. Evidence is presented that adsorbed C2H4, exposed to low doses of electrons, is selectively decomposed to adsorbed H and vinyl (C2H3). The latter leads exclusively to 1,3-butadiene (C4H6) in subsequent temperature-programmed desorption (TPD). Besides C4H6, higher doses of electrons lead to acetylene (C2H2), precursors to butene and tiny amounts of ethane, but no C1 or C3 products. In the presence of Cl, the parent ethylene is stabilized and the amount Of C4H6 produced in TPD is enhanced. For ethylene and coadsorbed D, there is, in the absence of electron irradiation, no reaction. With electron irradiation, isotopically labeled dihydrogen and partially deuterated ethylene and ethane form, but there are no C1, C3, or C4 products. We propose that bond-specific decomposition, e.g., C-H bond cleavage, to form vinyl occurs when the weakly bound ethylene is ionized by low-energy electrons. That further C-H bond cleavage occurs with much lower cross section and that there is no C-C bond cleavage may result from relatively effective quenching of excited ionic states of the strongly chemisorbed primary product, C2H3.