The adsorption structure of furan on Pd(1 1 1)

The structure of molecular furan, C4H4O, on Pd(1 1 1) has been investigated by O K-edge near-edge X-ray absorption fine structure (NEXAFS) and C 1s scanned-energy mode photoelectron diffraction (PhD). NEXAFS shows the molecule to be adsorbed with the molecular plane close to parallel to the surface, a conclusion confirmed by the PhD analysis. Chemical-state specific C 1s PhD data were obtained for the two inequivalent C atoms in the furan, the α-C atoms adjacent to the O atom, and the β-C atoms bonded only to C atoms, but only the PhD modulations for the α-C emitters were of sufficiently large amplitude for detailed evaluation using multiple scattering calculations. This analysis shows the α-C atoms to be located approximately 0.6 Å off-atop surface Pd atoms with an associated C–Pd bondlength of 2.13 ± 0.03 Å. Two alternative local geometries consistent with the data place the O atom in off-atop or near-hollow locations, and for each of these local structures there are two equally-possible registries relative to the fcc and hcp hollow sites. The results are in good agreement with earlier density functional theory calculations which indicate that the fcc and hcp registries are equally probable, but the PhD results fail to distinguish the two distinct local bonding geometries

A structural study of a C3H3 species coadsorbed with CO on Pd(1 1 1)

The combination of chemical-state-specific C 1s scanned-energy mode photoelectron diffraction (PhD) and O K-edge near-edge X-ray absorption fine structure (NEXAFS) has been used to determine the local adsorption geometry of the coadsorbed C3H3 and CO species formed on Pd(1 1 1) by dissociation of molecular furan. CO is found to adopt the same geometry as in the Pd(1 1 1)c(4 × 2)-CO phase, occupying the two inequivalent three-fold coordinated hollow sites with the C–O axis perpendicular to the surface. C3H3 is found to lie with its molecular plane almost parallel to the surface, most probably with the two ‘outer’ C atoms in equivalent off-atop sites, although the PhD analysis formally fails to distinguish between two distinct local adsorption sites

Adsorption and temperature-dependent decomposition of SO₂ on Cu(100) and Cu(111): A fast and high-resolution core-level spectroscopy study

The adsorption and temperature-dependent decomposition of SO2 on Cu(100) and Cu(111) have been studied by fast and high-resolution core-level photoemission. The analysis of the S 2p and O 1s data shows that molecular SO2 adsorption dominates at 170 K. On heating the SO2-covered surfaces to about room temperature, SO2 decomposes into SO+O+S. On further heating SO+O recombine to form SO2, which is the only species detected in corresponding temperature-programmed desorption (TPD) experiments. From the temperature- (time-) dependent S and O coverages a ‘‘TPD curve’’ can be constructed

The local adsorption structure of benzene on Si(001)-(2 × 1): a photoelectron diffraction investigation

Scanned-energy mode C 1s photoelectron diffraction has been used to investigate the local adsorption geometry of benzene on Si(001) at saturation coverage and room temperature. The results show that two different local bonding geometries coexist, namely the 'standard butterfly' (SB) and 'tilted bridge' (TB) forms, with a composition of 58 ± 29% of the SB species. Detailed structural parameter values are presented for both species including Si–C bond lengths. On the basis of published measurements of the rate of conversion of the SB to the TB form on this surface, we estimate that the timescale of our experiment is sufficient for achieving equilibrium, and in this case our results indicate that the difference in the Gibbs free energy of adsorption, ΔG(TB)−ΔG(SB), is in the range −0.023 to +0.049 eV. We suggest, however, that the relative concentration of the two species may also be influenced by a combination of steric effects influencing the kinetics, and a sensitivity of the adsorption energies of the adsorbed SB and TB forms to the nature of the surrounding benzene molecules

Photoelectron diffraction investigation of the structure of the clean TiO2(110)(1×1) surface

The surface relaxations of the rutile TiO2(110)(1×1) clean surface have been determined by O 1 s and Ti 2p3∕2 scanned-energy mode photoelectron diffraction. The results are in excellent agreement with recent low-energy electron diffraction (LEED) and medium energy ion scattering (MEIS) results, but in conflict with the results of some earlier investigations including one by surface x-ray diffraction. In particular, the bridging O atoms at the surface are found to relax outward, rather than inward, relative to the underlying bulk. Combined with the recent LEED and MEIS results, a consistent picture of the structure of this surface is provided. While the results of the most recent theoretical total-energy calculations are qualitatively consistent with this experimental consensus, significant quantitative differences remain

Kindlin-3 maintains marginal zone B cells but confines follicular B cell activation and differentiation

Integrin-mediated interactions between hematopoietic cells and their microenvironment are important for the development and function of immune cells. Here, the role of the integrin adaptor Kindlin-3 in B cell homeostasis is studied. Comparing the individual steps of B cell development in B cell-specific Kindlin-3 or alpha4 integrin knockout mice, we found in both conditions a phenotype of reduced late immature, mature, and recirculating B cells in the bone marrow. In the spleen, constitutive B cell-specific Kindlin-3 knockout caused a loss of marginal zone B cells and an unexpected expansion of follicular B cells. Alpha4 integrin deficiency did not induce this phenotype. In Kindlin-3 knockout B cells VLA-4 as well as LFA-1-mediated adhesion was abrogated, and short-term homing of these cells in vivo was redirected to the spleen. Upon inducible Kindlin-3 knockout, marginal zone B cells were lost due to defective retention within 2 weeks, while follicular B cell numbers were unaltered. Kindlin-3 deficient follicular B cells displayed higher IgD, CD40, CD44, CXCR5, and EBI2 levels, and elevated PI3K signaling upon CXCR5 stimulation. They also showed transcriptional signatures of spontaneous follicular B cell activation. This activation manifested in scattered germinal centers in situ, early plasmablasts differentiation, and signs of IgG class switch

The dimers stay intact: a quantitative photoelectron study of the adsorption system Si{100} (2x1)-C2H4

Using the technique of photoelectron diffraction in the scanned energy mode we show that the Si dimer separation on the Si{100} surface following the adsorption of ethene (ethylene) is 2.36(±0.21) A. This value is only very slightly larger than on the clean surface and shows that the dimer remains intact, thus providing a clear quantitative experimental resolution of a long controversy in the literature. The C-C and C-Si separations are 1.62±0.08 A and 1.90±0.01 A, respectively, the former indicating a bond order of less than one

SO₂-induced surface reconstruction of Cu(111): An x-ray-absorption fine-structure study

The interaction of SO2 with Cu(111) has been studied by x-ray-absorption fine-structure measurements above both the oxygen and sulfur K edge. The combined analysis of the data for the (SO2+O) phase formed on adsorption of SO2 at 170 K shows that the S atoms are located in hollow sites on a locally (100) reconstructed surface with a S-Cu bond length of 2.32±0.05Å. The SO2 molecular plane is oriented perpendicular to the surface. Within the plane the C2 axis is tilted with respect to the surface normal resulting in additional bonding to the substrate through one of the oxygen atoms. The corresponding O-Cu bond length measures 2.14±0.05Å. Due to bonding through only one of the oxygen atoms two different intramolecular bond lengths, 1.43±0.03 and 1.48±0.03Å, within the same molecule are observed. The coadsorbed atomic oxygen is located in bridge sites with a O-Cu bond length of 2.02±0.05Å. These results are consistent with a structural model where (100) units form “clock” reconstruction

Surface melting of Al(110) studied by surface extended X-ray absorption fine structure

Surface extended X-ray absorption fine structure (SEXAFS) measurements were used to study the surface melting of Al(110). The measurements were performed between 100 K and the bulk melting temperature Tm on Al(110) as well as on the reference system Al(111) which does not show surface melting. Based on a cumulant-expansion analysis of the SEXAFS data, surface melting of Al(110) was observed via an anisotropy of the Debye–Waller factor. This anisotropy can be explained by a residual order of the quasi-liquid which consists of intact rows or segments of them moving with liquid-like mobilities. The measurements also yield an Einstein temperature of 317 K and allow us to determine a linear thermal expansion coefficient of (2.7±0.5)×10−5 K−1