A HREEL investigation of adsorption and dissociation of NO on a Rh(110) surface

The adsorption and dissociation of NO on a Rh(110) surface in the temperature range from 100 to 300 K has been studied by means of high-resolution electron energy loss (HREEL) spectroscopy. At 100 K only one adsorption state of NO, assigned to bridge-bonded NO species, is observed at the whole NO coverage range. The N-O stretching frequency of this species increases from 1560 to 1710 cm-1 with increasing NO coverage. NO decomposition, which occurs readily at temperatures above 170 K has been studied for NO coverages less than 0.3 of the saturated NO coverage at 100 K. The HREELS data have shown that the fraction of NO molecules which undergo dissociation increases with increasing temperature and with decreasing initial NO coverage. For the highest NO coverages considered (0.3 of saturation at 100 K) all NO molecules decompose at 240 K. A variety of loss features are observed in the HREEL spectra after decomposition of different amounts of NO. These HREEL data are explained on the basis of comparison with the HREEL spectra measured for oxygen, nitrogen and mixed oxygen and nitrogen layers on Rh(110). It has been established that the variety of loss features observed after dissociation of NO is due to different oxygen states on the surface. The observed effect of the dissociation products on the N-O stretching frequencies have heen discussed considering the factors that can account for the blue-shifts observed in the presence of electronegative surface modifiers

Strain relaxation in small adsorbate islands: O on W(110)

The stress-induced lattice changes in a p(1x2) ordered oxygen layer on W(110) are measured by low-energy electron diffraction. We have observed that small oxygen islands show a mismatch with the underlying lattice. Our results indicate that along [1-10] the average mismatch scales inversely with the island size as 1/L for all oxygen coverages up to 0.5 ML, while along [001] it is significant only for the smallest oxygen islands and scales as a higher power of the inverse island size. The behaviour along [1-10] is described by a one-dimensional finite-size Frenkel-Kontorova model. Using this model, together with calculated force constants, we make a quantitative estimate for the change of surface-stress upon oxygen adsorption. The result is consistent with our ab-initio calculations, which give a relative compressive stress of -4.72 N/m along [1-10] and a minute relative tensile stress of 0.15 N/m along [001]. The scaling along [001] is qualitatively explained as an effect induced by the lattice relaxation in the [1-10] direction.Comment: 22 pages, 5 figure

Stress engineering at the nanometer scale: Two-component adlayer stripes

Spontaneously formed equilibrium nanopatterns with long-range order are widely observed in a variety of systems, but their pronounced temperature dependence remains an impediment to maintain such patterns away from the temperature of formation. Here, we report on a highly ordered stress-induced stripe pattern in a two-component, Pd-O, adsorbate monolayer on W(110), produced at high temperature and identically preserved at lower temperatures. The pattern shows a tunable period (down to 16 nm) and orientation, as predicted by a continuum model theory along with the surface stress and its anisotropy found in our DFT calculations. The control over thermal fluctuations in the stripe formation process is based on the breaking/restoring of ergodicity in a high-density lattice gas with long-range interactions upon turning off/on particle exchange with a heat bath.Comment: 6 pages, 4 figure

Direct mapping of 19F in 19FDG-6P in brain tissue at subcellular resolution using soft X-ray fluorescence

Low energy x-ray fluorescence (LEXRF) detection was optimized for imaging cerebral glucose metabolism by mapping the fluorine LEXRF signal of 19 F in 19 FDG, trapped as intracellular 19 F-deoxyglucose-6-phosphate ( 19 FDG-6P) at 1μm spatial resolution from 3μm thick brain slices. 19 FDG metabolism was evaluated in brain structures closely resembling the general cerebral cytoarchitecture following formalin fixation of brain slices and their inclusion in an epon matrix. 2-dimensional distribution maps of 19 FDG-6P were placed in a cytoarchitectural and morphological context by simultaneous LEXRF mapping of N and O, and scanning transmission x-ray (STXM) imaging. A disproportionately high uptake and metabolism of glucose was found in neuropil relative to intracellular domains of the cell body of hypothalamic neurons, showing directly that neurons, like glial cells, also metabolize glucose. As 19 F-deoxyglucose-6P is structurally identical to 18 F-deoxyglucose-6P, LEXRF of subcellular 19 F provides a link to in vivo 18 FDG PET, forming a novel basis for understanding the physiological mechanisms underlying the 18 FDG PET image, and the contribution of neurons and glia to the PET signal

Spectroscopic link between adsorption site occupation and local surface chemical reactivity

In this Letter we show that sequences of adsorbate-induced shifts of surface core level (SCL) x-ray photoelectron spectra contain profound information on surface changes of electronic structure and reactivity. Energy shifts and intensity changes of time-lapsed spectral components follow simple rules, from which adsorption sites are directly determined. Theoretical calculations rationalize the results for transition metal surfaces in terms of the energy shift of the d-band center of mass and this proves that adsorbate-induced SCL shifts provide a spectroscopic measure of local surface reactivity

Seeded x-ray free-electron laser generating radiation with laser statistical properties

The invention of optical lasers led to a revolution in the field of optics and even to the creation of completely new fields of research such as quantum optics. The reason was their unique statistical and coherence properties. The newly emerging, short-wavelength free-electron lasers (FELs) are sources of very bright coherent extreme-ultraviolet (XUV) and x-ray radiation with pulse durations on the order of femtoseconds, and are presently considered to be laser sources at these energies. Most existing FELs are highly spatially coherent but in spite of their name, they behave statistically as chaotic sources. Here, we demonstrate experimentally, by combining Hanbury Brown and Twiss (HBT) interferometry with spectral measurements that the seeded XUV FERMI FEL-2 source does indeed behave statistically as a laser. The first steps have been taken towards exploiting the first-order coherence of FELs, and the present work opens the way to quantum optics experiments that strongly rely on high-order statistical properties of the radiation.Comment: 24 pages, 10 figures, 37 reference

Oxidation of methanol on Ru catalyst: Effect of the reagents partial pressures on the catalyst oxidation state and selectivity

In situ core level photoelectron spectroscopy and mass spectrometry have been utilized to study the methanol oxidation on a model RuO2 catalyst at pressures ranging from 10-6 to 10-1 mbar. The experiments were carried out varying the O2/CH3OH molecular mixing ratio from 0.25 to 3.3 and the reaction temperature from 350 to 720 K. The Ru 3d5/2 and O 1s core level spectra were used to characterise the dynamic changes in the Ru oxidation state by exposing the oxide pre-catalyst to different reagents partial pressures and temperatures. Full oxidation to CO2 + H2O or partial oxidation to CO + H2O + H2 have been observed in the whole pressure range for specific reaction conditions, which preserve the oxide catalyst state or reduce the oxide to metallic Ru. The selective oxidation to formaldehyde is observed only at pressures in the 10-1 mbar range, catalyzed by a RuO_x surface oxide formed by partial reduction of the oxide pre-catalyst

Interaction of magnetic nanoparticles with U87MG cells studied by synchrotron radiation X-ray fluorescence techniques

International audienceSynchrotron radiation (SR) X-ray microscopy combined with X-ray fluorescence (XRF) microspectroscopy provides unique information that have pushed the frontiers of biological research, particularly when investigating intracellular mechanisms. This work reports an SR-XRF microspectroscopy investigation on the distribution and the potential toxicity of Fe 2 O 3 and CoFe 2 O 4 nanoparticles (NPs) in U87MG glioblastoma-astrocytoma cells. The U87MG cells exposed to NPs concentrations ranging from 5 to 250 mg/ml for 24 h were analyzed in order to monitor both morphological and chemical changes. The SR-XRF maps complemented with XRM absorption and phase contrast images have revealed different intracellular distribution patterns for the two nanoparticles types allowing different mechanism of toxicity to be deduced

Synchrotron soft X-ray imaging and fluorescence microscopy reveal novel features of asbestos body morphology and composition in human lung tissues

Background: Occupational or environmental exposure to asbestos fibres is associated with pleural and parenchymal lung diseases. A histopathologic hallmark of exposure to asbestos is the presence in lung parenchyma of the so-called asbestos bodies. They are the final product of biomineralization processes resulting in deposition of endogenous iron and organic matter (mainly proteins) around the inhaled asbestos fibres. For shedding light on the formation mechanisms of asbestos bodies it is of fundamental importance to characterize at the same length scales not only their structural morphology and chemical composition but also to correlate them to the possible alterations in the local composition of the surrounding tissues. Here we report the first correlative morphological and chemical characterization of untreated paraffinated histological lung tissue samples with asbestos bodies by means of soft X-ray imaging and X-Ray Fluorescence (XRF) microscopy, which reveals new features in the elemental lateral distribution. Results: The X-ray absorption and phase contrast images and the simultaneously monitored XRF maps of tissue samples have revealed the location, distribution and elemental composition of asbestos bodies and associated nanometric structures. The observed specific morphology and differences in the local Si, Fe, O and Mg content provide distinct fingerprints characteristic for the core asbestos fibre and the ferruginous body. The highest Si content is found in the asbestos fibre, while the shell and ferruginous bodies are characterized by strongly increased content of Mg, Fe and O compared to the adjacent tissue. The XRF and SEM-EDX analyses of the extracted asbestos bodies confirmed an enhanced Mg deposition in the organic asbestos coating. Conclusions: The present report demonstrates the potential of the advanced synchrotron-based X-ray imaging and microspectroscopy techniques for studying the response of the lung tissue to the presence of asbestos fibres. The new results obtained by simultaneous structural and chemical analysis of tissue specimen have provided clear evidence that Mg, in addition to Fe, is also involved in the formation mechanisms of asbestos bodies. This is the first important step to further thorough investigations that will shed light on the physiopathological role of Mg in tissue response to the asbestos toxicity