Hard x-ray photon-in-photon-out spectroscopy with lifetime resolution – of XAS, XES, RIXSS and HERFD

Spectroscopic techniques that aim to resolve the electronic configuration and local coordination of a central atom by detecting inner-shell radiative decays following photoexcitation using hard X-rays are presented. The experimental setup requires an X-ray spectrometer based on perfect crystal Bragg optics. The possibilities arising from non-resonant (X-Ray Emission Spectroscopy - XES) and resonant excitation (Resonant Inelastic X-Ray Scattering Spectroscopy – RIXSS, High-Energy-Resolution Fluorescence Detected (HERFD) XAS) are discussed when the instrumental energy broadenings of the primary (beamline) monochromator and the crystal spectrometer for x-ray emission detection are on the order of the core hole lifetimes of the intermediate and final electronic states. The small energy bandwidth in the emission detection yields line-sharpened absorption features. In transition metal compounds, electron-electron interactions as well as orbital splittings and fractional population can be revealed. Combination with EXAFS spectroscopy enables to extent the k-range beyond unwanted absorption edges in the sample that limit the EXAFS range in conventional absorption spectroscopy

Localized holes and delocalized electrons in photoexcited inorganic perovskites: Watching each atomic actor by picosecond X-ray absorption spectroscopy

We report on an element-selective study of the fate of charge carriers in photoexcited inorganic CsPbBr3 and CsPb(ClBr)3 perovskite nanocrystals (NCs) in toluene solutions using time-resolved X-ray absorption spectroscopy with 80 ps time resolution. Probing the Br K-edge, the Pb L3-edge and the Cs L2-edge, we find that holes in the valence band are localized at Br atoms, forming small polarons, while electrons appear as delocalized in the conduction band. No signature of either electronic or structural changes are observed at the Cs L2-edge. The results at the Br and Pb edges suggest the existence of a weakly localized exciton, while the absence of signatures at the Cs edge indicates that the Cs+ cation plays no role in the charge transport, at least beyond 80 ps. These results can explain the rather modest charge carrier mobilities in these materials.Comment: 19 pages, 3 figure

Direct observation of electron density reconstruction at the metal-insulator transition in NaOsO3

5d transition metal oxides offer new opportunities to test our understanding of the interplay of correlation effects and spin-orbit interactions in materials in the absence of a single dominant interaction. The subtle balance between solid-state interactions can result in new mechanisms that minimize the interaction energy, and in material properties of potential use for applications. We focus here on the 5d transition metal oxide NaOsO3, a strong candidate for the realization of a magnetically driven transition from a metallic to an insulating state exploiting the so-called Slater mechanism. Experimental results are derived from non-resonant and resonant x-ray single crystal diffraction at the Os L-edges. A change in the crystallographic symmetry does not accompany the metal-insulator transition in the Slater mechanism and, indeed, we find no evidence of such a change in NaOsO3. An equally important experimental observation is the emergence of the (300) Bragg peak in the resonant condition with the onset of magnetic order. The intensity of this space-group forbidden Bragg peak continuously increases with decreasing temperature in line with the square of intensity observed for an allowed magnetic Bragg peak. Our main experimental results, the absence of crystal symmetry breaking and the emergence of a space-group forbidden Bragg peak with developing magnetic order, support the use of the Slater mechanism to interpret the metal-insulator transition in NaOsO3. We successfully describe our experimental results with simulations of the electronic structure and, also, with an atomic model based on the established symmetry of the crystal and magnetic structure.Comment: 6 figure

An approach to verification and validation of MHD codes for fusion applications

We propose a new activity on verification and validation (V&V) of MHD codes presently employed by the fusion community as a predictive capability tool for liquid metal cooling applications, such as liquid metal blankets. The important steps in the development of MHD codes starting from the 1970s are outlined first and then basic MHD codes, which are currently in use by designers of liquid breeder blankets, are reviewed. A benchmark database of five problems has been proposed to cover a wide range of MHD flows from laminar fully developed to turbulent flows, which are of interest for fusion applications: (A) 2D fully developed laminar steady MHD flow, (B) 3D laminar, steady developing MHD flow in a non-uniform magnetic field, (C) quasi-two-dimensional MHD turbulent flow, (D) 3D turbulent MHD flow, and (E) MHD flow with heat transfer (buoyant convection). Finally, we introduce important details of the proposed activities, such as basic V&V rules and schedule. The main goal of the present paper is to help in establishing an efficient V&V framework and to initiate benchmarking among interested parties. The comparison results computed by the codes against analytical solutions and trusted experimental and numerical data as well as code-to-code comparisons will be presented and analyzed in companion paper/paper

Octahedral molybdenum cluster as a photoactive antimicrobial additive to a fluoroplastic

Finding methods that fight bacterial infection or contamination, while minimising our reliance on antibiotics is one of the most pressing needs of this century. Although the utilisation of UV-C light and strong oxidising agents, such as bleach, are still efficacious methods for eliminating bacterial surface contamination, both methods present severe health and/or environmental hazards. Materials with intrinsic photodynamic activity (i.e. a material's ability upon photoexcitation to convert molecular oxygen into reactive oxygen species such as singlet oxygen), which work with light within the visible photomagnetic spectrum could offer a significantly safer alternative. Here we present a new, bespoke molybdenum cluster (Bu4N)2[Mo6I8(n-C7F15COO)6], which is both efficient in the generation of singlet oxygen upon photoirradiation and compatible with the fluoropolymer (F23-L) known for its good oxygen permeability. Thus, (Bu4N)2[Mo6I8(n-C7F15COO)6]/F23-L mixtures have been solution-processed to give homogenous films of smooth and fibrous morphologies and which displayed high photoinduced antibacterial activity against four common pathogens under visible light irradiation. These materials thus have potential in applications ranging from antibacterial coatings to filtration membranes and air conditioners to prevent spread of bacterial infections

Layered Synthesis of Workpieces by the Method of Mig-Pulse Surface with the Use of Austenitic Metal-Core Wire with Nitrogen

Works were carried out on the synthesis of a workpiece using metal-cored wire with nitrogen (0,322 wt. %) using the mig-pulse-technology. It was possible to obtain a defect-free dense deposited metal with high strength and plastic characteristics.Работа выполнена при финансовой поддержке Министерства науки и высшего образования Российской Федерации (государственное задание № FSNM-2021-0011) и Российского фонда фундаментальных исследований (проект РФФИ 20-48-596006 р_НОЦ_Пермский край)

Quantum chemical calculations of X-ray emission spectroscopy

The calculation of X-ray emission spectroscopy with equation of motion coupled cluster theory (EOM-CCSD), time dependent density functional theory (TDDFT) and resolution of the identity single excitation configuration interaction with second order perturbation theory (RI-CIS(D)) is studied. These methods can be applied to calculate X-ray emission transitions by using a reference determinant with a core-hole, and they provide a convenient approach to compute the X-ray emission spectroscopy of large systems since all of the required states can be obtained within a single calculation removing the need to perform a separate calculation for each state. For all of the methods, basis sets with the inclusion of additional basis functions to describe core orbitals are necessary, particularly when studying transitions involving the 1s or- bitals of heavier nuclei. EOM-CCSD predicts accurate transition energies when compared with experiment, however, its application to larger systems is restricted by its computational cost and difficulty in converging the CCSD equations for a core-hole reference determinant, which become increasing problematic as the size of the system studied increases. While RI-CIS(D) gives accurate transition energies for small molecules containing first row nuclei, its application to larger systems is limited by the CIS states providing a poor zeroth order reference for perturbation theory which leads to very large errors in the computed transition energies for some states. TDDFT with standard exchange-correlation functionals predicts transition energies that are much larger than experiment. Optimization of a hybrid and short-range cor- rected functional to predict the X-ray emission transitions results in much closer agreement with EOM-CCSD. The most accurate exchange-correlation functional identified is a modified B3LYP hybrid functional with 66% Hartree-Fock exchange, denoted B66LYP, which predicts X-ray emission spectra for a range of molecules including fluorobenzene, nitrobenzene, ace- tone, dimethyl sulfoxide and CF3Cl in good agreement with experiment