Core-hole Spectroscopy Research Articles

Experimental and theoretical study of resonant core-hole spectroscopies of gas-phase free-base phthalocyanine.

We studied N 1s-1 inner-shell processes of the free base Phthalocyanine molecule, H2Pc, in the gas-phase. This complex organic molecule contains three different nitrogen sites defined by their covalent bonds. We identify the contribution of each site in ionized, core-shell excited or relaxed electronic states by the use of different theoretical methods. In particular, we present resonant Auger spectra along with a tentative new theoretical approach based on multiconfiguration self-consistent field calculations to simulate them. These calculations may pave the road towards resonant Auger spectroscopy in complex molecules.

Transition-potential coupled cluster II: optimisation of the core orbital occupation number

The issue of orbital relaxation in computational core-hole spectroscopy, specifically x-ray absorption, has been a major problem for methods such as equation-of-motion coupled cluster with singles and doubles (EOM-CCSD). The transition-potential coupled cluster (TP-CC) method is utilised to address this problem by including an explicit treatment of orbital relaxation via the use of reference orbitals with a fractional core occupation number. The value of the fractional occupation parameter λ was optimised for both TP-CCSD and XTP-CCSD methods in an element-specific manner due to the differences in atomic charge and energy scale. Additionally, TP-CCSD calculations using the optimised parameters were performed for the K-edge absorption spectra of gas-phase adenine and thymine. TP-CCSD reproduces the valence region well and requires smaller overall energy shifts in comparison to EOM-CCSD, while also improving on the relative position and intensities of several absorption peaks.

Accurate Core-Excited States via Inclusion of Core Triple Excitations in Similarity-Transformed Equation-of-Motion Theory.

The phenomenon of orbital relaxation upon excitation of core electrons is a major problem in the linear-response treatment of core-hole spectroscopies. Rather than addressing relaxation through direct dynamical correlation of the excited state via equation-of-motion coupled cluster theory (EOMEE-CC), we extend the alternative similarity-transformed equation-of-motion coupled cluster theory (STEOMEE-CC) by including the core-valence separation (CVS) and correlation of triple excitations only within the calculation of core ionization energies. This new method, CVS-STEOMEE-CCSD+cT, significantly improves on CVS-EOMEE-CCSD and unmodified CVS-STEOMEE-CCSD when compared to full CVS-EOM-CCSDT for K-edge core-excitation energies of a set of small molecules. The improvement in both absolute and relative (shifted) peak positions is nearly as good as that for transition-potential coupled cluster (TP-CC), which includes an explicit treatment of orbital relaxation, and CVS-EOMEE-CCSD*, which includes a perturbative treatment of triple excitations.

Progress in the Theory of X-ray Spectroscopy: From Quantum Chemistry to Machine Learning and Ultrafast Dynamics.

The development of high-brilliance third- and fourth-generation light sources such as synchrotrons and X-ray free-electron lasers (XFELs), the emergence of laboratory-based X-ray spectrometers, and instrumental and methodological advances in X-ray absorption (XAS) and (non)resonant emission (XES and RXES/RIXS) spectroscopies have had far-reaching effects across the natural sciences. However, new kinds of experiments, and their ever-higher resolution and data acquisition rates, have brought acutely into focus the challenge of accurately, quickly, and cost-effectively analyzing the data; a far-from-trivial task that demands detailed theoretical calculations that are capable of capturing satisfactorily the underlying physics. The past decade has seen significant advances in the theory of core-hole spectroscopies for this purpose, driven by all of the developments above and-crucially-a surge in demand. In this Perspective, we discuss the challenges of calculating core-excited states and spectra, and state-of-the-art developments in electronic structure theory, dynamics, and data-driven/machine-led approaches toward their better description.

Transition-potential coupled cluster.

The problem of orbital relaxation in computational core-hole spectroscopies, including x-ray absorption and x-ray photoionization, has long plagued linear response approaches, including equation-of-motion coupled cluster with singles and doubles (EOM-CCSD). Instead of addressing this problem by including additional electron correlation, we propose an explicit treatment of orbital relaxation via the use of "transition potential" reference orbitals, leading to a transition-potential coupled cluster (TP-CC) family of methods. One member of this family, in particular, TP-CCSD(12), is found to essentially eliminate the orbital relaxation error and achieve the same level of accuracy for the core-hole spectra as is typically expected of EOM-CCSD in the valence region. These results show that very accurate x-ray absorption spectra for molecules with first-row atoms can be computed at a cost essentially the same as that for EOM-CCSD.

Multiplet structure for perovskite-type Ba0.9Ca0.1Ti0.9Zr0.1O3 by core–hole spectroscopies

Core–hole spectroscopies such as x-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HR-EELS) in combination with multiplet calculation are important tools to elucidate the electronic structure of transition metal compounds. This work presents a comparison between the electronic structure obtained by XPS and HR-EELS for polycrystalline perovskite-type Ba0.9Ca0.1Ti0.9Zr0.1O3. Raman analysis suggests that Ca2+ cations could partly occupy the Ti4+ cations in the B-site of a perovskite structure with the tetragonal phase. A multiplet structure was determined by XPS for Ti 2p and by HR-EELS for Ca L2,3 and Ti L2,3 edges. Octahedral (Oh) symmetry in the crystal field (CF) effects reproduces the local distortion of TiO6 octahedra. The charge transfer (CT) effects were also considered to reproduce L3-edge EELS shoulders and the satellite in the Ti 2p XPS region. CF and CT parameters, 10 Dq, (charge transfer energy) Δ, and (Coulomb repulsion energy) Udd, are reported for future reference. The broadening of the Ti L2-edge suggests the presence of the Coster–Kronig electron decay process. Multiplet calculation in Oh symmetry for the Ca L2,3-edge could support the Raman interpretation.

Advanced Computation Method for Double Core Hole Spectra: Insight into the Nature of Intense Shake-up Satellites.

Double core hole spectroscopy is an ideal framework for investigating photoionization shake-up satellites. Their important intensity in a single site double core hole (ssDCH) spectrum allows the exploration of the subtle mix of relaxation and correlation effects associated with the inherent multielectronic character of the shake-up process. We present a high-accuracy computation method for single photon double core-shell photoelectron spectra that combines a selected configuration interaction procedure with the use of non-orthogonal molecular orbitals to obtain unbiased binding energy and intensity. This strategy leads to the oxygen ssDCH spectrum of the CO molecule that is in excellent agreement with the experimental result. Through a combined wave function and density analysis, we highlight that the most intense shake-up satellites are characterized by an electronic reorganization that opposes the core hole-induced relaxation.

Clocking Femtosecond Collisional Dynamics via Resonant X-Ray Spectroscopy.

Electron-ion collisional dynamics is of fundamental importance in determining plasma transport properties, nonequilibrium plasma evolution, and electron damage in diffraction imaging applications using bright x-ray free-electron lasers (FELs). Here we describe the first experimental measurements of ultrafast electron impact collisional ionization dynamics using resonant core-hole spectroscopy in a solid-density magnesium plasma, created and diagnosed with the Linac Coherent Light Source x-ray FEL. By resonantly pumping the 1s→2p transition in highly charged ions within an optically thin plasma, we have measured how off-resonance charge states are populated via collisional processes on femtosecond time scales. We present a collisional cross section model that matches our results and demonstrates how the cross sections are enhanced by dense-plasma effects including continuum lowering. Nonlocal thermodynamic equilibrium collisional radiative simulations show excellent agreement with the experimental results and provide new insight on collisional ionization and three-body-recombination processes in the dense-plasma regime.

The influence of charge transfers effects in monazite-type LaVO4 and perovskite-type LaVO3 prepared by sol-gel acrylamide polymerization

Core-hole spectroscopy such as X-ray absorption spectroscopy (XAS) is useful to determine the electronic structure of strongly correlated and strongly hybridized compounds such as vanadates. Monazite-type LaVO4 and perovskite-type LaVO3 are good candidates to elucidate the electronic structure through the vanadium L2,3 edge. LaVO4 was prepared by sol-gel acrylamide polymerization and solid-state reaction. LaVO3 was obtained by reduction of LaVO4 using Zr as gatherer. Monoclinic crystal phase for LaVO4 and orthorhombic crystal phase for LaVO3 were confirmed by the Rietveld refinement of X-ray diffraction patterns. XAS comparison between Vanadium L2,3 edge confirms the presence of V5+ for the monazite and V3+ for the orthorhombic perovskite. Multiplet calculations including crystal field and charge transfer effects (CTM) were performed in order to elucidate the tetragonal (D4h symmetry) parameters Dq, Ds and Dt, the charge transfer energy Δ, and d-d Coulomb repulsion energy U parameters. CTM confirms for LaVO3 the strong V 3d–O 2p hybridization with a significant contribution of covalent character due to the delocalization of 3d electrons. For LaVO4 this work suggest the reclassification of this band insulator as charge transfer insulator that shows a significant contribution of ionic character.

Polarization-assisted amplitude gating as a route to tunable, high-contrast attosecond pulses

Attosecond spectroscopy is a powerful technique for probing electron dynamics in fundamental systems. However, extending this method to cover a wide range of element-specific, core-hole transitions requires the availability of broadly tunable attosecond pulses. In this Letter, we present a new method for generating high-flux, high-contrast single attosecond pulses tunable across the range of 50–120 eV. The method is referred to as a Polarization ASSisted Amplitude GatE (PASSAGE) and uses a few-cycle driving pulse along with a partial polarization gate to extend the bandwidth of high harmonic emission in the temporally isolated, cut-off portion of the spectrum. The simplicity of this technique will help pave the way for implementing attosecond core–hole spectroscopy to probe more complex reactions and bring attosecond science into a multidisciplinary setting.

Probing chemical environment with molecular double core-hole electron spectroscopy

With the advent of X-ray free electron lasers and the development of elaborate multi-coincidence methods combined with conventional synchrotron radiation, the interest in double core-hole spectroscopy revived. To describe the measured data and provide the guideline for yet coming experiments, theoretical studies are required. In this paper we review previous theoretical works on double core-hole states and discuss new theoretical results for the XHm–YHn (X, Y=C, N, O, or F; m,n=0–3) molecules and their fluorine substituted compounds. We compute the single and double core-hole binding energies of these systems using different methods and show that the DFT results agree well with the results of other ab initio methods. In agreement with previous theoretical works, our study demonstrates that the changes in the ionization potential resulting from fluorine substitutions mainly arise due to changes in the ground state and not due to electron relaxation. Special emphasis is given to interatomic relaxation originating from creation of two core holes on different atomic sites. We demonstrate that there exists clear correlation between this quantity, the number of hydrogen atoms and the order of the bond between the heavy atoms in the systems studied.

Double core hole spectroscopy with synchrotron radiation

In the last five years, a strong experimental and theoretical activity has been dedicated to the study of inner-shell double photoionization. Almost simultaneously, between 2009 and 2011, different experiments were performed using two different light sources: well established third generation synchrotron light sources that exist for more than two decades and recently developed x-ray free electron lasers (X-FEL) that provide brightness of about 10 orders of magnitude higher. The high photon flux of X-FEL has made possible the successive absorption of many x-ray photons by inner-shell electrons in femtosecond timescale. The possibility to create multiple inner-shell vacancies with this new X-FEL light source has resuscitated “prophetic” 25 years old calculations by Cederbaum et al. [1,2] concerning double K-shell ionization that could be of great interest when conventional Electron Spectroscopy for Chemical Analysis (ESCA) finds some limitations. As a feedback, this has stimulated new theoretical developments. On the other hand, it was considered practically out of reach to observe such states with conventional third generation synchrotron light sources due to the very weak probability to eject two inner-shell electrons by a single photon when the dominant process, that could mask everything else, is single inner-shell ionization. However, due to the development of very efficient detection techniques, it was also possible to perform spectroscopy of double core-hole states with excellent accuracy using third generation synchrotron sources.In this paper we give an outlook on the basis of recently published results, of the complementarity of these light sources for double core-hole spectroscopy. Some new developments and results are also presented with a prospective of what could be done in the future with the technical breakthrough that will be necessary to improve our further understanding of chemical analysis.

Chemical analysis: Double core-hole spectroscopy with free-electron lasers

Free-electron lasers with their femtosecond pulse duration, high pulse energy and tunable photon energy in a regime from XUV to hard-X-ray have opened up several research avenues in physics, chemistry and biology. We have exploited the attributes of these new lasers by exploring intriguing non-linear effects such as double core-hole formation via multiphoton absorption. We have carried out X-ray two-photon photoelectron spectroscopy for molecular chemical analysis using the Linac Coherent Light Source. Our experimental work aimed to probe the chemical environment of specific atomic sites in several molecules and validate the theory of two-photon photoelectron spectroscopy. This work demonstrates that our methodology will be very effective with the future intense, high repetition rate as well as laser-seeded or self-seeded photon facilities.

Xclaim: A graphical interface for the calculation of core-hole spectroscopies

Xclaim (X-ray core level atomic multiplets) is a graphical interface for the calculation of core-hole spectroscopy and ground state properties within a charge-transfer multiplet model taking into account a many-body Hamiltonian with Coulomb, spin–orbit, crystal-field, and hybridization interactions. Using Coulomb and spin–orbit parameters calculated in the Hartree–Fock limit and ligand field parameters (crystal-field, hybridization and charge-transfer energy) the program calculates X-ray absorption spectroscopy (XAS), X-ray photoemission spectroscopy (XPS), photoemission spectroscopy (PES) and inverse photoemission (IPES). The program runs on Linux, Windows and MacOS platforms.

X-ray absorption and emission spectroscopies of X-bridged diiron phthalocyanine complexes (FePc)2X (X = C, N, O) combined with DFT study of (FePc)2X and their high-valent diiron oxo complexes.

μ-Nitrido diiron phthalocyanine [PcFe(+3.5)NFe(+3.5)Pc](0) is an efficient catalyst, able to catalyze the oxidation of methane under near-ambient conditions. In this work, we compared the properties of structurally similar μ-carbido (1), μ-nitrido (2), and μ-oxo (3) dimers of iron phthalocyanine. The goal was to discern the structural and electronic differences between these complexes and to propose a rationale for the exceptional activity of 2. Extended X-ray fine-structure spectroscopy, high-resolution X-ray emission spectroscopy, and resonant inelastic X-ray scattering were applied to study the geometry and electronic structure of iron species in the series 1-3. The data provided by core hole spectroscopies were compared to the results of DFT calculations and found to coherently describe the structural and electronic properties of 1-3 as having equivalent iron centers with formal iron oxidation degrees of 3, 3.5, and 4 for the μ-oxo, μ-nitrido, and μ-carbido dimers, respectively. However, the bond length to the bringing atom changed in an unexpected sequence Fe-O > Fe-N < Fe-C, indicating redox non-innocence of the brigding μ-carbido ligand in 1. According to the X-ray emission spectroscopy, the μ-nitrido dimer 2 is a low-spin compound, with the highest covalency in the series 1-3. The DFT-calculated geometry and electronic structures as well as core hole spectra of hypothetical high-valent oxo complexes of 1-3 were compared, in order to explain the particular catalytic activity of 2 and to estimate the prospects of spectroscopic observation of such species. It appears that the terminal Fe═O bond is the longest in the oxo complex of 2, due to the strong trans-effect of the nitrido ligand. The corresponding LUMO of the μ-nitrido diiron oxo complex has the lowest energy among the three oxo complexes. Therefore, the oxo complex of 2 is expected to have the highest oxidative power.

K-shell double core-hole spectroscopy in molecules

A great deal of attention has been devoted in the last few years to photoionization processes in isolated molecules leading to the formation of double core-hole (DCH) states. There are two main experimental avenues to induce such processes, namely single-photon absorption followed by the simultaneous ejection of two core electrons, and x-ray-induced multiphoton processes leading to the production of DCH states via the sequential absorption of two soft x- ray photons on a time scale on the order of the molecular Auger lifetime (4–8 femtoseconds for light elements). The formation of molecular two-site (ts) DCH states, in particular, shows great potential as a powerful tool for chemical analysis. A compelling motivation for the study of ts-DCH states is their ability to probe the local chemical environment more sensitively than either single core-hole (SCH) or single-site (ss) DCH states. The enhanced sensitivity originates from the fact that the double ionization potential (DIP) of ts-DCH states is directly coupled to induced changes in the valence charge distribution at the two different atomic sites. Here a review of the recent literature is presented on both types of experiments, and on the related theoretical work.

Extracting chemical information of free molecules from K-shell double core-hole spectroscopy

Using density functional theory (DFT) methods, we have calculated ionization potential (IP) for K-shell single core hole (SCH) creation and double ionization potential (DIP) for K-shell double core hole (DCH) creation for XHmYHn (X, Y=C, N, O, F, m,n=0–3), NX2CXO (X=H or F) and C60. For these molecules, we estimated the relaxation energies (a measure of the electron density flow to the core-hole site) and the interatomic relaxation energies (a measure of the electron density flow to the two core-hole sites) from the calculated IPs and DIPs. For XHmYHn, we find that the interatomic relaxation energy for the DCH states having two holes at X and Y atoms decreases with the increase in the bond order between X and Y. For NX2CXO (X=H or F), we find that the substitution of the hydrogen atoms by the fluorine atoms affects the initial-state-bonding shifts but less influences the relaxation energy. For DCH states having two holes at two carbon atoms in C60, we find that the interatomic relaxation energy decreases with the increase in the hole–hole distance.

Magnetic and resonant X-ray scattering investigations of strongly correlated electron systems

Resonant X-ray scattering is a method which combines high- Q → resolution X-ray elastic diffraction and atomic core-hole spectroscopy for investigating electronic and magnetic long-range ordered structures in condensed matter. During recent years the development of theoretical models to describe resonant X-ray scattering amplitudes and the evolution of experimental techniques, which include the control and analysis of linear photon polarization and the introduction of extreme environment conditions such as low temperatures, high magnetic field and high pressures, have opened a new field of investigation in the domain of strongly correlated electron systems. To cite this article: L. Paolasini, F. de Bergevin, C. R. Physique 9 (2008).

Non-quasiparticle effects in half-metallic ferromagnets

The unusual electronic structure of the half-metallic ferromagnets (HMF) is analysedtaking account of correlation effects (electron–magnon interaction, in particularspin-polaron effects). Special attention is paid to the so-called non-quasiparticle(NQP) (incoherent) states which arise in the minority- (majority-) spin gapabove (below) the Fermi level and which may make considerable contributionsto the electronic properties. First-principles calculations of the NQP states inHMF within the local-density approximation plus dynamical mean field theory(LDA+DMFT) are reviewed. These states can be probed, in particular, by spin-polarized scanningtunnelling microscopy. They also lead to observable effects in core–hole spectroscopy,nuclear magnetic relaxation and contribute to the temperature dependence of impurityresistivity, etc. The peculiarities of the transport properties of three- and two-dimensionalHMF are discussed, which are connected with the absence of one-magnon spin-flipscattering processes.

Direct observation of electron dynamics in the attosecond domain

Dynamical processes are commonly investigated using laser pump-probe experiments, with a pump pulse exciting the system of interest and a second probe pulse tracking its temporal evolution as a function of the delay between the pulses. Because the time resolution attainable in such experiments depends on the temporal definition of the laser pulses, pulse compression to 200 attoseconds (1 as = 10(-18) s) is a promising recent development. These ultrafast pulses have been fully characterized, and used to directly measure light waves and electronic relaxation in free atoms. But attosecond pulses can only be realized in the extreme ultraviolet and X-ray regime; in contrast, the optical laser pulses typically used for experiments on complex systems last several femtoseconds (1 fs = 10(-15) s). Here we monitor the dynamics of ultrafast electron transfer--a process important in photo- and electrochemistry and used in solid-state solar cells, molecular electronics and single-electron devices--on attosecond timescales using core-hole spectroscopy. We push the method, which uses the lifetime of a core electron hole as an internal reference clock for following dynamic processes, into the attosecond regime by focusing on short-lived holes with initial and final states in the same electronic shell. This allows us to show that electron transfer from an adsorbed sulphur atom to a ruthenium surface proceeds in about 320 as.