A.S.Y. Chan

Surface adsorption structures in 1-octanethiol self-assembled on Cu(111)

A normal incidence X-ray standing wave (NIXSW) and near-edge X-ray absorption fine structure (NEXAFS) study was performed on saturated monolayers of 1-octanethiol on Cu(111). NEXAFS around the S K-edge taken in normal and grazing incidence revealed that the S-C bond was oriented with respect to the Cu(111) surface to give a calculated orientation of the pendant hydrocarbon which was almost normal to the surface, similar to self-assembled alkanethiols on gold surfaces. Modulation in the S(1s) photoemission with variation in the energy of the X-ray standing wave about the (111) and (1¯11) Bragg energies was modelled to yield values of structural parameters: the coherent position, dhkl, and coherent fraction, fhkl, for the adsorbate with respect to the bulk Cu lattice planes. For the (111) reflection, d111 = 1.19 ± 0.10A˚, f111 = 0.70 ± 0.03, but for the (1¯11) reflection, f1¯11 was close to zero (f111 = 0.10 ± 0.05) and d1¯11 was indeterminate. The coherent fraction for the Cu substrate was high for both reflections studied (f111 = 0.95 ± 0.03;1¯11 = 0.92 ± 0.02). The structural parameters are rationalised in terms of penetration of the Cu by thiolate sulfur atoms to cause a surface layer reconstruction. The resultant adsorbate structure is either incommensurate or else consists of a large-mesh commensurate superlattice in which the S atoms exist at a number of different positions with respect to the (1¯11) planes of the bulk.

The local adsorption structure of SO2 on Ni(111) : a normal incidence X-ray standing wavefield determination

Abstract A normal incidence X-ray standing wavefield study of the structure of molecular SO 2 on Ni(111) has been conducted, using photoabsorption at both the O and S atoms and real-space site triangulation using {111} scatterer planes both parallel to, and at 70° to, the surface plane. Both O and S atoms are found to be in the vicinity of atop sites, although the S atoms are displaced significantly further from these high symmetry sites. S K-edge NEXAFS confirms an earlier finding that the molecule lies with its molecular plane parallel to the surface. The detailed sites can only be reconciled with a model in which the SO 2 molecules are centred close to hollow sites (with equal occupation of both types of hollow) and the internal conformation of the molecule, especially the OSO bond angle, is significantly different from that of the gas-phase molecule. Specifically, the OSO bond angle is estimated to be no more than 100°, while the data indicate an SO bond-length expansion of 5% or more. This change is attributed to the unusual π-bonding (for which there appears to be no analogue in coordination compounds) and thus partial occupation of the 3b 1 π∗ LUMO of the molecule.

Line of sight techniques: Providing an inventory of all species arriving at and departing from a surface

Line of sight (LOS) techniques comprise those methods in which species emanating from a surface (atoms, molecules, and radicals) undergo just a single pass through the ionization volume of a mass spectrometer before being pumped. This is achieved by enclosing the mass spectrometer within a cryoshield fitted with appropriate apertures, such that line of sight is established only between a patch on the sample surface (≈7 mm diameter) and the ionization volume. All LOS techniques are free from extraneous signals and have approximately equal detection probabilities for all species. Line of sight temperature programmed desorption, sticking probability (LOSSP), and product desorption (LOSPD) provide powerful and reliable ways of studying all aspects of surface kinetics, by allowing an inventory of all species arriving at and departing from a surface, for any combination of partial pressures, surface temperature, surface composition, and surface structure. Here we illustrate LOSSP and LOSPD using the reactions o...

A structural study of the interaction of methanethiol with Pt(111) using X-ray standing waves

In combination with surface characterisation by synchrotron radiation X-ray photoelectron spectroscopy, the normal incidence X-ray standing wave (NIXSW) technique has been applied to a determination of the structure of surface phases formed on Pt(1 1 1) by reaction with methanethiol. On surfaces heated to > approximate to500 K, producing only coadsorbed atomic S and C, the S atoms are found to occupy fcc hollow sites (directly above Pt atoms in the third layer) in a geometry essentially identical to that of simple ordered S overlayer phases on Pt(1 1 1) with a S-Pt layer spacing of 1.67 Angstrom, but with possible fractional co-occupation of a complex S phase. On a surface annealed to approximate to223 K only a surface methanethiolate (CH3S-) species is believed to be present, the favoured model involves a tilted off-atop bonding such that the S atoms are located offset from the fcc hollow sites, with frustrated rotational vibrations of large amplitude, although an alternative model based on co-occupation of atop and fcc hollow sites is also consistent with the NIXSW data

X-ray standing waves at surfaces

Adatoms immersed in an x-ray standing wave at a surface betray their position within the wave by the way they absorb the x-rays; feebly when positioned at the nodes, strongly when positioned at the antinodes. The elemental (and chemical) identity of the adatoms are easily monitored using the binding energies of the photoelectron or Auger electron emissions, while the intensities of these emissions provide the information needed to determine the atomic positions relative to the crystalline substrate which formed the standing wave. By using normal incidence Bragg diffraction to generate the standing wave, the technique is applicable to the rather imperfect crystalline samples and standard manipulators used in most surface science studies. Examples of structural studies from a range of systems will be drawn from recent work carried out at the SRS in Daresbury to illustrate the strengths, and weaknesses, of this structural technique. Specifically, the structure of reactive intermediates (SiHx) formed by chemical reaction of silane on Cu(111); the structure of a physisorbed molecule (ClCH2CH2F) on Cu(111); an example of how chemically shifted Auger peaks may be useful for chemical shift XSW (chloroform on a chlorinated copper surface), and a system which presents many difficulties when studied by this technique, methyl thiolate on Au(111).

Adsorption, decomposition, and stabilization of 1,2-dibromoethane on Cu(111)

The molecular adsorption and thermal reaction of 1,2-dibromoethane (DBE) on Cu(111) has been studied using ultraviolet photoelectron spectroscopy, work function change (ΔΦ) measurements, low energy electron diffraction, and line-of-sight temperature programmed desorption. At 100 K, DBE adsorbs molecularly, exhibiting a decrease in work function of 0.86 eV at the completion of the monolayer. Layer-by-layer growth is observed up to two molecular layers, with further adsorption leading to the growth of tower crystallites. The first layer of DBE undergoes thermal decomposition at 125 K to give gaseous ethene and chemisorbed bromine in a (√3×√3)R30° structure, which increases the work function of the surface by 0.58 eV. Chemisorbed bromine from partial decomposition of the first layer is found to stabilize the remaining DBE molecules in the decomposing layer, thereby arresting complete decomposition until a higher temperature. Subsequent decomposition of the stabilized DBE leads to ethene emission at 145 and 1...