Robert G. Jones

An electron-yield EXAFS study of anodic-oxide and hydrated-oxide films on pure aluminium

Abstract The molecular structures of the amorphous films formed on pure aluminium by anodic oxidation in sodium tartrate and phosphoric acid electrolytes, which are known to give quite different oxide morphologies, have been investigated using the technique of electron-yield EXAFS. The Al-0 ‘bond length’ derived from the EXAFS is found to be different for these two types of film, which suggests different states of aluminium-oxygen coordination. The two types of film also show quite different behaviour when immersed in hot water. The tartrate-formed oxide shows only slight morphological change and a correspondingly small change in Al—O bond length, whereas the phosphoric acid-formed film undergoes an extensive change of morphology and bond length. These results are discussed in relation to the structural chemistry of the hydration process.

Thermodynamic measurements for N2 adsorption on Ni(100)

Abstract We investigated the adsorption of molecular N2 on Ni(100) by LEED and thermodynamic measurements. From our data isosteric heats and the entropies in the adsorbed layer are calculated and compared to results for N2 on Ni(110) and supported nickel particles. Saturation coverage of N2 on Ni(100), as characterized by a c(2 × 2) LEED pattern, corresponds very closely to saturation coverage of N2 on Ni(110). Near saturation coverage on Ni(100) we observe a drop in the heat of adsorption and an strong increase in the differential entropy of the adsorbed layer. We believe this observation is being caused by the presence of antiphase boundaries in the c(2 × 2) structure which cause a softening of vibrational modes in the surface layer.

Sampling depths in total yield and reflectivity SEXAFS studies in the soft X-ray region

Abstract Both total photoelectron yield and reflectivity measurements have been shown to display EXAFS and to possess some surface sensitivity. Using aluminium foils with natural oxide films and anodically prepared films of different thicknesses we have investigated the viability and surface specificity of the methods in the vicinity of the Al K-edge (1560 eV). We find that oxide films of only ~ 130 A thickness lead to EXAFS dominated by the oxide structure in the total yield mode, and develop a simple model to account for the depth dependence of sampling in this mode which provides a reasonable match of the experimental data.

The formation of a surface iodide on Ni{100} and adsorption of I2 at low temperatures

Iodine adsorption on clean Ni[100] has been investigated using low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). At temperatures below 340 K. a saturated surface of adsorbed iodine atoms in a c(2 × 2) structure is observed. Adsorption of iodine on clean Ni{100} at temperatures in exces of 370 K forms a structure identified as a single layer of the layered compound NiI2 on the metal substrate. Solid iodine is shown to grow epitaxially on both the c(2 × 2) chemisorbed surface and the surface iodide at temperatures less than 185 K. Heating to 185 < T < 226 K leaves a physisorbed molecular iodine layer, while on returning to room temperature the original c(2 × 2) or iodide is restored.

The adsorption of I2 on Ni{100} studied by AES, LEED and thermal desorption

Abstract Iodine adsorbs on Ni{100} at room temperature to form a c(2 × 2) structure. It begins to desorb on heating to just above room temperature with the surface evolving through a 1 1 tan θ − 1 1 tan θ LEED structure as the concentration decreases. Complete desorption is only achieved for temperatures 1000 K indicating an activation energy for desorption which is strongly coverage dependent. Surface structures are proposed which are consistent with both the LEED and surface coverage data, and comparisons are drawn between this system and other halogen adsorption systems.

An unusual adsorption site for methoxy on Al(111) surfaces

The adsorption site of the methoxy (CH3O) species on Al(111) has been investigated using the technique of normal incidence standing X-ray wavefield absorption. By recording the X-ray absorption of the O atom of this species at both the (111) and (111)-normal incidence Bragg scattering conditions (at normal and 70.5 degrees incidence to the surface) the adsorption site was obtained by simple real-space triangulation. The species is adsorbed in a three-fold symmetry hollow site with an O-Al layer spacing of 0.70+or-0.10 AA, (as found for chemisorbed oxygen), but the site occupied is the HCP hollow directly above an Al atom in the second layer, and not the FCC site (above an Al atom in the third layer) which is occupied by chemisorbed oxygen. This appears to be the first example of an adsorbate with a clear preference for this HCP hollow site on an otherwise clean FCC (111) surface.

Core level photoemission study of the adsorption of iodine on Ni{100}

Abstract Synchrotron radiation photoemission from the Ni 3p and I 4d core levels has been studied for iodine adsorbed on Ni{100} in a range of different phases. In the range of chemisorbed structures involving overlayer compression from coverages θ of about 0.3 to 0.4 the I 4d photoemission binding energies decrease by ~0.3 eV, while further compression to a c(2 × 2) structure leads to a further decrease by ~0.4 eV. Low temperature adsorption of molecular iodine shows a binding energy of 0.1 eV lower still. In the chemisorbed coverage range these core level shifts are found to be substantially less than the I-I repulsive energy interactions and are associated with the through-metal component of this interaction, while the residual component concealed from the photoemission experiments is attributed to through-space overlap interaction. Other processes which may contribute to the observed binding energy shifts, including Coulomb interactions in both initial and final states, are discussed in some detail.

A structural study of the Al(111)( square root 3* square root 3)R30 degrees -Rb phase at different temperatures

Normal-incidence standing x-ray wavefield (NISXW) measurements have been made of the local adsorption site of Rb on Al(111) surfaces, particularly in an ordered ( square root 3* square root 3)R30 degrees phase, as a function of the sample temperature during adsorption or subsequent annealing. The results confirm the a top-site occupation for low-temperature (around 150 K) preparation, but show that room-temperature preparation leads to a structure having Rb atoms in surface substitutional sites. The overall structural situation is therefore essentially the same as that found previously by low-energy electron diffraction LEED for the Al(111)( square root 3* square root 3)R30 degrees -K phases. However, experiments involving annealing of the low-temperature prepared surface to room temperature indicate that only a small part of the surface easily transforms to the higher-temperature form, and indeed there is evidence that even in room-temperature preparations some fraction of the adsorbed atoms may remain in atop sites. The apparent conflict of this result with that from recent photoemission core-level shift and LEED data is discussed.

A surface EXAFS study of a surface iodide phase on Ni{100}

Surface EXAFS data from a surface iodide phase formed on Ni{100} are compared with data from bulk NiI2. Two independent analyses are presented. In the first, the nearest neighbour Ni-I bond length change is extracted by Fourier filtering using methods previously used for SEXAFS analysis. The second method uses a comparison of calculated and experimental EXAFS based on calculated scattering phase shifts. Both analyses show the surface phase to involve a contraction of the NiI bond length by 0.04 ± 0.02 A, while the second analysis also shows a similar contraction of the II bond length.

A SEXAFS study of several surface phases of iodine adsorption on Ni{100} : II. The breakdown of Fourier filtering single shell analysis

Fourier filtering single shell analyses (the currently established method of analysis) of SEXAFS data for four surface phases of iodine adsorption on Ni{100}, and for bulk NiI2, are presented. Although reasonable I-Ni bondlength changes (< 0.10 A) are deduced for the different surface phases, the results are shown to be unreliable due to the role of I-I scattering in the NiI2 and surface iodide phases which is not satisfactorily separated by this method. This problem does not, however, prevent the adsorption site in the c(2 × 2) chemisorbed phase being shown to be either a hollow or bridge site, with the atop site being excluded. The possible influence of second shell scattering on previous SEXAFS analyses is discussed.