Photoelectron diffraction determination of adsorbate structures

Scanned-energy mode photoelectron diffraction (PhD) is a well-known method to determine quantitatively the local structure of adsorbates at surfaces. In this thesis, it has been employed to determine the adsorption site of a selection of molecules on surfaces. The adsorption on Cu(110), of methoxy (CH3O), an intermediate in the catalytic decomposition of methanol (CH3OH), has been studied. O 1s PhD spectra show the strongest modulation at 30° and 40° polar emission angles, both in the [1 1 0] azimuth, which is consistent with a bridge position adsorption site in the [1 1 0] azimuth. The subsequent analysis, as well as parallel DFT studies, confirms two bridge adsorption sites, with different bond lengths to the underneath copper atoms. A tilt of the molecules of 37° in the [1 1 0] azimuth is also observed, with the carbon atoms pointing in opposite directions for every adsorption site. This tilt creates a zig-zag model, which fits with an old STM [1] study. Formate (HCOO), a surface intermediate of the catalytic decomposition of formic acid (HCOOH), has been studied on two different faces of copper, Cu(110) and Cu(111). Although the adsorption sites obtained for both surfaces is similar, namely a short-bridge site slightly off atop, a significant difference of = 0.1 Å in the copper-oxygen bond lengths is found, being 1.99 Å for Cu(111) and 1.90 Å for Cu(110). In this thesis, it is demonstrated that it is possible, though very challenging, to perform PhD successfully under higher pressures. Methanol oxidation on Cu(110) has been studied under reaction conditions. At temperatures below = 450 K, the adsorption sites of methoxy and formate, the most important surface intermediates of this reaction, have been proved to be similar as in the previous studies performed in ultra high vacuum. A recent investigation of two different organic molecules, azobenzene (C12H10N2) and aniline (C6H7N), on rutile TiO2(110) and anatase TiO2(101) surfaces with scanning tunneling microscopy (STM) [2] indicates that both molecules lead the formation of the same superstructure, believed to be of a common species, phenyl imide (C6H5N). PhD has been exploited to determine the local adsorption site of adsorbed species formed by both molecules on rutile TiO2(110). N 1s photoelectron diffraction data are almost identical for both molecules, providing further support for a common surface species with the same, or a closely similar. Additional NEXAFS results support these results, implying that the local adsorption site of azobenzene and aniline is indeed the same. PhD results, which show the largest modulation amplitude at normal emission, suggests that the phenyl imide bonds via the N atoms atop a five-fold coordinated surface Ti atom, with the molecular plane tilted with respect to the surface normal, with a N-Ti bond length of 1.77 Å. [1] F. Leibsle, S. Francis, S.Haq, and M. Bowker, Aspects of formaldehyde synthesis on Cu(110) as studied by STM, Surf. Sci. 318, 46 (1994). [2] S.-C. Li, and U. Diebold, Reactivity of TiO2 rutile and anatase surfaces toward nitroaromatics, J. Am. Chem. Soc. 134, 64 (2010)

Quantitative adsorbate structure determination under catalytic reaction conditions

Current methods allow quantitative local structure determination of adsorbate geometries on surfaces in ultrahigh vacuum (UHV) but are incompatible with the higher pressures required for a steady-state catalytic reactions. Here we show that photoelectron diffraction can be used to determine the structure of the methoxy and formate reaction intermediates during the steady-state oxidation of methanol over Cu(110) by taking advantage of recent instrumental developments to allow near-ambient pressure x-ray photoelectron spectroscopy. The local methoxy site differs from that under static UHV conditions, attributed to the increased surface mobility and dynamic nature of the surface under reaction conditions

Qualification of piezo-electric actuators for the MADMAX booster system at cryogenic temperatures and high magnetic fields

We report on the qualification of a piezo-based linear stage for the manipulation of positions of dielectric discs in the booster of the MADMAX axion dark matter search experiment. A first demonstrator of the piezo drives, specifically developed for MADMAX, was tested at room temperature as well as at cryogenic temperatures down to 4.5 K and inside strong magnetic fields up to 5.3 T. These qualification measurements prove that the piezo-based linear stage is suited for MADMAX and fulfills the requirements.Comment: 13 pages, 9 figures, 2 table

Identifying the Azobenzene/Aniline reaction intermediate on TiO2-(110) : a DFT Study

Density functional theory (DFT) calculations, both with and without dispersion corrections, have been performed to investigate the nature of the common surface reaction intermediate that has been shown to exist on TiO2(110) as a result of exposure to either azobenzene (C6H5N═NC6H5) or aniline (C6H5NH2). Our results confirm the results of a previous DFT study that dissociation of azobenzene into two adsorbed phenyl imide (C6H5N) fragments, as was originally proposed, is not energetically favorable. We also find that deprotonation of aniline to produce this surface species is even more strongly energetically disfavored. A range of alternative surface species has been considered, and while dissociation of azobenzene to form surface C6H4NH species is energetically favored, the same surface species cannot form from adsorbed aniline. On the contrary, adsorbed aniline is much the most stable surface species. Comparisons with experimental determinations of the local adsorption site, the Ti–N bond length, the molecular orientation, and the associated C 1s and N 1s photoelectron core level shifts are all consistent with the DFT results for adsorbed aniline and are inconsistent with other adsorbed species considered. Possible mechanisms for the hydrogenation of azobenzene required to produce this surface species are discussed

Uracil on Cu(110) : a quantitative structure determination by energy-scanned photoelectron diffraction

The local adsorption site of the nucleobase uracil on Cu(110) has been determined quantitatively by energy-scanned photoelectron diffraction (PhD). Qualitative inspection of the O 1s and N 1s soft x-ray photoelectron spectra, PhD modulation spectra, and O K-edge near-edge x-ray adsorption fine structure indicate that uracil bonds to the surface through its nitrogen and oxygen constituent atoms, each in near atop sites, with the molecular plane essentially perpendicular to surface and aligned along the close packed [1 (1) over bar0] azimuth. Multiple scattering simulations of the PhD spectra confirm and refine this geometry. The Cu-N bondlength is 1.96 +/- 0.04 angstrom, while the Cu-O bondlengths of the two inequivalent O atoms are 1.93 +/- 0.04 angstrom and 1.96 +/- 0.04 angstrom, respectively. The molecule is twisted out of the [1 (1) over bar0] direction by 11 +/- 5 degrees

V-doped TiO2(110) : quantitative structure determination using energy scanned photoelectron diffraction

The surface structure of a novel vanadium-titanium dioxide epitaxial film (Ti1-xVxO2, x ~ 0.2) has been explored using V4+ 2p and Ti4+ 2p energy-scanned photoelectron diffraction (PhD). The determined structure is a rutile TiO2(110)-like surface, with V atoms substitutionally replacing some Ti atoms. The results show no evidence for significant preferential occupation by V atoms of any specific surface or sub-surface sites. LEED shows a (1x2) reconstruction to be present on the surface, and the PhD simulations do favour this being the dominant surface termination, although the reliability factor for simulations for a (1x1) termination falls just within the variance of the value for the preferred (1x2) structure. The V3+ and Ti3+ species were observed to occupy the same sites as the V4+ and Ti4+ species; V5+ species do not appear to occupy a single well-defined structural site

Does methanol produce a stable methoxy species on Ru(0001) at low temperatures?

Soft X-ray photoelectron spectroscopy (SXPS) and energy-scanned photoelectron diffraction (PhD) have been used to study the surface species produced by exposure of Ru(0001) to methanol at ~ 150 K. SXPS shows a single surface species is formed at sub-monolayer coverages with an O 1s peak binding energy of ~ 532.6 eV, 2.8 eV greater than that of chemisorbed atomic oxygen. O 1s PhD data from this species shows no significant modulations, in contrast to simulated PhD spectra from a methoxy species occupying a three-fold coordinated hollow site, as predicted by earlier density functional theory calculations, or atop or bridging sites. By contrast, PhD data from the O 1s of the atomic oxygen species in the Ru(0001)(2 × 1)–O phase are consistent with the oxygen atoms occupying ‘hcp’ hollow sites (above second-layer Ru atoms) at a RuO bondlength of 2.01 ± 0.02 Å, essentially identical to previous structure determinations of this phase. O 1s PhD recorded at normal emission from adsorbed CO are also consistent with the known CO atop adsorption species. We conclude that the methanol-derived surface molecular species is not methoxy in a well-defined local site on the surface, but is consistent with clusters of intact methanol identified in a recent infrared spectroscopy investigation