Fluorine surface speciation in South Andean volcanic ashes

Fresh volcanic ash samples collected during the eruptions of the Hudson (1991), Chaitén (2008), Puyehue (2011), Calbuco (2015) and Copahue (2016) volcanoes were analyzed in order to identify the fluorine (F) bearing phases. XPS and HR-TEM was used to determine the solid speciation of F. In order to decrease the surface differential charging of samples, XPS data have been collected using an electron flood gun during spectral acquisition. Ash leaching at different pH conditions was also performed with the aim of understanding the factors that control the fluoride release to aqueous reservoirs. The results indicates that in the studied Patagonian ashes, F is concentrated onto the ash surface, likely in the form of Fsingle bondNa and Fsingle bondCa salts but also within the glass lattice forming Fsingle bondSi and Fsingle bondAl bonds. Although the main F-source in the studied volcanic ashes is the aluminosilicate glass, this phase is stable at the neutral pH that dominates the aqueous reservoirs, thus the contribution to the pool of dissolved fluoride is minor. Higher contributions are associated with soluble fluoride salts. In addition, the presence of coatings of Fsingle bondCa compounds inhibits the release of fluoride within the volcanic glass lattice, at least during the initial stages of the ash-water interaction.Fil: Bia, Gonzalo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Borgnino Bianchi, Laura Carolina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Zampieri, Guillermo Enrique. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche | Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Garcia, Maria Gabriela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentin

Band dispersion in the deep 1s core level of graphene

Chemical bonding in molecules and solids arises from the overlap of valence electron wave functions, forming extended molecular orbitals and dispersing Bloch states, respectively. Core electrons with high binding energies, on the other hand, are localized to their respective atoms and their wave functions do not overlap significantly. Here we report the observation of band formation and considerable dispersion (up to 60 meV) in the $1s$ core level of the carbon atoms forming graphene, despite the high C $1s$ binding energy of $\approx$ 284 eV. Due to a Young's double slit-like interference effect, a situation arises in which only the bonding or only the anti-bonding states is observed for a given photoemission geometry.Comment: 12 pages, 3 figures, including supplementary materia

Atomic Sulfur Formation Mechanism on 3‑Mercaptopropanoic Acid Derivative Self-Assembled Monolayers: Understanding the C–SBond Cleavage

Self-assembled monolayers (SAMs) of ω-carboxylic acid thiols are very important in the surface modification of metals, especially on gold surfaces. Indeed, the 3-mercaptopropanoic acid (MPA) and its ester or amide derivatives are widely used for SAM-based sensor design. It was already shown that MPA does not suffer C−S bond scission when adsorbed on Au. On the other hand, in this work we demonstrate that its simplest derivative, methyl 3-mercapto propionate (Me-MPA), is prone to form significant amounts of atomic sulfur when adsorbed on Au. The MPA derivatives are more sensible than MPA itself to alkaline solutions, and its SAM-based sensors will rapidly degrade given atomic sulfur. In this work, we study the simplest MPA derivative Me-MPA SAMs on preferentially oriented Au(111) surfaces by XPS and electrochemical measurements. It was found that the desulfuration of Me-MPA depends on its preparation conditions (grown from ethanol or toluene solution) and on its post-treatment with alkaline solution. In order to explain the S−C bond scission on Me-MPA SAMs, we discuss different reaction mechanisms. We concluded that the reaction mechanism involves an E1cB elimination pathway (β-elimination). This reaction mechanism also explains the desulfuration behavior of other important related molecules like L-cysteine and glutathione.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Electrochemical Self-Assembly of Alkanethiolate Molecules on Ni(111) and Polycrystalline Ni Surfaces

In this work, the electrochemical formation of alkanethiolate self-assembled monolayers (SAMs) on Ni(111) and polycrystalline Ni surfaces from alkanethiol-containing aqueous 1 M NaOH solutions was studied by combining Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), electrochemical techniques, and density functional theory (DFT) calculations. Results show that alkanethiolates adsorb on Ni concurrent with NiO electroreduction. The resulting surface coverage depends on the applied potential and hydrocarbon chain length. Electrochemical and XPS data reveal that alkanethiolate electroadsorption at room temperature takes place without S-C bond scission, in contrast to previous results from gas-phase adsorption. A complete and dense monolayer, which is stable even at very high cathodic potentials (-1.5 V vs SCE), is formed for dodecanethiol. DFT calculations show that the greater stability against electrodesorption found for alkanethiolate SAMs on Ni, with respect to SAMs on Au, is somewhat related to the larger alkanethiolate adsorption energy but is mainly due to the larger barrier to interfacial electron transfer present in alkanethiolate-covered Ni. A direct consequence of this work is the possibility of using electrochemical self-assembly as a straightforward route to build stable SAMs of long-chained alkanethiolates on Ni surfaces at room temperature.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Estudio comparativo de la adsorción de S sobre Au(111) y Ag(111) con XPS, AES, TOF-DRS y LEED

Here we study the adsorption of S on the (111) Surface of the noble metals Au and Ag using XPS, AES, LEED and TOF-DRS. Despite the similarities of the two surfaces, all the techniques showed different behaviours in both surfaces from the beginning of the adsorption. XPS and AES were used to determine the dependence of the S coverage with dosing time, whereas LEED allowed us to identify the ordered phases of the S/Au and S/Ag systems. The S coverage in Ag grows continuously with dose, while for Au there is saturation of the S signals in q ~ 0.5ML. Analysis of the intensity and shape of the TOF-DRS signals provide evidence about the coexistence of Ag atoms with S atoms in the top most layer for all coverages, whereas in Au these effects can be discarded up to q ~ 0.3ML.Fil: Tosi, Ezequiel. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Ruano, G.. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Salazar Alarcón, Leonardo. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Martiarena, María Luz. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Sánchez, Esteban Alejandro. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Grizzi, Oscar. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Zampieri, Guillermo Enrique. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Periodic signals from the Circinus region: two new cataclysmic variables and the ultraluminous X-ray source candidate GC X-1

The examination of two 2010 Chandra ACIS (Advanced CCD Imaging Spectrometer) exposures of the Circinus galaxy resulted in the discovery of two pulsators: CXO J141430.1-651621 and CXOU J141332.9-651756. We also detected 26 ks pulsations in CG X-1, consistently with previous measures. For ̃40 other sources, we obtained limits on periodic modulations. In CXO J141430.1-651621, which is ̃2 arcmin outside the Circinus galaxy, we detected signals at 6120 ± 1 s and 64.2 ± 0.5 ks. In the longest observation, the source showed a flux of abut 1.1 × 10^-13 erg/cm^2/ (absorbed, 0.5-10 keV) and the spectrum could be described by a power law with photon index Γ ≃ 1.4. From archival observations, we found that the luminosity is variable by ≈50 per cent on time-scales of weeks to years. The two periodicities pin down CXO J141430.1-651621 as a cataclysmic variable of the intermediate polar subtype. The period of CXOU J141332.9-651756 is 6378 ± 3 s. It is located inside the Circinus galaxy, but the low absorption indicates a Galactic foreground object. The flux was ≈5 × 10^-14 erg/cm^2/s in the Chandra observations and showed ≈50 per cent variations on weekly/yearly scales; the spectrum is well fitted by a power law with Γ ≃ 0.9. These characteristics and the large modulation suggest that CXOU J141332.9-651756 is a magnetic cataclysmic variable, probably a polar. For CG X-1, we show that if the source is in the Circinus galaxy, its properties are consistent with a Wolf-Rayet (WR) plus black hole (BH) binary. We consider the implications of this for ultraluminous X-ray sources and the prospects of Advanced LIGO and Virgo. In particular, from the current sample of WR-BH systems, we estimate an upper limit to the detection rate of stellar BH-BH mergers of ̃16 y^-1

The ULTraS project: Understanding the X-ray variable and transient sky

Time variability is ubiquitous in the X-ray sky and is a unique probe of a wide diversity of astrophysical processes. The aim of the ULTraS (Understanding the Xray variable and transient sky) project is to exploit the scientific results and products from the EU/FP7-funded project EXTraS (a systematic characterisation of time variability in all XMM-Newton sources) and from its more recent developments. In particular, we have addressed a series of science cases, from the search for rare phenomena to the characterization of the properties of source classes, on a large variety of astrophysical objects, ranging from nearby, ultracool stars to exploding supernovae in distant galaxies. Results described here prove the effectiveness and the potential for discovery of our systematic temporal characterization. A full exploitation of the temporal information in the XMM-Newton/EXTraS archive requires a different approach, based on artificial intelligence – this seems definitely the perspective we should adopt to cope with the high data volume expected from future X-ray facilitie

Notas de Clase de Física de Superficies

Se analizan los aspectos más importantes de la estructura cristalina y estructura electrónica en superficies ordenadas puras (i.e. sin adsorbatos)