X-ray Standing Wavefield Absorption Research Articles

The Structure of Atomic Sulfur Phases on Au(111)

The structural phases formed by atomic sulfur on Au(111) due to reaction with molecular S2 have been investigated by qualitative low-energy electron diffraction (LEED), scanning tunneling microscopy, and normal incidence X-ray standing wavefield absorption (NIXSW) combined with X-ray photoelectron spectroscopy (XPS). Three phases are identified with increasing coverage, namely, a newly identified (5 × 5) phase, a ( × )R30° phase, and a “complex” phase. The (5 × 5) phase, with a LEED pattern having the appearance of a “split-spot” ( × )R30° pattern, is interpreted in terms of local ( × )R30° ordering within a (5 × 5) ordered domain structure. The S atoms in the (5 × 5) phase occupy fcc hollow sites 1.56 A above the outermost extended Au(111) bulk atomic scatterer plane. A specific model of the ordering in this phase is proposed that, together with the observed marginal stability of the true, long-range-ordered, ( × )R30° phase, indicates significant short-range S−S repulsion and probably compressive surfac...

A structural study of disordered sulfur overlayers on Cu(1 1 1)

The structure of disordered chemisorbed sulfur overlayers on Cu(1 1 1) have been investigated using normal incidence X-ray standing wavefield absorption (NIXSW). The NIXSW data indicates that within the coverage range studied, 0.13 < θ < 0.30 ML, all the disordered layers contain the same structural sub-units. The NIXSW data are consistent with a previously proposed model, which suggests that at room temperature the disordered layers contain both sulfur adatoms and mobile Cu 3S 3 clusters.

Self-assembly of an aromatic thiolate on Cu(1 0 0): The local adsorption site

The long-range ordered p(2 × 2) self-assembled monolayer formed by dosing 2-mercaptobenzoxazole (MBO, C 7H 5NOS) on the Cu(1 0 0) surface at room temperature has been structurally characterised by means of normal incidence X-ray standing wavefield absorption (NIXSW) at both (2 0 0) and (1 1 1) reflections. The results show that the S atom, through which the adsorbed molecular thiolate species bonds to the surface, has an interlayer spacing of 1.33 ± 0.05 Å relative to the extended bulk scatterer planes (2 0 0) parallel to the surface and occupies the four-fold coordinated hollow site, with no evidence of major reconstruction. These results are entirely consistent with previous findings for a number of thiolates formed on (1 0 0) noble metal surfaces. The role of the head-group to surface potential corrugation and intermolecular interactions in determining the adsorption site and ordering of this aromatic molecular adsorbate is discussed.

Non-dipole effects in high-energy photoelectron emission; identification and quantification using X-ray standing waves

The dipole approximation is routinely used in photoelectron emission studies of gas phase and condensed matter, and is generally thought to be adequate up to photon energies of tens of kiloelectron-volts. There is, however, now ample evidence from theory and experiment that this approximation routinely breaks down at much lower energies, in some cases as low as hundreds of electron-volts. The breakdown is most significant in influencing the angular dependence of photoelectron emission and was first identified in gas-phase experiments. A particular consequence is the introduction of a forward–backward asymmetry into the angular dependence relative to the direction of photon propagation. This effect also has a proved important in surface structure determination using photoelectron-monitored X-ray standing wavefield absorption, and a variant of this technique has been used to determine the quadrupolar asymmetry parameter values for 1s emission from many low atomic number atoms.

The bonding of acetone on Cu(1 1 1)

The structure and bonding of acetone on Cu(1 1 1) has been probed using a combination of temperature programmed desorption, UV photoelectron spectroscopy (UPS) and normal incidence X-ray standing wavefield absorption (NIXSW). Acetone was found to interact very weakly with Cu(1 1 1). This is evidenced by UPS data, which showed no significant perturbation of the electronic structure of acetone upon adsorption. NIXSW data is consistent with acetone adsorbing to the surface via the oxygen with an atop local registry in a η 1(O) configuration. The observed Cu–O distance of 2.73 ± 0.16 Å is close to the expected van der Waals separation (2.93 Å). The local registry determined in the present study is consistent with that predicted in previous theoretical work.

A TPD and NIXSW investigation of furan and tetrahydrofuran adsorption on Cu [formula omitted

Normal incidence X-ray standing wavefield absorption (NIXSW) and temperature programmed desorption (TPD) have been used to study the bonding of both furan and tetrahydrofuran (THF) on Cu(1 1 1). THF was found to adopt an atop adsorption site, with furan adopting a displaced atop position. We attribute this displacement to the effects of adsorbate–adsorbate interactions, the magnitude of which are significant compared to the strength of the furan to copper interaction. The adsorption of THF is significantly stronger than that of furan, which we propose is due to the influence of C–H⋯Cu interactions between THF and the copper surface.

The structure of methanol and methoxy on Cu(1 1 1)

The local registry of methoxy and methanol adsorbed on Cu(1 1 1) has been studied using normal incidence X-ray standing wavefield absorption (NIXSW). Methanol adsorbs to the surface with its oxygen atom directly above a Cu atom, with a Cu–O separation of 2.69 ± 0.19 Å. This distance is only slightly shorter than that expected for a purely van der Waals interaction (2.96 Å), which indicates that the molecule is only weakly chemisorbed to the surface. In contrast to methanol, methoxy is strongly chemisorbed on Cu(1 1 1). The NIXSW data can be modelled assuming methoxy adopts threefold hollow adsorption sites, in agreement with other studies. The NIXSW data is consistent with methoxy populating both FCC and HCP hollow sites, this contradicts previous work which suggests that the species only occupies FCC type hollows. The results of our studies have been compared with previous theoretical predictions for the structure of methanol and methoxy on Cu(1 1 1), and differences between experiment and theory have been highlighted.

The structure of disordered chemisorbed oxygen on Cu [formula omitted

The structure of a disordered chemisorbed oxygen layer on Cu(1 1 1) has been investigated using normal incidence X-ray standing wavefield absorption (NIXSW). The NIXSW data indicates that the oxygen atoms within the overlayer have more than one well-defined height with respect to the copper surface. The NIXSW data has been compared to previous proposed structural models for the disordered overlayer. We believe that the model that is most consistent with our data is the oxygen overlayer having a Cu 2O-like structure.

The structure of a coadsorbed layer of thiophene and CO on Cu([formula omitted

The influence of coadsorbed CO on the structure and bonding of thiophene (C4H4S), an aromatic heterocycle, on Cu(111) has been investigated using a combination of near edge X-ray absorption fine structure spectroscopy, normal incidence X-ray standing wavefield absorption (NIXSW) and temperature programmed desorption. The coadsorption of CO does not induce an ordering of the disordered chemisorbed thiophene layer. This lack of ordering contrasts with the behaviour of related benzene coadsorption systems where CO induces ordering of disordered layers. The results of NIXSW measurements show that CO and thiophene both adopt atop adsorption sites within the coadsorbed overlayer, the same site adopted by both in pure layers. The orientation of the thiophene within the coadsorbed layer is also similar to that adopted in a pure layer. However, the CO molecules within the coadsorbed layer are tilted, which contrasts with a linear geometry observed in pure CO layers of a similar coverage. It is suggested that the lack of any significant cooperative effects between the CO and thiophene within the coadsorbed overlayers is due to the relatively weak adsorbate–substrate interactions.

Chemical-shift X-ray standing wavefield determination of the local structure of methanethiolate phases on Ni( [formula omitted

By monitoring the X-ray absorption through the chemically-shifted components of the S 1s photoemission signal, normal-incidence X-ray standing wavefield absorption at the (1 1 1) and ( 1 ̄ 1 1) scatterer planes has been used to determine the local adsorption geometry of the two distinct methanethiolate (CH 3S–) species which occur on Ni(1 1 1) following exposure to methanethiol. The species which is favoured at low temperatures is found to occupy either mixed hollow or bridge sites on a non-reconstructed Ni(1 1 1) surface, whereas that seen at higher temperatures is shown to involve Ni surface layer reconstruction and the data are consistent with hollow site adsorption on a reduced density outermost Ni layer. The relative merits of alternative reconstruction models based on that which occurs due to methanethiolate adsorption on Cu(1 1 1), or the (5√3×2)rect. phase formed by atomic S on Ni(1 1 1), are discussed. Both of these models are based on local square or `pseudo-(1 0 0)' outermost Ni layers. Co-adsorbed atomic sulphur, to which the methanethiolate species decompose at higher temperatures, appears to occupy mainly fcc hollow sites at low temperatures, but is partially converted to the local geometry of the ordered reconstructed (5√3×2)rect.-S phase after higher temperature annealing.

Non-dipole effects in photoelectron-monitored X-ray standing wave experiments: characterisation and calibration

The results are reported of a series of experiments designed to characterise and calibrate the role of non-dipolar (notably electric quadrupole–dipole interference) effects in the angular distribution of photoelectron emission on structural studies of surfaces using normal incidence X-ray standing wavefield absorption. The values of the resulting backward/forward asymmetry parameter (relative to the photon propagation direction) have been measured for 1s emission at photon energies around 3 keV for first and second row elements from C to Cl, and are compared with recent theoretical determinations. The influence of additional non-dipole interference terms in the coherent X-ray standing wave has also been determined for the case of Al 1s emission and found to be weak, typically introducing systematic errors in atomic positions of only around 0.02–0.04 Å.

Effects of Substituents on the Structure and Bonding of Thiophene on Cu(111)

The local registry and orientation of chemisorbed 3-methylthiophene and 3-methoxythiophene on Cu(111) has been determined using normal incidence X-ray standing wavefield absorption and near edge X-ray absorption fine structure spectroscopy, respectively. 3-methylthiohene and 3-methoxythiophene have, within experimental error, identical local registries (atop), Cu−S separations (2.57 ± 0.03 and 2.55 ± 0.05 Å), and orientations (24 ± 4° and 26 ± 4°). These results are placed in context with previous studies of thiophene and 3-chlorothiophene and provide quantitative information on how simple substituents affect the adsorption structure of the thiophene ring.

The structure of acetate and trifluoroacetate on Cu(1 1 1)

The structure of acetate and 1,1,1-trifluoroacetate on Cu(1 1 1) has been determined using normal incidence X-ray standing wavefield absorption (NIXSW). Our results show that the carboxylate groups of both species bond in a bidentate form, with oxygen atoms adopting displaced atop positions on neighbouring copper atoms, and their heights above a non-relaxed copper surface measuring 1.81±0.03 and 1.91±0.05 Å for acetate and trifluoroacetate, respectively. These distances are very similar to those found in previous studies employing other surface structural techniques. In the case of trifluoroacetate the NIXSW data are consistent with a freely rotating CF 3 group. The structure of chemisorbed trifluoroacetate is identical, within experimental error, to that found in inorganic trifluoroacetate μ 2-copper complexes, that display considerable ionic bonding character. From the similarity between the structures of the observed surface species and the inorganic complexes we infer the existence of a significant ionic component in the chemisorption bonds.

A Complete Structural Study of the Coverage Dependence of the Bonding of Thiophene on Cu(111)

The influence of coverage on the structure and bonding of thiophene within chemisorbed overlayers on Cu(111) has been investigated. Normal incidence X-ray standing wavefield absorption (NIXSW) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS) were used to determine the adsorption site and orientation of thiophene. In the low coverage phase the thiophene molecule forms a π-bonded species, adopting an atop adsorption site with a Cu−S separation of 2.62 ± 0.03 A with the ring inclined by 26 ± 5°. On increasing coverage, thiophene undergoes a compressional phase transformation to a more weakly bound sulfur lone-pair-bonded species. In this new phase the thiophene still occupies an atop site but the Cu−S separation has increased by 0.2 A and the ring is now inclined at 44 ± 6° to the surface.

Adsorption Structures of 1-Octanethiol on Cu(111) Studied by Scanning Tunneling Microscopy

The interaction of 1-octanethiol with Cu(111) at room temperature in ultrahigh vacuum has been studied using scanning tunneling microscopy (STM). Two structural phases are observed, a “honeycomb” phase and a pseudo-(100) reconstructed surface phase, which are essentially equivalent to methanethiolate structures observed on Cu(111) in a recent study using STM. The pseudo-(100) structure can also be reconciled with the results of a prior study of this phase using normal-incidence X-ray standing wavefield absorption, which indicated the presence of adsorbate-induced reconstruction but was unable to establish further details of the local structure. We discuss the similarity of the behavior of octanethiol and methanethiol on Cu(111), contrast this with alkanethiol behavior on Au(111), and discuss the role of S−substrate interactions in the structures of these self-organized monolayers.

Atomic quadrupolar photoemission asymmetry parameters from a solid state measurement

A method for measuring the asymmetry parameter gamma arising from dipole-quadrupole interference in core-level photoemission is proposed which is based on condensed thin films and exploits the influence of this asymmetry in photoemission monitoring of x-ray standing wave field absorption. The high density of material in this method offers some advantages over a conventional gas-phase measurement. Results are presented for C, O, and F 1s photoemission at photon energies around 3 keV. The combined nondipole asymmetry factor (delta+gamma/3) for non- s-states can be determined in a similar way.

A structural study of methanethiolate adsorbed on Cu(100)

The interaction of methanethiol, CH3SH, with Cu(100) has been studied by S 1s photoemission, S K-edge near-edge x-ray absorption fine structure and normal-incidence standing x-ray wavefield absorption at both (200) and (111) reflections. The results indicate that a single methanethiolate species, CH3S-, is formed and this is bonded to the unreconstructed surface via the S atoms which adopt the fourfold symmetric hollow site. This conclusion confirms the results of a recent near-edge and surface extended x-ray absorption fine structure study, but contrasts with the established adsorbate-induced reconstruction produced by this species on Cu(111).

A structural study of formate on Cu(111)

We have performed the first structural study of formate adsorbed on Cu(111). The local registry of formate has been determined using normal incidence X-ray standing wavefield absorption (NIXSW). The oxygen atoms of the formate were located in atop sites, with NIXSW measurements made using the (1̄11) planes suggesting that they could be statically displaced by up to 0.33Å, and with the formate bridging two copper atoms. This adsorption site is identical to that found for formate on Cu(110) and Cu(100) by Woodruff and co-workers. The oxygen–(111) plane separation determined by NIXSW was 1.89Å. This would, assuming that the oxygen atoms are displaced by 0.33Å from a perfect atop position and no relaxation of the underlying substrate, infer a upper limit for the CuO separation of 1.92±0.04Å. This value compares with CuO separations of 1.98±0.04Å for formate on both Cu(100) and Cu(110), and the average value of 1.97Å observed for all known μ2-formate–copper complexes.

The structure of (√3×√3)R30° iodine on Pd (111) surface studied by normal incidence X-ray standing wavefield absorption

The surface structure of (√3×√3)R30° I-Pd (111) has been investigated by normal incidence X-ray standing wave absorption. The measurements show that the site of adsorption is at the three-fold fcc hollow, with a layer spacing of 2.16 Å above the surface. The location of the I atoms at the fcc hollow site is consistent with halogen adatom adsorption on the neighbouring fcc (111) transition metal surfaces.

Probing the adsorption structure of a multifunctional organic molecule: a NIXSW and NEXAFS study of 3-chlorothiophene on Cu(111)

The adsorption structure of 3-chlorothiophene on Cu(111) has been investigated using a combination of normal incidence X-ray standing wavefield absorption (NIXSW) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. For coverages up to saturation of the chemisorbed layer, the 3-chlorothiophene bonds through the S atom. The S atom adsorbs in an atop site with a Cu–S distance identical, within experimental error, to that observed for thiophene on the same substrate. From a combination of NEXAFS and NIXSW, thiophene was found to adsorb with the aromatic ring flat lying. From NIXSW measurements the S–Cl axis was found to be inclined by 12±2° from the surface. Whilst NEXAFS data suggested an orientation of 23±8° for the aromatic ring of 3-chlorothiophene. The Cl atom interacted only weakly with the substrate, with a Cl–surface distance longer than the Cu–Cl van der Waal separation.