Surface dynamics using pulsed electron beams

Line of sight time of flight has been used to measure the translational energy of ethene formed by dissociative electron attachment of adsorbed dihalocarbons (XCH2CH2Y) on Cu(111) at 100 K. A pulsed electron beam was used to generate low energy secondary electrons at the surface, which were the active agents in initiating the decomposition,XCH2CH2Y(ads)+e−→XCH2CH2(ads)+Y(chem)+e−,XCH2CH2(ads)→C2H4(g)+X(chem). 2-dichloroethane (DCE) adsorbed with a spacer layer of chemisorbed chlorine between it and the Cu(111)surface, the TOF data could be fitted using three Maxwell distributions with fast (960 K), slow (180 K) and diffusion (‘20 K’) components. For a single monolayer of DCE adsorbed directly on the clean Cu(111) surface the fast (1230 K) and slow (225 K) components increased in temperature, indicating that the copper surface was affecting the reaction. For 1-bromo-2-chloroethane the results were the same, consistent with both molecules dissociating via a common intermediate, ClC2H4(ads). For 1,2-dibromoethane the intermediate is different, BrC2H4, and decomposition of a monolayer of this molecule on clean Cu(111) exhibited fast (1850 K) and slow (270 K) components, both higher than the corresponding temperatures for DCE. The dynamics of these reactions and the origin of the two Maxwell distributions are discussed in terms of the energy available within the radical, and from the formation of the chemisorbed halogen when the radical dissociates.