Surface Science 271 (1992) 452-467 North-Holland surface science ....... Low energy electron induced chemistry: C2HsC1 on Ag(111) X.-L. Zhou, P.M. Blass 1, B.E. Koel 2 and J.M. White * Department of Chemistry and Biochemistry, Univer~i'y of Texas, Austin, TX 78712, USA Received 27 September 1991; accepted for publication 24 January 1992 The chemistry, induced by 50 eV electrons, of CzHsCi on clean, CI- and D-covered Ag(lll) at 100 K has been studied using TPD, XPS, UPS and A~b. For monolayer (ML) coverage, the electron induced dissociation (EID) cross section has an upper limit of (6 ___1)x 10 -16 cm 2. For coverages up to 1 ML, CzH s radicals and C2H 6 and CzH 4 molecules, but no H z or Ci-containing species, desorb during electron exposure. At the surface, H and CI atoms, CzH,, (x = 3-5) radicals, and CzH 4, C4H6, C4H 8 and C4H m 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, C2H 4, C4H 6, C4H 8 and C4Hto, desorb, whereas H atoms and CzH x (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 Cl's or C3's. The only other TPD product is AgCI (750 K) and, unless the electron dose is very high, TPD to 950 K leaves a clean surface. In the presence of coadsorbed CI, the TPD production of saturated hydrocarbons is suppressed and the desorption of olefins, CzH4, C4H 6 and C4H 8, shifts to a lower temperature. When atomic D is preadsorbed, C-D bonds form both during irradiation and during TPD, producing reaction-limited C2HzD2, C2H4D 2 and C2HsD , and desorption-limited C4HsD 2 and C4HgD. Multilayers are different. AgC! desorption is sharply suppressed above 2 ML and not detected above 4 ML initial C2HsC! coverage. During electron irradiation, H2, C2H4, C2H5, C2H 6, C4H 8, C4Hlo, CI, HCI and C2HsCI desorb. The post-EID TPD products below 350 K are desorption-limited HCi, H2, C2H 4, C4H 6, C4H 8 and C4HIo, and reaction-limited C2H 6 and C4H 6. At higher temperatures, HCI and H,, 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 yCI z (x > 2) species. I. Introduction In a companion paper [1], we reported low energy (50 eV) electron induced dissociation (EID) of methyl chlor!de (CH3CI) adsorbed on clean, CI- and D-coveled Ag(lll) at 100 K. We found that EID of both submonolayer and multi- layer CH3CI on Ag(lll) occurs very efficiently. EID results in both retention and desorption of products. For 1 IVlL, electron-driven desorption is dominated by CH 3 radicals; CH4, C2H 5 and C2H 6, but no H2, are also observed. During TVrn H2 ' ~u ,'~ ~ ,,, l, L,, ~x~4' ~2H4 ' 1-16, 6, L- 2 C3H C3H s, Present address: Department of Chemistry, University of Toronto, Ontario, Canada M5S 1A1. 2 Department of Chemistry, University of Southern Califor- nia, Los Angeles, CA 90089, USA. * To whom correspondence should be addressed. C4H s and C4HI0 desorb below 350 K and AgC! above 700 K. After TPD to 950 K, surface carbon is found only for very high electron fluence EF > 3.37 × 1016 e-. For multiiayers, electron-driven desorption includes, in addition to the species noted for 1 ML, H 2, CI, HCI and CH3CI. Below 350 K, the only new TPD product is HCI. How- ever, above 400 K new H 2 and HCI TPD peaks appear and surface carbon is left after TPD. In this paper, we report a similar study of ethyl chloride. As reported earlier [2], C2HsCI adsorbs molecularly at 100 K and desorbs at 124 K (multi- layer) and i54 K (monoiayer) with no detectable thermal decomposition. The EID orocess for C2HsCI, while distinctive, does show similarities to that for CH3CI. For 1 ML C2HsCI on clean Ag(lll), the EID cross section equals that for CH3CI and is no larger than (6 _+ 1) × 10 -~6 cm z. C 2 alkyl fragments and C 2 hydrocarbons, but no 0()39-6028/92/$05.00 ~.~ 1992 - Elsevier Science Publishers B.V. All rights reserved