The influence of CO adsorption on the surface composition of cobalt/palladium alloys

AM is grateful for funding from SASOL Technology UK Ltd (PhD studentship); JG is grateful for the award of a fellowship from the Knut and Alice Wallenberg Foundation. TEJ and AGT acknowledge EPSRC funding of postdoctoral research (EP/E047580/1). The MEIS facility was funded via EPSRC grant EP/E003370/1.Segregation induced by the adsorption of gas phase species can strongly influence the composition of bimetallic surfaces and can therefore play an important role in influencing heterogeneous catalytic reactions. The addition of palladium to cobalt catalysts has been shown to promote Fischer Tropsch catalysis. We investigate the adsorption of CO onto bimetallic CoPd surfaces on Pd{111} using a combination of reflection absorption infrared spectroscopy and medium energy ion scattering. The vibrational frequency of adsorbed CO provides crucial information on the adsorption sites adopted by CO and medium energy ion scattering probes the surface composition before and after CO exposure. We show that cobalt segregation is induced by CO adsorption and rationalise these observations in terms of the strength of adsorption of CO in various surface adsorption sites.PostprintPeer reviewe

Methylthiolate-induced reconstruction of Ag(1 1 1): A medium energy ion scattering study

Medium energy ion scattering (MEIS), using 100 keV H+ incident ions, has been used to investigate the structure of the Ag(1 1 1)(√7 × √7)R19° –CH3S surface phase. The results provide the first direct evidence that this structure does involve substantial reconstruction of the Ag surface layer. The measured absolute scattered ion yields and blocking curves are in generally good agreement with a specific structural model of the surface based on a reconstructed layer containing 3/7 ML Ag atoms, previously suggested on the basis of scanning tunnelling microscopy (STM) and normal incidence X-ray standing wave (NIXSW) studies. However, the MEIS data indicate that any rumpling of the thiolate layer, is small, and probably 0.2 Å. This value is smaller than the amplitude suggested in the STM and NIXSW studies, but could be entirely consistent with the earlier experimental data

Path to AWAKE : evolution of the concept

This paper describes the conceptual steps in reaching the design of the AWAKE experiment currently under construction at CERN. We start with an introduction to plasma wakefield acceleration and the motivation for using proton drivers. We then describe the self-modulation instability - a key to an early realization of the concept. This is then followed by the historical development of the experimental design, where the critical issues that arose and their solutions are described. We conclude with the design of the experiment as it is being realized at CERN and some words on the future outlook. A summary of the AWAKE design and construction status as presented in this conference is given in Gschwendtner et al. [1]

Coaxial impact collision ion scattering spectroscopy of semiconductor and metal surfaces

SIGLEAvailable from British Library Document Supply Centre-DSC:DXN005860 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

A medium energy ion scattering study of the structure of Sb overlayers on Cu(111)

Using 100 keV H+ ions, medium energy ion scattering (MEIS) has been applied to a study of Sb overlayers on Cu(111). Upon deposition of 10 ML of Sb at room temperature, considerable intermixing of Sb and Cu occurs in thick (100 Å) overlayers, forming a partially ordered alloy phase, of composition Cu3Sb. Heating to 200°C produces a second ordered alloy phase with reduced Sb content, whilst heating to 305°C diffuses excess Sb into the bulk and produces an ordered Cu(111)R30°Sb surface phase with 0.33 ML Sb. Comparison of blocking curves obtained from this phase with the results of simulations for a range of model structures leads to the conclusion that the Sb atoms substitute outermost layer Cu atoms, but this surface alloy layer is laterally displaced so that all constituent atoms occupy hcp hollow sites relative to the underlying Cu(111) substrate. In this stacking faulted surface layer, the layer spacings of the Sb and Cu atoms relative to the outermost underlying Cu surface layer have been determined to be 2.52±0.13 Å and 2.05±0.09 Å respectively. Some aspects of the methodology for the application of MEIS to surface structure determination and structural parameter precision estimation are addressed

The UK MEIS facility: a new future at the IIAA, Huddersfield

The MEIS facility, formerly at STFC’s Daresbury Laboratory and used by a wide range of UK and foreign research groups, has moved to the IIAA at the University of Huddersfield. It has been fitted with a new 200 keV ion accelerator. MEIS is a powerful tool for the structural and compositional characterisation of nanolayers, including depth profiling. Further to basics of the MEIS technique, examples of its analytical capability are presented from the area of Ion beam crystallography but mainly from Depth profiling analysis

Structural studies of two- and three-dimensional dysprosium silicides using medium-energy ion scattering

Medium-energy ion scattering has been used to determine the atomic structure of two-dimensional and three-dimensional (3D) dysprosium silicide films on the Si(1 1 1) surface. A quantitative study of the ion yield from the dysprosium has enabled the positions of the atoms in the top three layers of the Si(1 1 1)1×1–Dy to be precisely determined. For the case of the Si(1 1 1)(√3×√3)R30°–Dy 3D silicide surface the experimental blocking curves are in agreement with simulations for the structural model determined by surface X-ray diffraction for the Si(1 1 1)(√3×√3)R30°–Er 3D silicide surface. A contraction of 2.0±0.6% in the c-axis lattice constant compared to bulk dysprosium disilicide is found

A medium-energy ion scattering investigation of the structure and surface vibrations of two-dimensional YSi2 grown on Si(111)

Medium-energy ion scattering has been used to determine the atomic structure of two-dimensional yttrium silicide on silicon (1 1 1). A full quantitative analysis of the atomic positions of the Si atoms in the top bilayer yields a model similar to that previously suggested in the literature with a Si1–Si2 vertical spacing of 0.80 ± 0.03 Å, but with the Si bilayer relaxed slightly further away from the Y layer (Si2–Y vertical spacing of 1.89 ± 0.02 Å). Observing the effects of the top bilayer vibrations yields a model with significant enhancement

Trends and strain in the structures of two-dimensional rare-earth silicides studied using medium-energy ion scattering

The surface structures of the two-dimensional (2D) rare-earth (RE) silicides formed by Gd and Tm on Si(111) have been determined using medium-energy ion scattering (MEIS). These data have been taken together with those from earlier MEIS studies of the 2D silicides formed by Dy, Ho, Er, and Y to investigate the existence of trends in the structural parameters of this class of surfaces. It was found that the Si-RE bond length associated with the surface bilayer in the 2D silicides followed the same trend as the bond length in the bulk silicide compounds which is simply related to the size of the RE atom. In contrast, changes in the Si-RE layer spacing in the 2D silicides were found to be too large to be accounted for by a size effect alone. It was found that the strain that results from the expansion or compression of the overlayer to fit the Si(111) surface is compensated by changes in the Si-RE layer spacing. Finally, there is a systematic variation in the rumple of the surface bilayer in the 2D RE silicides which appears to increase with the magnitude of the lattice mismatch suggesting that strain in the overlayer results in a weakening of the Si-Si bond in the surface bilayer