Christine L. A. Lamont

The structure of oxygen on Cu(100) at low and high coverages

The local adsorption structure of oxygen on Cu(1 0 0) has been studied using O 1s scanned-energy mode photoelectron diffraction. A detailed quantitative determination of the structure of the 0.5 ML (√2×2√2)R45°-O ordered phase confirms the missing-row character of this reconstruction and agrees well with earlier structural determinations of this phase by other methods, the adsorbed O atoms lying only approximately 0.1 A above the outermost Cu layer. At much lower coverages, the results indicate that the O atoms adopt unreconstructed hollow sites at a significantly larger O–Cu layer spacing, but with some form of local disorder. The best fit to these data is achieved with a two-site model involving O atoms at Cu–O layer spacings of 0.41 and 0.70 A in hollow sites; these two sites (also implied by an earlier electron-energy-loss study) are proposed to be associated with edge and centre positions in very small c(2×2) domains as seen in a recent scanning tunnelling microscopy investigation.

Dynamics of atomic adsorbates: hydrogen on Cu(111)

Abstract We have measured the absolute changes induced in the far infrared reflectivity by H and D atoms adsorbed on Cu(111). The strongest features in the spectra are the dipole forbidden parallel vibrational modes which give rise to anti-absorption peaks with an intensity almost 10 times higher than that of the dipole allowed perpendicular modes. The dynamics can be explained by excitation of electron-hole pairs in the substrate. The energy relaxation time of the parallel vibration is 1 ps, but the linewidth is larger by a factor of 10 probably due to lateral tunneling of the adsorbates and dephasing processes.

Argon plasma-induced modifications at the surface of polycarbonate thin films

Abstract The chemical modification of the surfaces of polycarbonate films by microwave-induced argon plasma treatment has been followed by X-ray absorption spectroscopy (XAFS) and core level photoemission spectroscopy (XPS). Measurements have been made on thin films of both ex-situ spin-coated bis-phenol-A polycarbonate with phenol endgroups and the model compound bis-phenol-A polycarbonate ( n = 1) with tert-butyl phenyl endgroups evaporated in-situ in UHV on single crystal surfaces. A preferential orientation of a monolayer of the model compound on a Ag{111} surface was determined from the angular dependence of the XAFS spectra. Spectral features were assigned to contributing transitions within different moieties. Relatively mild microwave Ar plasma activation of the surfaces of both types of films led to a decrease in intensity of carbonyl features in both XAFS and XPS. Combined with the observation of new spectral features in XPS the data can be interpreted in terms of a formation of ester and ether groups in the near-surface region with possible cross-linking after desorption of CO or CO 2 .

The wavelength dependence of the photodissociation cross-section of CH3Br chemisorbed on Cu{111}

Abstract The photodissociation of methylbromide adsorbed on Cu{111} has been investigated as a function of the photon energy between 2.6 and 6.0 eV using a Xe arc lamp. The photodissociation products remaining on the surface, identified using HREELS, are CH3 (in a C3v configuration) and Br. In subsequent thermal desorption CH4, C2H4, H2 and CuBr are detected. The cross-section curve as a function of the wavelength shows an onset at ~ 2.6 eV and a maximum of 1.2 × 10−20 cm2 at 3.5 eV. The results are compared with those obtained on a Pt{111} surface and both sets of data are interpreted within the framework of a one-step charge transfer model involving electron excitation from the substrate d-band into the σ∗ CBr antibonding orbital.

A vibrational spectroscopic study of the adsorption and dehydrogenation of cyclic C6 hydrocarbons on the (1 × 1) and (5 × 20) surfaces of Pt{100}

The dehydrogenation of cyclohexane and cyclohexene has been investigated on the (1 × 1) and (5 × 20) surface modifications of Pt{100}. In each case dehydrogenation to adsorbed benzene occurs, although the reaction has a very low probability for cyclohexane adsorbed on the (5 × 20) surface at low temperature, despite the presence of softened CH stretching modes in the vibrational spectra. The surface electronic structure appears to influence the reaction pathway of the dehydrogenation process. On the (5 × 20) surface a conformational change of the adsorbate appears to occur, while on the (1 × 1) surface a stable intermediate can be isolated. Measurements made on isomers of cyclohexadiene support the suggestion that this intermediate has the formula C6H9.

The local adsorption geometry of CO and NH3 on NiO (100) determined by scanned-energy mode photoelectron diffraction

The local adsorption structures of CO and NH3 on NiO(1 0 0) have been determined by C 1s and N 1s scanned-energy mode photoelectron diffraction. CO adsorbs atop Ni surface atoms through the C atom in an essentially perpendicular geometry (tilt angle 12±12°) with a C–Ni nearest-neighbour distance of 2.07±0.02 A. NH3 also adsorbs atop Ni surface atoms with a N–Ni distance of 2.06±0.02 A. These bondlengths are only very slightly longer than the comparable values for adsorption on metallic Ni surfaces. By contrast theoretical values obtained from total energy calculations, which exist for CO adsorption on NiO(1 0 0) (2.46 A and 2.86 A) are very much longer than the experimental value. Similar discrepancies exist for the N–Ni nearest-neighbour bondlength for NO adsorbed on NiO(1 0 0). Combined with the published measurements of the desorption energies, which also exceed the calculated bonding energies, these results indicate a significant failure of current theoretical treatments to provide an effective description of molecular adsorbate bonding on NiO(1 0 0).

Photoelectron diffraction study of the Ag(110)-(2×1)-O reconstruction

The structure of the (2×1)-O adsorption phase on Ag(110) has been determined using scanned-energy mode photoelectron diffraction. The oxygen atoms have been found to occupy the long-bridge site and are almost coplanar with the top layer of silver atoms. The best agreement between multiple-scattering theory and experiment has been obtained for a missing-row (or equivalently an ‘added-row’) reconstruction in which the first-to-second and second-to-third interlayer spacings are 1.55±0.06 A and 1.33±0.06 A, respectively. Alternative buckled-row and unreconstructed surface models can be excluded.

Quantitative determination of the local adsorption structure of carbonate on Ag(110)

The local geometry of carbonate (CO3) on Ag(1 1 0), formed by the reaction of CO2 with preadsorbed oxygen, has been investigated using C Is scanned-energy mode photoelectron diffraction. The carbonate species is essentially planar and adsorbs almost parallel to the surface in an off-atop site relative to an outermost layer Ag atom, at a C-Ag layer spacing of 2.64 +/- 0.09 Angstrom, with a well-defined azimuthal orientation. This geometry is best understood in terms of the added-row model proposed by Guo and Madix in which additional Ag atoms lie adjacent to the carbonate, such that bonding can occur through at least one of the oxygen atoms. The distance between this oxygen and its nearest neighbour Ag adatom is 1.90 +/- 0.42 Angstrom. The C-O distances are in the range 1.26-1.30 Angstrom. While the symmetry of the carbonate in the optimum structure is reduced, the D-3h symmetry of the isolated species lies within the limits of Precision

Structure determination of methanethiolate on unreconstructed Cu(111) by scanned-energy mode photoelectron diffraction

The local structure of methanethiolate, CH3S–, on an unreconstructed Cu(1 1 1) surface at low temperature, has been investigated by S 2p and C 1s scanned-energy mode photoelectron diffraction, with chemical state sensitivity. 71(+14/−16)% of the methanethiolate was found to occupy bridge sites, 29±14% to occupy fcc hollow sites and 0+19% to occupy hcp hollow sites. In the bridge site the layer spacing of the sulphur atom to the outermost substrate layer is 1.87±0.03 A giving a Cu–S bondlength of 2.27±0.03 A. The methanethiolate adsorbed in the fcc hollow site has a Cu–S layer spacing of 1.73±0.04 A, corresponding to the same bondlength of 2.27±0.04 A. The S–C bondlength was found to be 1.92±0.10 A. These conclusions are consistent with the results of previous X-ray standing wave and scanning tunnelling microscopy studies for a common model involving co-occupation of bridge and hollow sites, although differing relative occupations and long-range ordering are thought to arise from different preparation conditions. The new data favour a model in which the S–C bond axis of the bridge-bound thiolate is tilted by 45±12° away from the surface normal in the azimuth directed towards the fcc hollow site.

Aluminium metallisation of argon and oxygen plasma-modified polycarbonate thin film surfaces

Abstract The influence of plasma treatment on the metallisation of polycarbonate surfaces was studied using X-ray absorption spectroscopy (XAFS) and core level X-ray photoelectron spectroscopy (XPS). Thin films of two different molecules were chosen: bis-phenol-A polycarbonate with phenol endgroups (P-PC) prepared ex situ by the spin-coating technique onto MoTe 2 {0001}surfaces, and the model compound bis-phenol-A polycarbonate ( n =1) with tert -butyl phenyl endgroups (tBP-PC) evaporated in situ in UHV onto Cu{110}, Ag{100} and Ag{111} surfaces with film thicknesses of up to several monolayers. Surfaces of untreated samples and of samples which were pre-treated with either an inert argon or a reactive oxygen microwave plasma were metallised with Al (evaporated by electron beam heating) at film thicknesses ranging from the sub-monolayer region up to several monolayers. For the untreated surface, XAFS and XPS spectra suggest that the Al reacts with the carbonate groups leading to a breaking of the CO double bonds (and/or a reduction in bond order) as well as formation of Al oxide, Al hydroxide and Al–O–C linkages. A study of the time-dependent oxidation of the evaporated Al leads to the conclusion that Al slowly diffuses to the reactive sites in the first few subsurface layers of the polymer. Argon plasma treatment of samples leads to a reduction in the number of carbonyl groups in the near surface region. After metal deposition a higher ratio of metallic, non-reacted, Al was observed covering the polycarbonate surface and the diffusion rate into the polymer bulk seems to be higher than in the case of the untreated surface. Oxygen plasma treatment leads to the creation of additional CO containing species which also react with the Al in the subsequent metallisation process. Here, the ratio of oxidised Al on the polymer surface is higher than observed for untreated and argon plasma pre-treated polymer surfaces.