D. Aberdam

Structural changes of a Pt(111) electrode induced by electrosorption of oxygen in acidic solutions: A coupled voltammetry, LEED and AES study

Abstract In this paper, we describe the apparatus which allows us to study by LEED and AES the surface of a single crystal electrode before and after it is submitted to voltammetric experiments sheltered from atmospheric air. This apparatus is used to contribute to the study of the old problem of platinum electrode “activation”. Firstly, we determine which voltammogram shape does actually correspond to the first hydrogen electrochemical adsorption-desorption process onto a clean, well ordered Pt(111) surface. Secondly, we study by LEED and AES the changes in the Pt(111) electrode surface structure induced by a few electrochemical oxygen adsorption- desorption cycles.

X-ray absorption in relation to valency of iridium in sputtered iridium oxide films

Abstract Electronic and structural changes induced by the charge storage reaction due to proton insertion in sputtered iridium oxide films (SIROFs) have been investigated by in situ X-ray absorption spectroscopy at the L3 edge of iridium atoms in 1 M H2SO4. The iridium valency is shown to increase from 3 to 3.85 when the potential varies from −0.2 to +1 V(SCE). In XANES spectra, the white line peak height and energy position decrease with insertion. The fine structures of the spectra have been analyzed and simulated in view of structural parameter extraction. A correspondence curve is established between the interatomic Ir–O distance in the first shell and the iridium valency. A strong decrease of this distance is observed with the oxidation state of iridium accompanied by a conspicuous decrease of the Debye–Waller factor.

Theory of angular dependence in electron emission from surfaces: Application to Auger emission from several metal surfaces

Abstract An exact multiple scattering description of electron emission from inner sources is given using the same matrix formalism as applied in LEED theory, and some approximations are discussed. The model is derived for sources localized on atomic sites and represented in terms of spherical waves but it may be adapted for other types of inner sources. This model is used to study the anisotropy of the Auger emission from aluminum and some transition metals (Fe, Ni, Cu). The results obtained show that: (a) The intra-layer multiple scattering term dominates the calculated angular dependence. (b) The anisotropy due to multiple scattering is very important but too strong compared to experiment; this may be related to the over simplified model used for the source wave function, (c) The isotropic emitter does not account for experimental results (d) The intrinsic anisotropy of the emitter is of first importance in the observed angular dependence. Thus we conclude that not only a full multiple scattering description must be used, but also a realistic description of the Auger source is needed in particular to give account of the experimental behaviour of aluminum and of transition metals.

Angular resolved Auger emission from crystal surfaces: New experimental results and predictions from a simple model

Abstract In a first part, new angular resolved CVV Auger emission results (including line shape after background substraction and anisotropy measurements) are reported concerning the Al L 23 VV transition on clean Al(001) and Al(110), the Ni M 23 VV transition on clean Ni(001), and the Ni M 23 VV and S L 23 VV on the Ni(001)-(11,11)-S system. In a second part we discuss a semi-quantitative analysis of the Auger emission anisotropy with a model which includes at the same time the final state effects due to multiple elastic scattering of the ejected Auger electron on its escape path and a description of the Auger source in solids as realistic as possible. The band aspect of the valence states id described as s, p, d, … like contributions at the emitters lattice site, and we use atomic results for the matrix element angular momentum dependence. In a manner consistent with this model we have analyzed the available anisotropy measurements on Al(001), Al(110), Cu(001), Cu(110), Cu(111), Fe(001) and Ni(001)-(11,11)-S. The results are consistent for various faces of the same material and correspond, moreover, to the model predictions, indicating that the most important aspects of the Auger emission anisotropy are included. In a third part, on the Ni(001)-(11,11)-S system, we investigate the sensitivity of the emission anisotropy of adsorbate Auger transitions to structural parameters such as the adsorption site coordinance and the adsorbate-substrate distance. We find a large sensitivity in both cases, comparable to LEED. Moreover, satisfactory agreement is found only when the structural parameters are given the values known from LEED studies and when the angular momentum predicted by the above model is used for the ejected Auger electron.

SEDEM, a software package for EXAFS data extraction and modelling.

A software package for extended X-ray absorption fine structure (EXAFS) data extraction and modelling, running on DOS- or Windows-operated PCs, is described. This package is written with the aim of giving the user a tool to undertake all steps of data processing and modelling, rather than making use of the most recent programming facilities. However, it remains easy to use, and self-explanatory to those who have already worked with EXAFS. It is divided into two main executable pieces of software. The first one is used to extract the EXAFS k(n)chi(k) function from the data and isolate the shell contributions by Fourier filtering. A tool to sum the spectra before EXAFS extraction is provided. The second one is designed to model the EXAFS spectra or the shell contributions, using amplitude and phase data either from McKales tables, computed from the FEFF program or extracted from experimental reference spectra. This modelling program allows either an optimization of the simulation by a least-mean-square gradient algorithm, with a statistical evaluation of the result of optimization, or, in the case of a single shell, a direct determination of the four main parameters (neighbour distance and number, energy shift and Debye-Waller factor) by decorrelation of the phase and amplitude. In the presence of anharmonicity, the cumulant expansion of the radial distribution of distances is obtained from the phase and amplitude decorrelation. This package is in use at the Collaborative Research Group on Interfaces (CRG-IF) bending-magnet #32 X-ray line (BM-32) at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France.

LEED and AES study of electrosorption of copper on a platinum (110) surface

Abstract Using an experimental set up designed to transfer a sample from an UHV chamber into an electrochemical cell, and conversely, without contact with atmosphere, we have studied the underpotential deposition (electrosorption) of copper on a platinum (110) surface. Thanks to the deposition of a droplet of high purity water on the (110) platinum surface, still in UHV, the level of contamination of the surface after forth and back transfer is quite moderate, as revealed by AES. The LEED pattern after copper electrosorption exhibits the same (2×1) periodicity as the clean platinum (110) face, with some linear disorder. Quantitative AES using calibration by clean platinum and copper single crystals correlates reasonably well with coulommetric measurements of the amount of copper adsorbed.

Ordered overlayer of lead obtained by underpotential deposition on Pt(100)

Abstract A direct observation of an ordered metallic overlayer is reported. Lead has been under-potentially deposited on platinum (100) from a 0.1 M HClO 4 + 10 −3 M Pb 2+ solution and vacuum transferred to a LEEDAES system. Auger analysis shows that the lead overlayer has been transferred without alteration and with nearly no loss in coverage. LEED observation indicates that the lead atoms are organized according to a c(2 × 2) ordered superstructure on Pt(100) (1 × 1).

Crystallography by LEED: IV. Application of the iso-intensity map method to the Ni(100)-(11, 11)-S system☆☆☆

Abstract In the present study, we illustrate in the case of the Ni(001) and Ni(001)-(1 1 , 11)-S surfaces, the application of a procedure of surface analysis by LEED using iso-intensity maps for the specular beam together with a very fast LEED program based on the RFS method. This procedure has several advantages: first, iso-intensity maps prevent one from leaving out important experimental information. Second, the fast LEED program allows one to study a large number of beams previously selected from the maps for their particular interest, thus avoiding finding a wrong result, as could hapen if one looks only at a few beams. It comes out from this study that the adsorption distance d = 1.3 A of the sulfur atoms is confirmed with the help of a cluster type superposition potential for the sulfur layer. The agreement between experiment and theory is generally excellent, except for a very strong experimental peak which is not well reproduced by calculation, whatever the changes in the physical or structural parameters. This has not been reported previously and is similar to what has been found by the same authors for the aluminium (001) surface.

Electrochemical surface alloying of silver on Pt(100) : LEED and Auger study

Abstract Under potential deposition (UPD) of silver on Pt(100) exhibits two sharp peaks, each corresponding to the deposition of almost one monolayer; the first one is located 0.48 V above the Ag+/Ag equilibrium potential Eb for the bulk deposit, the second one just above this potential. After repeated Ag adsorption-desorption cycles, down to Eb new peaks can be observed between the two initial ones, which imply some modifications of the composition or structure of the surface. On the one hand, a study of the dependence of the amplitude of platinum and silver Auger lines on the polar angle of incidence of the primary electron beam gives evidence for surface alloying, when the electrode has been submitted to these repeated cycles. On the other hand, LEED examination of the surface emersed from the cell always exhibit the geometry of a primitive Pt(100) surface net, had it been subjected to silver adsorption-desorption cycles or not.

A LEED data reduction method: Description and application to the relaxation of aluminum (001) and (110) surfaces

Abstract A LEED data reduction method is described and applied to Al (001) surface. The method consists of averaging rotation diagrams (azimuthal intensity profiles) obtained at constant electron energy between 500 and 1000 eV in the whole range of the colatitude angle of incidence. The method avoids certain difficulties, stemming e.g. from the energy dependence of the mean optical potential, that are present in other methods in which data obtained at different energies are averaged. The averaged data are analyzed using the two-beam Darwin theory. Values for the surface normal relaxation and mean optical potential are obtained from the study of the position, width and profiles of the averaged peaks. The (100) surface of aluminum is found not relaxed while the (110) surface is contracted. The values found for the mean optical potential are similar to those obtained from other sources. The precision of the averaging method for surface structure is not better than that of direct calculation. Experimental factors limiting the precision of the method (surface roughness, background interference due to Kikuchi lines) are specified.