4d Core Hole Research Articles

All-XUV Pump-Probe Transient Absorption Spectroscopy of the Structural Molecular Dynamics of Di-iodomethane

In this work, we use an extreme-ultraviolet (XUV) free-electron laser (FEL) to resonantly excite the I: 4d5/2–σ* transition of a gas-phase di-iodomethane (CH2I2) target. This site-specific excitation generates a 4d core hole located at an iodine site, which leaves the molecule in a well-defined excited state. We subsequently measure the time-dependent absorption change of the molecule with the FEL probe spectrum centered on the same I: 4d resonance. Using ab initio calculations of absorption spectra of a transient isomerization pathway observed in earlier studies, our time-resolved measurements allow us to assign the timescales of the previously reported direct and indirect dissociation pathways. The presented method is thus sensitive to excited-state molecular geometries in a time-resolved manner, following a core-resonant site-specific trigger.Received 9 June 2020Revised 27 February 2021Accepted 10 May 2021DOI:https://doi.org/10.1103/PhysRevX.11.031001Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Open access publication funded by the Max Planck Society.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasPhotodissociationPhysical SystemsMoleculesTechniquesTransient absorption spectroscopyAtomic, Molecular & Optical

Atomic origin of 3d94 f 1 configuration in La3+ solids

We have studied the excited electronic structure of LaBr3(Ce) scintillator by soft x-ray spectroscopy such as x-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES). The La 3d XAS and XES spectra of LaBr3(Ce) are compared with those of other La3+ solids (LaF3, La2O3, and La metal). From this comparison, it turns out that the La 3d XAS and XES spectra from all the La3+ solids considered here appear at almost the same energy, even though the corresponding binding energies of the 3d core holes determined by XPS (x-ray photoelectron spectroscopy) are very different. As a result, we argue that the atomic nature of the configuration created by x-ray absorption process in La3+ solids is maintained via the localized 4 f 1 state, which screens the 3d core holes differently from one La3+ solid to another.

Inner-shell photoionization and core-hole decay of Xe and XeF2.

Photoionization cross sections and partial ion yields of Xe and XeF2 from Xe 3d(5/2), Xe 3d(3/2), and F 1s subshells in the 660-740 eV range are compared to explore effects of the F ligands. The Xe 3d-ϵf continuum shape resonances dominate the photoionization cross sections of both the atom and molecule, but prominent resonances appear in the XeF2 cross section due to nominal excitation of Xe 3d and F 1s electrons to the lowest unoccupied molecular orbital (LUMO), a delocalized anti-bonding MO. Comparisons of the ion products from the atom and molecule following Xe 3d photoionization show that the charge-state distribution of Xe ions is shifted to lower charge states in the molecule along with production of energetic F(+) and F(2+) ions. This suggests that, in decay of a Xe 3d core hole, charge is redistributed to the F ligands and the system dissociates due to Coulomb repulsion. The ion products from excitation of the F 1s-LUMO resonance are different and show strong increases in the yields of Xe(+) and F(+) ions. The subshell ionization thresholds, the LUMO resonance energies, and their oscillator strengths are calculated by relativistic coupled-cluster methods and agree well with measurements.

Soft-X-ray emission spectroscopy study of transition intensities from the valence band to 3D core-hole of lanthanide oxides

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

4d core hole formation in silver and its role in Ag − secondary ion formation/survival

The low energy mono-atomic secondary ion intensities of Ag − over the transient regions from Ag under 1–5 keV Cs + primary ion impact are compared. These energies are used since they appear to be above and below the collisional excitation threshold for 4d core hole formation. The lack of any work function dependence in both suggests that (a) Ag − forms via resonance charge transfer of electrons from the broad 5s–5p band to Ag atoms bearing 4d core holes (Auger is disregarded for energetic reasons), and that (b) the route via which 4d core holes are formed, whether it be via collective oscillations or collisional excitation, is of minimal importance. The similarity in X-ray induced plasmon loss features from clean and Cs implanted Ag confers with these results.

Strong electron correlation in Ca 2p, Sr 3d and Ba 4d core hole states investigated by means of photoelectron spectroscopy and MCDF calculations

Many-electron (correlation) effects play a dominant role in inner-shell ionization processes in free alkaline earth metal atoms. In this work calcium 2p, strontium 3d and barium 4d photoionization processes have been investigated with emphasis on the correlation effects. In order to understand the details of the spectra, a series of ab initio calculations based on multiconfiguration Dirac–Fock method have been performed. Theoretical predictions are compared with high-resolution synchrotron radiation induced electron spectra and the capability of the multiconfigurational computations in reproducing the experiment is discussed.

Band symmetries of GaSe(0001) studied by spin-resolved electron spectroscopy using circularly polarized radiation

We have studied the symmetry properties of the bands of the layered semiconductor GaSe along the $\ensuremath{\Delta}$ direction by means of spin-resolved electron spectroscopy using circularly polarized synchrotron radiation and by means of a relativistic LAPW band-structure calculation. The photoelectron spectra show in the main a pair of weakly dispersing peaks caused by direct transitions from valence-band states with symmetries ${\ensuremath{\Delta}}_{9}^{5},$ ${\ensuremath{\Delta}}_{9}^{6}$ and ${\ensuremath{\Delta}}_{7}^{5},$ ${\ensuremath{\Delta}}_{8}^{6}.$ Our data determine the spin-orbit splitting of these valence bands to be $\ensuremath{\Delta}{E}_{\mathrm{so}}=0.3\ifmmode\pm\else\textpm\fi{}0.1\mathrm{eV}.$ We find a gap in the unoccupied bands along $\ensuremath{\Delta}$ about 8 eV above the fundamental gap. The analysis of the photoelectron spin polarization reveals that also unoccupied states with low symmetries ${\ensuremath{\Delta}}_{7}^{5},$ ${\ensuremath{\Delta}}_{8}^{6}$ and ${\ensuremath{\Delta}}_{9}^{5},$ ${\ensuremath{\Delta}}_{9}^{6}$ are involved in excitation and emission. Spin resolved spectra measured at Auger electrons following the decay of Ga 3d core holes confirm that p derived unoccupied states exists below the vacuum level.

Ionization of the xenon fluorides

The noble-gas atom xenon can bind fluorine atoms and form several stable compounds. We study the electronic structure of the xenon fluorides (XeFn, n=2, 4, 6) by calculating their ionization spectra using a Green’s function method, which allows one to treat many-body effects at a high level. Our focus is on the valence region and on the Xe 4d core hole. We observe a sensitive dependence of the spectra on the number of fluorine ligands. Systematic line shifts are uncovered and explained. In the Xe 5s and F 2s inner-valence regimes, from XeF2 to XeF6, the usefulness of the one-particle picture of ionization is found to become progressively worse. Moreover, adding the electronegative fluorine ligands seems to enhance—not suppress—the Auger decay of a Xe 4d core hole.

X-ray photoelectron spectroscopy investigation of the initial oxygen adsorption sites on the LaB6(100) surface

X-ray photoelectron spectroscopy was used to investigate the initial stages of oxygen adsorption on the (100) surface of a single crystal of lanthanum hexaboride. Numerous previous studies had not resolved the issue of whether oxygen adsorbs at lanthanum sites, boron sites, or both. We find that oxygen adsorption markedly alters the La3d lineshapes, whereas the B1s peak is unaffected. On the clean surface the La3d3/2 peak is split into two components at binding energies of 854.7 and 851.8eV, a splitting that is typical of rare-earth metals and their compounds. The two components are associated with two different final states. In one final state the 3d core hole is poorly screened (854.7eV) and in the other it is well-screened (851.8eV). The relative intensity of the two components is known to be very sensitive to the chemical environment of the rare earth atom and a 10L O2 exposure at room temperature produces a large increase in the relative intensity of the well-screened component. Annealing the surface to 600°C and then to 700°C produces sharp c(2×2) and p(2×1) LEED patterns respectively. The La3d peaks associated with the two LEED patterns are similar to those observed after the initial 300K 10L O2 exposure, indicating oxygen bonding to La in both overlayer structures. Thus while the XPS data clearly reveal oxygen adsorption at La sites, there is no indication of adsorption at boron sites for low O2 exposures. More extensive oxidation at higher temperatures shows formation of both boron and lanthanum oxides.

Interplay between intra-atomic multiplet coupling and interatomic hybridization in core-level spectroscopy

Abstract Interplay between atomic multiplet and solid-state hybridization often plays an important role in the core-level spectroscopy of f and d electron systems. Here a short review is given of theoretical analyses of core-level spectra in rare-earth oxide systems, where the effects of the intra-atomic multiplet coupling and interatomic hybridization are taken into account with the impurity Anderson model. Some examples of theoretical calculations are shown for 3d X-ray photoemission spectra (XPS) and 4d X-ray absorption spectra (XAS) of CeO 2 and for 4d XPS of Pr 2 O 3 . For 4d XPS of heavy rare-earth systems, the effect of term-dependent lifetime of the 4d core hole is shown to be essential in explaining the spectral shape. A recent analysis of resonant X-ray emission spectra for CeO 2 is also given.

Decay of core holes in cesium chloride studied by the luminescence spectroscopy

The relaxation of the electronic excitations created in the CsCl single crystal after the Auger decay of Cs + 4d core hole was studied by the measurements of crossluminescence (CL), STE emission, excitation spectra and decay kinetics in the energy range 70–140 eV and at temperatures from 10 to 350 K using synchrotron radiation from the SRS storage ring. CL decay time at hv > 80 eV decreased with temperature in the range 10–150 K, then increased with it up to 350 K, but did not reach the value observed at 10 K. At 140 K < T < 350 K and excitation energies hv > 80 eV, a molecular-type band structure with the separation between the peaks of ~ 0.24 eV superimposed over the CL emission spectrum was observed similar to the CN − spectrum in other alkali halides. Possible relaxation channels for the electronic excitations involved are discussed.

Comparison of magnetic linear dichroism in 4f photoemission and 4d–4f photoemission from Gd on Y(0001)

Magnetic linear dichroism (MLD) in 4d–4f resonant and 4f nonresonant photoemission (PE) is studied from thin epitaxial gadolinium films. In an angle resolved and high-energy resolution mode, experiments were conducted with the electric-field vector of the incident light perpendicular to the sample magnetization. Our results show a significant difference in behavior of MLD in resonant PE as compared to that in nonresonant PE. Off-resonance, the MLD signal is dominated by a negative feature at the low binding energy side of the peak. Near the 4d–4f resonance maximum, the MLD displays a plus–minus shape, with a negative signal at the low binding energy side of the 4f peak and a positive signal at the high binding energy side. Analysis of MLD in 4d–4f resonant PE may provide insight into interactions of the 4d core hole with the 4f core level in the intermediate state.

Resonant photoemission spectra at the 4f and 5p levels of Tm across the 4d-4f absorption threshold.

The decay of the 4d core hole in the rare-earth metal thulium, following resonant absorption into the 4f level, has been monitored by photoelectron spectroscopy and compared with calculations predicting the 4f and 5p electron distribution curves. The Tm, deposited on a cooled Ag surface to stabilize both trivalent and divalent states, shows three strong peaks in the total electron yield spectrum. The first absorption peak ${(}^{3}$${\mathit{H}}_{6}$) results in a resonant enhancement of predominantly the 5s and 5p emission, together with an increase in the trivalent 4f emission. The weaker absorption peak ${(}^{3}$${\mathit{H}}_{5}$) leads to a relatively small increase in the emission through the trivalent 4f state. The last absorption peak ${(}^{3}$${\mathit{G}}_{5}$) shows a strong enhancement of two specific channels, at higher binding energy, within the trivalent 4f emission. The divalent 4f emission is only enhanced near the $^{3}$${\mathit{H}}_{6}$ resonance.

The X-ray photoemission spectra of Nd(OH) 3, Sm(OH) 3, Eu(OH) 3 and Gd(OH) 3

Experimental XPS spectra of lanthanide trihydroxides (Ln(OH) 3, where Ln is Nd, Sm, Eu or Gd) at Ln 3+5p, 5s, 4d, 4p, 4s, and 3d and OH − 2σ, 3σ and 1π levels are reported. A process of computation obtained by combining Wendin's quasi-particle method and the Johansson-Martenssen method with first-order correction is employed in this work to explain the observed physical phenomena. The derived model implies that the spectral function of a long lifetime quasi-particle is a Gaussian function, locates the atomic orbital energy, corrects the variation in binding energy due to the correlation energy, calculates semi-empirically the dipole relaxation energy shift, and accounts for the atom-to-solid core level shift. Valence band XPS spectra, Ln 3+ 4d, 4p, 4s, 3d and experimental and theoretical binding energies of Nd 3+, Sm 3+, Eu 3+ and Gd 3+ in Ln(OH) 3 are tabulated. It is conspicuous that the 4f and 5d orbital collapses are responsible for the interesting features of three giant Coster-Kronig fluctuations, 5 s 1 ⇌ 5 p 2md 1 (m = 5, 6, 7, …), 4 p 1 ⇌ 4 d 2mf 1 (m = 4, 5, 6, …) and 4 s 1 ⇌ 4 p 14 d 1mf 1 (m = 4, 5, 6, …), and the “satellite complex” in the 3d core hole spectra.

Surface core-level shifts of some 4d-metal single-crystal surfaces: Experiments and ab initio calculations.

High resolution measurements are reported of the surface core-level shift of the 3d level for the Rh(111), Rh(110), Pd(111), Pd(110), and Ag(111) single-crystal surfaces. These measurements and earlier ones for the Mo(110), Rh(100), and Pd(100) surfaces are analyzed by ab initio calculations of the surface core-level shift. The calculations are found to reproduce well the trends of the experimental shifts with the 4d metal and with the crystal plane. The comparison between these experimental and theoretical results demonstrates the importance of proper inclusion of final-state effects for accurate calculations of surface core-level shifts. A core hole in a surface atom is found to be better screened than one in a bulk atom for the 4d metals to the left of Pd in the Periodic Table. The use of the Z+1 approximation to describe the core hole is investigated both by explicit use of this approximation and by performing calculations for 1s and 3d core holes, respectively. The Z+1 approximation is found to be well obeyed in the case of Ag whereas for the rest of the 4d transition metals it is less precise, introducing errors of typically 0.1 eV.

Electronic structure and core level photoemission spectra in TiO2 compounds

The authors first calculate a standard 'linear muffin-tin orbital' band structure (including combined correction and exchange correlation) for TiO2 compounds in rutile structure. The outcoming bands, especially the last valence (mostly O 2p states) and first conduction (mostly Ti t2g states) bands, are fairly hybridized. The authors then use the information contained in the previously considered density of states to build a simplified electronic structure of TiO2 in order to interpret the Ti core-level photoemission spectra in the simplest way. To achieve this they use the filled band impurity Anderson model which treats the Ti 3d electron-electron correlations and they incorporate the 3d-core hole interaction. Although the model is quite crude from the point of view of the one-electron band structure calculation, it contains many-electron features which can be solved exactly and which are sufficient to roughly describe the satellite structure observed in the core photoemission spectra of TiO2 (as well as related insulating compounds like Li43/Ti53/O4 or TiF4, for example).

The NVV Auger electron spectrum of the HI molecule

The electron-beam-excited NVV Auger electron spectrum of HI is presented together with a new recording of the X-ray excited photoelectron spectrum of the I 4d core lines. It is shown that the initial I 4d core hole state is vibrationally excited. The NVV Auger electron spectrum is dominated by transitions to the pi -2 dicationic states of 3 Sigma -, 1 Delta and 1 Sigma + symmetry. Vibrational structure is resolved in all Auger transitions and the equilibrium geometry of both the initial core-hole state and the final dicationic states have been determined. A splitting of the 3 Sigma 1- and 3 Sigma 0- states of 220 meV is observed. The dissociation of the pi -2 dicationic states in terms of Coulomb explosion is discussed and tentative potential curves are given.

THEORY OF X-RAY EMISSION SPECTRA IN Ce COMPOUNDS

X-ray emission spectra (XES) corresponding to the Ce 5p + 3d transition in insulating Ce compounds are studied theoretically with the impurity Anderson model. It is shown that the XES reflects the dynamical relaxation through the hybridization between 4f and valence band states, as well as the lifetime of 3d core hole.

Influence of valence band states on the core hole screening in lanthanide perovskite compounds

Investigation on core level and valence band photoelectron spectra of lanthanide perovskite samples (Ag x La1−xCoO3;x<0,3) and reinvestigations of metallic lanthanum and La-oxide layers are reported. Differences in preparation method during perovskite formation are leading to different quality of sample purity and different electronic properties. This could be followed by changes in the density of states near the Fermi energy. On the other hand the screening mechanism after ionization of a La 3d core hole is affected by the relative energy position of occupied valence band states with respect to the empty La 4f* level in the core ionized state. Changes in La 3d line profile are discussed as changes in multi electron processes (e.g. “shake up” or “energy gain”) during photoionization. We attribute the two electron process during photoemission of La 3d electrons to the well screened energy gain transition in LaCoO3, whereas the two electron process results in a shake up transition in La2O3 and La(OH)3.

4f STATE WITH THE 3d CORE HOLE OF La, Sm AND SmB6

Mesure de l'emission M 4,5 de Sm, des spectres d'isochromate caracteristiques de Sm et du compose a valence mixte SmB 6 , et du spectre d'emission fluorescente M 4,5 de La dans le lanthane