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Surface Science 108 (1981) 205-224 North-Holland Publishing Company

205

THE Pt( 100) (5 X 20) + (1 X 1) PHASE TRANSITION: A STUDY BY RUTHERFORD BACKSCATTERING, NUCLEAR MICROANALYSIS, LEED AND THERMAL DESORPTION SPECTROSCOPY P.R. NORTON, J.A. DAVIES, D.K. CREBER *, C.W. SITTER and T.E. JACKMAN * Atomic Energy of Canada Limited, Research Company, Chalk River Nuclear Laboratories, Chalk River, Ontario, Canada KOJ I.JO

Received 30 October 1980; accepted for publication 10 March 1981

The reconstruction exhibited by clean Pt(lOO) surfaces [(5 X 20) LEED pattern] is removed by the adsorption of CO. Rutherford backscattering (RBS) indicates that 1.65 f 0.05 X 10” Pt atoms cmm2 move into registry with the bulk upon adsorption of 6.4 + 0.4 X 1014 CO molecules cme2 (0 = 0.50 + 0.03 monolayers). The data indicate that some atoms in the second and perhaps even subsequent layers must be displaced by 20.01 nm in the reconstructed surface. By contrast, only 1.3 t 0.1 X 1015 Pt atoms cm-* move back into registry upon adsorption of Hz or D2, and the LEED pattern also indicates that residual reconstruction remains. The stability of the CO-covered, H-covered and “almost clean” (1 X 1) surfaces (the latter prepared by NO and Hz treatments with a residual H-coverage of -1 X lOi H atoms cmm2) was investigated by RBS. The CO-covered surface starts to reconstruct only when the CO coverage drops below 0.5 monolayers (T 2450 K) while the H-covered surface (produced by adsorption on the (5 X 20) surface) reconstructs rapidly at T 2350 K, by which temperature the adsorbed hydrogen coverage drops below -0.2 monolayers. The “almost clean” surface reconstructs at T 2390 K and the data indicate that the process exhibits an activation energy of 88 + 17 kJ mol-’ . The absolute coverages of CO and D were determined by nuclear micro- analysis (NMA) and excellent agreement was achieved between the LEED and NMA data. The saturation CO coverage was found to be 0.77 + 0.03 monolayers, consistent with the observed c(4 X 2) LEED pattern. Deuterium (and hence hydrogen) coverages of 1.54 f 0.1 X 10ls D (H) atoms cm-’ (0 = 1.20 + 0.08) were found at saturation at -150 K and the hydrogen adsorbed on the (1 X 1) surface was more strongly bound than that resulting from adsorption on the (5 X 20) surface.

Surface Science 108 (1981) 225-234 North-Holland Publishing Company

225

ADSORPTION OF HYDROGEN ON RHODIUM; COMPARISON WITH HYDROGEN ADSORPTION ON PLATINUM AND IRIDIUM V.V. GORODETSKII *, B.E. NIEUWENHUYS **, W.M.H. SACHTLER and G.K. BORESKOV *** Gorlaeus Laboratoria, Rijkuniversiteit Leiden, P.O. Box 9502, 2300 RA Leiden, The Netherlands

Received 10 December 1980; accepted for publication 10 March 1981

The adsorption of hydrogen on Rh has been studied (i) on a single crystal tip using field electron microscopy, and (ii) on a filament carrying this tip, using thermal desorption spectros- copy. The results are compared to those of other Group VIII metals. An isosteric heat of adsorption of 19 kcal/moIe was found at low coverage, decreasing slightly with increasing cover-

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age. This heat is substantially lower than that on Ru and II, determined by the same method. The work function increases by 0.4 eV, a value comparable to data reported for Ni and Ru, but significantly larger than those of Ir and Pt. An electropositive state of hydrogen as observed for Pt and II was not found for Rh. A small fraction of the adsorbed hydrogen is not desorbed at temperatures where other transition metal surfaces are completely denuded. This &-hydrogen which is desorbed only at 600-800 K, is tentatively assigned to a subsurface species.

Surface Science 109 (1981) 235-252 North-Holland Publisliing Company

2

THEINFLUENCEOFSTRUCTURALDEFECTSONTHEADSdRPTIONOF SIMPLEMOLECULES(H2,CZHZ,CZH4,CZH6,CO,NO)ONRHENIUMSINGLE CRYSTALS R. DUCROS, M. HOUSLEY *, G. PIQUARD and M. ALNOT LARIGS, Laboratoire Maurice Letort, CNRS, B.P. 104, F-54600 Villers-les-h’ancy, France

Received 24 October 1980; accepted for publication 24 February 1981

Using Thermal Programmed Desorption (TPD), Low Energy Electron Diffraction (LEED) and Auger Electron Spectroscopy (AES) we have studied the adsorption of hydrogen-con- taining molecules (Ha, Ca Ha, Ca H4, Ca He) and oxygen-containing molecules (CO and NO) on two vicinal planes of the Re(0001) surface. The two surfaces are designated thus:

ReS114(0001)(10i1)1, ReS16(0001)(1671)1.

The structural defects have little effect on the adsorption of hydrogen and the hydrocarbons. They are more influential in the case of the oxygen-containing molecules. This is particularly true for CO; on the kink sites the CO molecules can completely dissociate whereas only a partial dissociation is possible on the steps. These results should be viewed in relation to the strong bond energy between carbon and oxygen in a CO molecule of 256 kcal/mole and the great affinity of oxygen for rhenium; ERe_0 = 127 kcal/mole.

Surface Science 108 (1981) 253-270 North-Holland Publishing Company

253

CHARGEEXCHANGEINATOM-SURFACESCATTERING:THERMAL VERSUSQUANTUMMECHANICALNON-ADIABATICITY R. BRAKO * and D.M. NEWNS **

Institute of Theoretical Physics, Chalmers University of Technology, S-412 96 Gtiteborg, Sweden

Received 16 October 1980; arcepted for publication 24 February 1981

35

We consider the ionic fraction in the reflected beam when an atom or ion is scattered from a metal surface at finite temperature. Our starting point is the zero temperature theory of Blandin, Nourtier and Hone, applicable to a non-interacting Anderson model description of the coupling between the valence level of the atom and the metal electron states, in which the energy and lifetime broadening of the valence level are explicitly time-dependent as a result of the classical motion of the atom centre of mass. This model is generalised to give an exact