A500

Surface Science 289 (1993) 30-38 North-Holland

Reaction of C H 4 with substitutional F e / N i ( l l l ) Hong Yang and Jerry L. Whitten Department of Chemistry, North Carolina State University, Raleigh, NC 27695-8204, USA Received 4 November 1992; accepted for publication 4 January 1993

The reaction of c n 4 with a substitutional Fe/Ni(111) surface is treated using a many-electron embedding theory, modelling the lattice as a 41-atom, three layer cluster. Ab initio valence orbital configuration interaction (multiple parent) calculations carried out on a local surface region permit an accurate description of bonding at the surface. The 3d orbitals are explicitly used for the Fe atom and six nearest neighbor Ni atoms in the local surface region. The calculated activation energy for CH 4 dissociation at an atop Fe site to produce CH 3 and H coadsorbed on the surface is 5.7 kcal/mol, compared with 16.7 kcal/mol on Ni(lll). The reaction of CH 4 ~ CH3(ads)+ H(ads) is predicted to be 4 kcal/mol exothermic on the Fe /N i ( l l l ) surface and the immediate dissociation products are CH 3 and H coadsorbed at across atom three-fold sites. Across bond dissociation is energetically very unfavorable. The bonding properties of coadsorbed CH 3 and H on the Fe /Ni ( l l l ) surface are similar to those on the Ni( l l l ) surface. Compared to the clean Ni(lll), the substitutional Fe /N i ( l l l ) surface is very effective in activating CH 4.

Surface Science 289 (1993) 39-46 North-Holland

Thermal and electron-induced behavior of d6-benzene-chromium-tricarbonyl adsorbed on Ag(111) R.D. Ramsier and J.T. Yates, Jr. Surface Science Center, Department of Chemistry, University of Pittsburgh, lhttsburgh, PA 15260, USA

Received 24 September 1992; accepted for publication 15 January 1993

Deuterated benzene-chromium-tricarbonyl (C6D6Cr(CO)3) adsorbed on Ag( l l l ) has been studied with electron-stimulated desorption, temperature-programmed desorption and Auger electron spectroscopy. CoD6Cr(CO) 3 adsorbs non-dissociatively at 90 K on this surface, reversibly desorbing near 300 K (monolayer) and 275 K (multilayers) without thermal decomposition. Low-energy (220 eV) electron bombardment of monolayer C6D6Cr(CO) 3 induces the decomposition of the parent molecules by breaking Cr-CO or Cr-C6D 6 bonds, and the production of surface-bound benzene-CO-Cr complexes. The total decomposition cross section for this process is 2.7 + 0.1 x 10-17 cm 2. These new surface complexes produced by electron impact thermally decompose predominantly near 325 K, liberating gas-phase CO and benzene and depositing Cr on the Ag(l l l ) surface.

Surface Science 289 (1993) 47-67 North-Holland

Quantitative analyses of RHEED patterns from MBE grown GaAs( 001) -2 x 4 surfaces Y. Ma a, S. Lordi a, P.K. Larsen u and J.A. E a d e s a "Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA b Philips Research Laboratories, P.O. Box 80000, 5600 JA Eindhoven, Netherlands Received 26 October 1992; accepted for publication 6 January 1993

A multislice formalism of Cowley and Moodie [Acta Crystallogr. 10 (1957) 609] with a recently developed edge-patching method has been applied to quantitative analyses of RHEED patterns from MBE grown GaAs(001)-2 x 4 surfaces. The analyses are based on the ordering of visually estimated spot intensities of several observed RHEED patterns from the GaA,K001)-2 x 4 surfaces. The surface structure is proved to be a dimerized vacant 2 × 4 reconstruction missing one dimer of every four along the [110] direction, which is consistent with previous STM observations. The relaxation of the top layer is found to be about ~ = + 8.0% (0.113 ,~). The results give an overall Debye-Waller factor of B = 0.35 /~2 and a crystal absorption of a = 0.1 for the conditions at which the observed RHEED patterns were acquired. The effects of dimer twist on the intensities of RHEED patterns are briefly assessed. This indicates that twist of dimers on the GaAs(001)-2 x 4 surface is unlikely. Errors involved in the calculations are also discussed.