Local symmetry in liquid metals probed by x-ray absorption spectroscopy

The nature of local point symmetry in simple monoatomic liquids has been a fundamental open question for almost 40 years of computational and experimental studies. We present original results of local ordering in different monoatomic liquids obtained by x-ray absorption spectroscopy (XAS), exploiting its high sensitivity to short-range ordering. We used Reverse Monte Carlo (RMC) analysis (RMC-GNXAS package) for obtaining structural models compatible with XAS experimental data (and diffraction techniques for long-range ordering). Pair, bond-angle distributions and suitable indicators for point symmetry are calculated for different liquid metals

Microstructural defects in nanocrystalline iron probed by x-ray-absorption spectroscopy

An EXAFS (extended x-ray-absorption fine structure) study of nanocrystalline iron prepared by high-energy ball milling is presented. Accurate data analysis has been performed using a recently developed ab initio multiple-scattering method (GNXAS). The crystalline iron EXAFS spectrum, taken as reference, has been analyzed taking proper account of the important multiple-scattering signal. Results of the data analysis are shown to be in good agreement with known structural values. The dramatic decrease of the first-neighbor coordination number found in nanocrystalline iron, as a function of the milling time, is shown to be related to the presence of a large defect density. The corresponding decrease of the coordination numbers of the second and further neighbors, as well as the significant decrease of the multiplicities of the three-atom configurations, confirms this interpretation. A simple model which takes into account the presence of atomic defects is shown to lead to marked reduction of coordination numbers and three-atom multiplicities in agreement with the experimental data. Selected models without lattice defects are shown to be unable to give a quantitative explanation of the intensity reduction of the EXAFS signal found in milled nanocrystalline iron. An in situ annealing of the sample (up to about 1100 °C) milled for the longer time (32 h) is shown to reduce considerably the density of the defects, as expected, although no complete recovering of the crystalline order is found. The transition from α iron to γ iron has been directly observed for such nanocrystalline material

Short-range order of compressed amorphous GeSe2

The structure of amorphous GeSe2 (a-GeSe2) has been studied by means of a combination of two-edges X-ray absorption spectroscopy (XAS) and angle-dispersive X-ray diffraction under pressures up to about 30 GPa. Multiple-edge XAS data-analysis of a-GeSe2 at ambient conditions allowed us to reconstruct and compare the first-neighbor distribution function with previous results obtained by neutron diffraction with isotopic substitution. GeSe2 is found to remain amorphous up to the highest pressures attained, and a reversible 1.5 eV red-shift of the Ge K-edge energy indicating metallization, occurs between 10 GPa and 15 GPa. Two compression stages are identified by XAS structure refinement. First, a decrease of the first-neighbor distances up to about 10 GPa, in the same pressure region of a previously observed breakdown of the intermediate-range order. Second, an increase of the Ge-Se distances, bond disorder, and of the coordination number. This stage is related to a reversible non-isostructural transition involving a gradual conversion from tetra- to octa-hedral geometry which is not yet fully completed at 30 GPa

Saturation phenomena for ultra short Free Electron Laser pulses

Saturable absorption was recently observed in transmission measurements above the LII,III edge of pure Al thin films using ultra short x-ray pulses at a free-electron-laser (FEL) facility . [1] The high fluence reachable by FEL pulses, the shortness of the pulse duration, and the typical lifetime of the excited state, are all important factors enabling observation of the phenomenon. We devised a simplified theoretical model describing the saturation phenomenon using a three- channel model containing ground, excited and relaxed states. This phenomenological model explicitly includes the interaction between the solid and photon field in semi cl assical way, and the resulting non-linear coupled equation is solved numerically. We successfully applied this model to recent experimental results obtained using FEL radiation. [2,3

Band Gap Implications on Nano-TiO2 Surface Modification with Ascorbic Acid for Visible Light-Active Polypropylene Coated Photocatalyst

The effect of surface modification using ascorbic acid as a surface modifier of nano-TiO2 heterogeneous photocatalyst was studied. The preparation of supported photocatalyst was made by a specific paste containing ascorbic acid modified TiO2 nanoparticles used to cover Polypropylene as a support material. The obtained heterogeneous photocatalyst was thoroughly characterized (scanning electron microscope (SEM), RAMAN, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and Diffuse Reflectance Spectra (DRS) and successfully applied in the visible light photodegradation of Alizarin Red S in water solutions. In particular, this new supported TiO2 photocatalyst showed a change in the adsorption mechanism of dye with respect to that of only TiO2 due to the surface properties. In addition, an improvement of photocatalytic performances in the visible light photodegration was obtained, showing a strict correlation between efficiency and energy band gap values, evidencing the favorable surface modification of TiO2 nanoparticles

First-principles study of amorphous germanium under pressure

In this paper we present the study of the modifications of structural propertiesof a-Ge due to the application of external pressure. The investigations are carried out in the framework of density functional theory via first-principles molecular dynamics (FPMD)

b-Initio Molecular Dynamics Simulation of High Temperature GeO2

We made a series of molecular dynamic simulations to get a representative, high temperature GeO2 system to eventually be quenched down to room temperature so as to obtain an amorphous system to be studied under pressure for comparison to experimental results. We carefully generated an initial configuration such that all simulations have entirely been carried out by ab-initio methods in the range 10K-3000K

Impacto psicológico del COVID-19 en universitarios de la Universidad de La Laguna

El presente estudio explora el impacto psicológico que ha supuesto la pandemia de COVID-19 en una muestra de 147 estudiantes de la Universidad de la Laguna. El objetivo es investigar cuáles han sido los efectos producidos por la pandemia en el ámbito educativo. Los estudiantes cumplimentaron de manera online el Cuestionario de Impacto Psicológico del Coronavirus en estudiantes de la Universidad de La Laguna. Los resultados indican que los miedos mas comunes están relacionados con el contagio o muerte de los familiares. Así mismo, la mayoría de alumnos han valorado positivamente esta situación pues, además de haber podido adaptarse a la metodología online, han aprendido a valorar más las relaciones interpersonales. Aún así es importante destacar que la mayoría de los estudiantes indican que su concentración se ha visto afectada, debido a las clases no presenciales. De esta manera podemos concluir que la pandemia de COVID-19 ha supuesto una alteración importante en la vida estudiantil de esta muestra de alumnado.This project studies the psychological impact caused by COVID-19 pandemic in a sample of 147 students from the University of La Laguna. The aim is to investigate the effects that the pandemic brought out in the educational field of the students. The Survey of Psychological Impact caused by Coronavirus in students from University of La Laguna was filled out online. According to the results, the most common fears are related to the transmission of the disease and the death of relatives. Most of these students viewed this situation positively in two ways: they have successfully dealt with the online teaching methodology, and they have learnt to appreciate interpersonal relationships. However, it must be emphasized that most of them ensure their attention span has been disrupted due to telematic lessons. This approach proves that COVID-19 pandemic entails a significant alteration in the student's life of this sample

Spectrometer for X-ray emission experiments at FERMI free-electron-laser

A portable and compact photon spectrometer to be used for photon in-photon out experiments, in particular x-ray emission spectroscopy, is presented. The instrument operates in the 25\u2013800 eV energy range to cover the full emissions of the FEL1 and FEL2 stages of FERMI. The optical design consists of two interchangeable spherical varied-lined-spaced gratings and a CCD detector. Different input sections can be accommodated, with/without an entrance slit and with/without an additional relay mirror, that allow to mount the spectrometer in different end-stations and at variable distances from the target area both at synchrotron and at free-electron-laser beamlines. The characterization on the Gas Phase beamline at ELETTRA Synchrotron (Italy) is presented

SEI Growth and Depth Profiling on ZFO Electrodes by Soft X-Ray Absorption Spectroscopy

The evolution of the solid electrolyte interface (SEI) in carbon‐coated ZnFe2O4 (ZFO‐C) anodes is studied by soft X‐ray absorption spectroscopy (XAS). Experiments probe locally the SEI growth in the 2–100 nm range, using both total electron (TEY) and total fluorescence (TFY) yield techniques. XAS analysis shows that the SEI grows preferentially around the ZFO‐C nanoparticles