2p3d Rixs

[Leto Corrales]

Afull day lecture and hands-on analysis session on Charge Transfer Multiplet Calculations for X-ray Absorption Spectroscopy (CTM4XAS) will be held at SSRL/SLAC on May 24, 2011. CTM4XAS is a semi-empirical program to analyze transition metal L- and M-edge transitions by evaluating the effects of crystal field and charge transfer parameters on the atomic multiplets. CTM4RIXS is an interactive tool that calculates the two-dimensional Resonant Inelastic X-ray Scattering (RIXS) planes within the charge transfer multiplet model developed in the CTM4XAS software. 2p3d, 3p3d, 1s2p and 1s3p RIXS spectra are included. The instructor will be one of the the authors of the program, Professor Frank de Groot from Utrecht University, The Netherlands.

Transition metal L2,3 edges cannot be calculated with one-electron codes such as WIEN2K, FEFF, or DFT-based codes. The main reason for the deviation from the density of states is the strong overlap of the core wave function with the valence wave functions. CTM4XAS explicitly calculates this strong overlap between the core and valence level wave functions and additionally includes the core and valence spin-orbit coupling and the effects of strong correlations within the charge transfer model.

CTM4XAS has been developed by Eli Stavitski and Frank de Groot, based on the original charge transfer multiplet code developed by Theo Thole. It also includes the calculation of Electron energy loss spectroscopy (EELS), other XAS, X-ray photoemission (XPS) and X-ray emission (XES).

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In 2p3d resonant inelastic x-ray scattering (RIXS) one scans through the 2p X-ray absorption edge and measures the low energy excitations, including phonons, magnons, plasmons and orbitons. The present experimental resolution of 20 meV allows the detailed observation of the electronic and magnetic structure. One can distinguish different types of RIXS experiments:

As we approach the completion of the first phase of the Sirius project, which includes a 3 GeV 250 pm.rad storage ring operating at 100 mA and 14 beamlines, the Brazilian Synchrotron Light Laboratory (LNLS) of CNPEM is gearing up to construct and provide the second set of beamlines to the user community. This will expand the range of energies and techniques available on Sirius and push the limits of synchrotron science in Latin America. The second phase of Sirius will feature ten new beamlines that complement the existing lineup of available techniques, as well as three new x-ray bioimaging beamlines that will be connected to the Biosafety Level 4 laboratories of the Orion project. Moreover, as part of the Sirius Phase 2, there will be a 3.5-fold increase in the electron beam current, directly improving the experiments on all beamlines. In my presentation, I will go over the scope of these projects and discuss the planning and status of these activities.

X-ray absorption (XAS) and emission (XES) spectroscopies have imposed as powerful techniques to investigate structural and chemical dynamics of metal ions hosted in porous materials, such as zeolites, for applications in the field of selective redox catalysis [1]. Analysis of the XANES and EXAFS regions in XAS spectra offers a highly complementary view with respect to diffraction-based methods guaranteeing a unique sensitivity to the local electronic and structural properties of metal centers. These are often disorderly distributed in the crystalline matrix, and occur as dynamic mixtures of different species, responding to the physico-chemical environment while undergoing a rich redox chemistry mediated by host-guest interactions. Continuous instrumental developments at synchrotron sources today enable in situ/operando XAS studies at high time and energy resolution, allowing to monitor such dynamic systems with unprecedented accuracy. In parallel, the combination with XES-based approaches greatly enhances sensitivity to the ligands of the metal centers, allowing discrimination of almost isoelectronic atomic neighbors which is difficulty achieved by XAS. In this contribution, the potential of these methods, empowered by advanced data analysis strategies and synergic integration with multi-technique laboratory characterization and computational modelling, will be exemplified by selected research results.

Presented case studies will focus on Cu-exchanged zeolites for both deNOx applications via NH3-assisted Selective Catalytic Reduction [2] as well as for C-H bond activation in light alkanes [3]. Here, the potential of Multivariate Curve Resolution (MCR) of time-resolved XANES datasets, XAS/XES combination and EXAFS Wavelet Transform analysis will be highlighted, to accurately quantify condition/composition-dependent metal speciation and therein establish robust structure-activity relationships, essential to design improved catalysts.

Abstract: The work that will be presented focuses on the development and analysis of nanocomposites consisting of NiSi2 nanocrystals embedded in single-crystalline Si, with potential applications in nanotechnology. Current preparation methods face challenges such as nanocrystal agglomeration and the need for complex instrumentation. The work proposes an alternative method involving a Ni-doped SiO2 thin film deposited on Si(001) wafers through the sol-gel process. Thermal treatments induce Ni diffusion, leading to the formation of ordered and isolated NiSi2 nanoplates into silicon wafers. Several techniques, including STEM and GISAXS, are employed to study the nanocomposite.

Resonant and Non-resonant Phenomena in RIXS. Spectroscopic aspects of RIXS Resonant and non-resonant Integrated 2p3d RIXS (FY detection) Removing the obscuring silent majority. resonant inelastic x-ray scattering/spectroscopy. 2p XAS: 2p3d integrals & 2p spin-orbit coupling. 3d.

This Transition in Ce is archetypical of localization-delocalization phenomena encountered in f-electron systems. This isostructural phase transition accompanied by a large volume contraction (14%) is a manifestation of subtle interactions between f levels self-consistently embedded in a sea of conduction electrons, this transition is commonly described by either a Mott transition or a Kondo hybridization.

Important information about this transition is contained not only in the occupation number, nf, but also in the probability of double occupancy of the f sites. It is well known that the f states are clearly identifiable in spectroscopies (absorption, XAS or photoemission, XPS). Many proofs of the mixed-valent behaviour has been accumulated unambiguously from such experiments. Well separated features, each assigned to different f states, indicate that the f configurations are mixed in the ground state. But the different screenings of the core hole by the f 0>, f 1> and f 2> states, lead to strong difficulties to estimate the various f-electron weights and therefore the degree of hybridisation of the f electron at the origin of the heavy-fermion-like behaviour of Ce.

Recently, Resonance Inelastic x-ray Scattering (RIXS) has emerged as a means of probing the mixed-valent behaviour in rare earths (RE) systems in considerable details. The experiment consists in measuring the 3d->2p decay following a resonant excitation close to the RE-L2,3 edges (2p->5d transition).

The RIXS process benefits from the selective resonant enhancement of the different states. Previous series of experiments [1] have revealed a well-defined feature associated with the f2> configuration, which is normally hidden in XAS spectra. The f 1/f 2 ratio, as a function of temperature, closely resemble the magnetization loop of the transition induced by chemical pressure in Ce-Th alloy. However, electron interactions with the dopant element necessarily intervene in the Ce-4f electronics properties.

transition, while the feature 4f0 progressively builds up at high energy. These spectral changes are consistent with the early results obtained by RKKW. The XAS final states split into multiple components because the 2p core-hole Coulomb potential acting on the mixed-valent ground state c0f 0> + c1f 1> + c2f 2>, where cn 2 represent the weight of the individual components. Note that in these experiments the 4f 2 component is masked by the 2p3/2 core-hole lifetime.

In figure 2, we can follow the evolution of the 2p3d-RIXS, measured at E0=5718 eV, as the pressure is increased. A striking increase (40%) in the 4f 2/4f 1 intensity ratio is observed as the system passes the transition. By carrying out full multiplet calculations within the Anderson impurity model we obtain the simulated XAS and RIXS represented in Figs. 1b and 2, the spectra are well reproduced throughout the transition.

The main effect is the sharp decrease in the 4f 1 component to the advantage of the 4f 0-related feature, which gains intensity as Ce becomes more -like. Formally, the transfer of spectral weight from 4f 1(5d1) configuration toward a more 4f0(5d 2) configuration in the phase can be understood as a partial delocalisation of the 4f electrons. The increased contribution from 4f2 component at high pressure stresses the growing interaction between 4f and conduction band electrons, a characteristic feature of the Kondo-like behavior, and consequently an increase of the hybridization parameter.

The picture that arises from RIXS analysis at the transition in Ce metal is that of the coexistence of competing effects : partial delocalization of the 4f electrons through band formation with the conduction band states on the one hand, and reduced electron-electron correlations on the other hand that allow the system to accommodate stronger on-site repulsion. The quantitative analysis shows that nf deviates from unity as a direct consequence of non-zero hybridization : the occupation number passes from 0.97 (TK=70K) in the phase to 0.81 (TK=1700K) in the phase, assuming a coupling parameter, proportional to the hybridization energy, of 100meV.

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