Multielectron Excitations Research Articles

Two-color pump-probe study and internal-energy dependence of Rydberg-state excitation inC60

Excitation of Rydberg states in isolated ${\mathrm{C}}_{60}$ is studied by time-resolved photoelectron spectroscopy in a femtosecond two-color pump-probe experiment. The relaxation time for electron-electron interaction is determined to be approximately $100\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ with the ${t}_{1g}(\mathrm{LUMO}+1)$ orbital being considered to define the doorway state in a nonadiabatic multielectron excitation process. The internal energy stored in vibrational modes of the ${\mathrm{C}}_{60}$ at $770\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is found to support the excitation process very efficiently while in ``cold'' ${\mathrm{C}}_{60}$ $(80\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, no significant Rydberg population is detected.

Double-Electron Excitation Channels at the Ca2+ K-Edge of Hydrated Calcium Ion

To investigate the effect of multielectron transitions on the EXAFS data analysis at the Ca K-edge, we collected spectra of Ca2+ in aqueous solution. The anomalous peaks detected in the experimental data have been assigned to the simultaneous excitations of the [1s3p] and [1s3s] electrons. An accurate determination of the hydration geometry of the Ca2+ ion has been performed. The hydrogen atoms have also been included in the EXAFS data analysis, and a Ca−H distance of 3.10 Å has been obtained. The influence of multielectron excitations on the quantitative extraction of the structural parameters from the EXAFS spectrum has been investigated. Omission of double-electron excitation edges from the atomic background significantly worsens the quality of the EXAFS fits and results in unreliable values of the Ca−O coordination number and Debye−Waller factor. A proper determination of the coordination geometry of calcium compounds can be obtained only if multiexcitations effects are accounted for.

Adsorption mechanisms of trivalent gold on iron- and aluminum-(oxy)hydroxides. Part 1: X-ray absorption and Raman scattering spectroscopic studies of Au(III) adsorbed on ferrihydrite, goethite, and boehmite

Gold adsorption products on powdered ferrihydrite, goethite, and boehmite samples, prepared by reacting Au(III)-Cl solutions ([Au] = 4.2 × 10 −5-9.0 × 10 −3 M; [Cl] = 0.017–0.6 M) with these adsorbents at pH values of 4 to 9 and Au adsorption densities ranging from 0.046 to 1.53 μmol/m 2 were characterized using Au-L III XAFS spectroscopy. The solutions (before and after uptake) were investigated by Raman scattering to determine speciation and by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) to determine solution composition. We present an analysis of several effects that are observed in the Au L III-edge XAFS spectra, including X-ray beam-induced photo-reduction, multi-electronic excitations, disorder effects, and multiple scattering, that would complicate interpretation of the spectra if not accounted for. A combination of methods (spectral deconvolution, principal component analysis, spectral inversion, and wavelet analysis) was used to identify and quantify these effects, to characterize the nature of mixed ligands around gold, and to distinguish between multiple-scattering features and features due to next-nearest neighbors in the XAFS spectra. Analysis of the Au-L III XAFS spectra showed that Au(III) is present as square-planar Au(III)(O,Cl) 4 complexes in the aqueous solutions and on the surfaces of the Al/Fe-(oxy)hydroxide adsorption samples with dominantly O ligands at pH > 6 and mixed O/Cl ligands at lower pH values. The EXAFS-derived Au-O and Au-Cl distances are 2.00(2) and 2.28(2) Å, respectively, and the magnitudes of the Debye-Waller factors and third cumulants from anharmonic analyses indicate very little thermal or positional disorder around Au(III) in the adsorption samples. Iron second neighbors are present around Au in the Au(III)/ferrihydrite and Au(III)/goethite adsorption samples, with Au-Fe distances of 3.1(1) and 3.3(1) Å. In boehmite, two sets of Au-Al distances were detected at 3.0(1) and 3.2(1) Å. A reverse Monte Carlo study of the XAFS spectroscopic data suggests the presence of a continuum of edge-shared AuO 4-FeO 6 distances, which cannot be described correctly by a classical model of these data in which only a mean distance (although severely under-estimated) is derived.

K-edge x-ray absorption spectra of Cs and Xe

X-ray absorption spectrum of cesium vapor in the K-edge region is measured in a stainless steel cell. The spectrum is free of the x-ray absorption fine structure signal and shows small features analogous to those in the spectrum of the neighbor noble gas Xe. Although the large natural width of the K vacancy $(g10\mathrm{eV})$ washes out most of the details, fingerprints of multielectron excitations can be recognized at energies close to Dirac-Fock estimates of doubly excited states $1s4(d,p,s)$ and $1s3(d,p).$ Among these, the $1s3p$ excitation 1000 eV above the K edge in both spectra is the deepest double excitation observed so far. Within the K-edge profile, some resolution is recovered with numerical deconvolution of the spectra, revealing the coexcitation of the $5(p,s)$ electrons, and even the valence $6s$ electron in Cs. As in homologue elements, three-electron excitations, either as separate channels or as configuration admixtures are required to explain some spectral features in detail.

Development of XAFS theory.

A major goal of theoretical simulations of X-ray absorption fine structure (XAFS) is to provide calculations for the interpretation and analysis of experimental data in terms of geometrical and electronic information. The extended region or EXAFS (50-2000 eV above an absorption edge) contains geometric information about the pair distribution function, i.e. distances to the nearest neighbors and their orientation. The theory of EXAFS is now well understood and has been recently reviewed [Rehr & Albers (2000). Rev. Mod. Phys. 72, 621-654]. The near-edge region (0-50 eV above the edge) or X-ray absorption near-edge structure (XANES) probes the states just above the Fermi level, and contains important electronic information, e.g. the electronic density of states (DOS). This data can be used to obtain the number of electrons or holes in the electronic configuration and spin and orbital moments on a particular atom via sum rules. XANES calculations with our ab initio code FEFF8 [Ankudinov et al. (1998). Phys. Rev. B, 58, 7565-7576] usually give semi-quantitative agreement with experiment, and permits the interpretation of XANES in terms of DOS. However, fully quantitative calculations remain a challenge. Several effects still need to be considered to treat the XANES region. These include non-spherical parts of the scattering potential and many-body effects such as multi-electron excitations, core-hole effects and local field effects (screening of the X-ray field).

A systematic study of embedded atom EXAFS: the (2 × 1)O/Cu(110)reconstruction as an ideal prototype system

A systematic analysis of the embedded atom EXAFS (AXAFS) effectis presented. This effect is explained by the backscattering of thephotoelectron at interstitial charge densities. The reconstructed (2 × 1)O/Cu(110)system is an ideal prototype system in which to study the angular dependence ofthe AXAFS because oxygen–copper rows are formed resulting in a C2symmetry. The scattering potential is non-spherical because of the highdirectionality of the O–Cu bonds. The high signal-to-noise ratio of theexperimental data enables us to clearly identify the AXAFS contribution. Here wepresent the definite angular dependence of the experimental AXAFS for the firsttime, giving a unique opportunity to measure the anisotropy of the localembedded atom potential. The angular dependence demonstrates that theAXAFS effect cannot be mimicked by multi-electron excitations or experimentalartifacts for this system. We compare our experiments to theoretical calculationswithin the muffin-tin approximation and show that future full-potentialcalculations are needed to model the angular dependence determined.

Understanding the Effects of Concentration on the Solvation Structure of Ca2+ in Aqueous Solution. I: The Perspective on Local Structure from EXAFS and XANES

X-ray absorption fine structure (XAFS) spectroscopy was used to probe the effects of concentration on the first-shell structure of Ca2+ in aqueous solution. Measurements were carried out under ambient conditions using a bending magnet beamline (sector 20) at the Advanced Photon Source, Argonne. The Ca K-edge EXAFS spectrum for 6 m CaCl2 yielded no evidence for the formation of significant numbers of Ca2+−Cl- contact ion pairs even at such high concentration, a result confirmed by comparison with the data for a dilute (0.2 m) reference solution of the perchlorate. A mean coordination number of 7.2 ± 1.2 water molecules and an average Ca−O distance of 2.437 ± 0.010 Å were determined for 6 m CaCl2, and these parameters are also consistent with earlier EXAFS measurements on dilute Ca2+ solutions. Comparison of the pre-edge and near-edge (XANES) spectrum against those for various references, including the crystalline hydrates, provided further confirmation of the lack of change in the Ca2+ first-shell structure and symmetry. Our measurements help clarify the earlier results of modeling thermodynamic data that imply that some significant structural change occurs at high salt concentration. Taken together, our results suggest the formation of Ca2+−OH2−Cl- solvent-shared ion pairs, rather than Ca2+−Cl- contact ion pairs, is most likely responsible for the unusual thermodynamic behavior of this system. The EXAFS spectrum for an even more concentrated (9.2 m CaCl2) hexahydrate melt, however, did indicate the presence of some contact ion pairs. The new results agree closely with those of an earlier X-ray diffraction study, and serve to further aid interpretation of the aqueous solutions data. On a technical note, a previously unreported multielectron excitation edge at k = 10.2 Å-1 was detected in the EXAFS spectra and assigned to the KLII,III transition. Inclusion of this new transition, along with the other known (KMII,III and KMI) transitions, in the background correction procedure significantly improved the quality of EXAFS fits. Further improvements resulted from the inclusion of Ca−H single scattering paths to treat the protons on the tightly bound water molecules. A Ca−H distance of 2.97 Å was obtained, which is in excellent agreement with the results of neutron scattering measurements (reported in part II). This appears to be the most convincing evidence to date for the detection of proton backscattering in EXAFS measurements of the local structure around ions in aqueous solution.

Continuous Cauchy wavelet transform analyses of EXAFS spectra: A qualitative approach

To better understand the extended X-ray absorption fine structure (EXAFS) spectroscopic information obtained for complex materials such as those encountered in Earth materials, we propose to use the Continuous Cauchy Wavelet Transform (CCWT). Thanks to this method, EXAFS spectra can be visualized in three dimensions: the wavevector (k), the interatomic distance uncorrected for phase-shifts (R’), and the CCWT modulus (corresponding to the continuous decomposition of the EXAFS amplitude terms). Consequently, more straightforward qualitative interpretations of EXAFS spectra can be performed, even when spectral artifacts are present, such as multiple-scattering features, multi-electronic excitations, or noise. More particularly, this method provides important information concerning the k range of each EXAFS contribution, such as next nearest-neighbors identification. To illustrate the potential of CCWT analyses applied to EXAFS spectra, we present experimental and theoretical spectra obtained for thorite and zircon at the Th LII and Zr K edges, respectively. Then, we present CCWT analyses of EXAFS spectra collected for amorphous materials of geochemical and environmental interest, including sodium trisilicate glass and an aqueous chloride solution, at the Mo K and Au LIII edges, respectively. Further studies based on CCWT phase terms are underway, in order to quantitatively characterize anharmonic information from EXAFS contributions.

In situgrowth and thermal treatment of nanostructured carbon produced by supersonic cluster beam deposition: An electron spectroscopy investigation

Supersonic cluster beam deposition (SCBD) has proved to be a powerful technique for assembling nanostructured materials with tailored physicochemical properties. Here we report a series of spectroscopic investigations of the growth process, from the early stages up to thick layers, of carbon films obtained by the deposition of carbon clusters. In order to perform the characterization in situ we have used an ultrahigh vacuum SCBD apparatus directly connected with an electron spectrometer. Carbon clusters have been deposited on Ag(100) and $\mathrm{Si}(100)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}1)$ surfaces. The electronic structure of the thus-obtained samples has been studied via valence-band and core-level photoemission. Multielectron excitations were studied by means of electron energy-loss spectroscopy. Thermal annealing of nanostructured carbon films predeposited onto the Si(100) surface promotes the formation of a SiC-like film. This process initiates at temperatures lower than those observed when closed-cage fullerene precursors are used.

Evolution of KL satellite of F in CaF2

AbstractPhotoexcited F Kα satellite spectra of CaF2 were studied as a function of excitation energy from threshold. The observed spectra show that the intensity evolution of the KL satellite line is not a monotonic function increasing with excitation energy, in contrast to the prediction from the conventional shake model, but has a small bump. Comparison with the Hartree–Fock calculations indicates that the origin of the bump can be ascribed to resonances of multi‐electron excitation processes in photoabsorption. Copyright © 2003 John Wiley & Sons, Ltd.

Failure of the Quasiparticle Picture of X-ray Absorption?

The Golden rule expression for x-ray absorption spectra (XAS) is typically calculated within a one-particle (quasiparticle) approximation and generally leads to good agreement between theory and experiment. The fact that a quasiparticle approximation works fairly well is surprising, since it neglects satellite excitations and intrinsic losses due to a suddenly created core-hole. The resolution of this paradox requires physics beyond the independent particle approximation. This is discussed here using an effective Green's function formulation based on a quasi-boson model that takes interference between inelastic losses into account. This approach shows that inelastic excitations such as multi-electron excitations tend to be suppressed, and that the XAS is given by a broadened quasiparticle particle approximation, together with weak satellite structure and edge singularity effects.

Multielectron excitations in x-ray absorption spectra of Rb and Kr

In the K-edge x-ray absorption of rubidium vapour and gaseous krypton,comprehensive spectra of collective excitations down to the relative probability of 5 ×10−5 are extracted. After removal of the asymptotic Victoreen trend, the region ∼ 100eV above the edge exhibits a steep decrease of the absorption coefficientattributed to core relaxation and post-collision interaction in the Auger decay.With an exponential model for the decrease, the entire spectrum of multielectronexcitations is recognized as a succession of consecutive resonant, shake-up andshake-off channels. They can be identified as coexcitations of electrons fromsuccessively deeper subshells, from 5s down to 3s. The identification of excitedstates is aided by a quantitative modelling of subshell contributions and bynatural-width deconvolution. The valence and subvalence coexcitations are shownto follow the pattern of the lighter homologues potassium and argon.

Symmetry-dependent multielectron excitations near the C 1s ionization threshold and distortion of the shape resonance in CO(2).

Satellite bands accompanying the C 1s photoline for the CO2 molecule parallel to the electric vector of the incident radiation E are found to be more intense than those for CO2 perpendicular to E in the shape resonance region. This indicates that multielectron excitations are caused in part by the interaction of the outgoing C 1s photoelectron with the valence electrons. The photoelectron-impact valence excitations couple with the C 1s single-hole ionization and distort the shape resonance significantly. We assign the broad resonance at approximately 312 eV to a distorted Sigma(u) shape resonance.

Structure and Chemical Transformation in Cerium Oxide Nanoparticles Coated by Surfactant Cetyltrimethylammonium Bromide (CTAB): An X-ray Absorption Spectroscopic Study

Cerium oxide nanoparticles coated by cetyltrimethylammonium bromide (CTAB) were prepared using reverse micelles and microemulsion method. The as-prepared nanoparticles (25 °C) were annealed at 200, 350, 500, and 700 °C for 2 h. Their particle sizes are, respectively, 230, 50, 2, 6, and 10 nm as identified by transmission electron microscopy and X-ray diffraction patterns. A chemical transformation from amorphous Ce3+ component to crystalline Ce4+ component was found in the temperature range of 200 °C to 400 °C. X-ray absorption spectroscopy was used to reveal the electronic and atomic structures. The intermediate spectroscopic valence of formally tetravalent compounds and four features A, B, C, and D were detected from the X-ray absorption near edge structure (XANES) spectra of Ce-LIII absorption for samples annealing at 350, 500, and 700 °C. As in trivalent bulk Ce(NO3)3·6H2O, the Ce3+ characteristic was also found for samples annealing at 25, 200, and 350 °C. Multielectron excitation effect on the EXAFS spectra was eliminated. Local atomic structures of these nanoparticles were probed by using extended X-ray absorption fine structure (EXAFS) technique. The obtained structural parameters for bulk Ce(NO3)3·6H2O were in excellent agreement with its crystallographic data. For nanoparticles annealed at 500 °C and 700 °C, similar atomic structures with bulk CeO2 were found. For the as-prepared nanoparticles without annealing (25 °C), a single shell (10 × 2.59 Å) is sufficient to describe its local structures, while two shells had to be used for the nanoparticles annealed at 200 °C and 350 °C. One shell is corresponding to the Ce3+ component. Another is corresponding to the Ce4+ component. All Ce4+ components have the same structural parameters except different Debye−Waller factors or atom-pair distribution functions. A core−shell model was used to explain the structural parameters around cerium for the two samples annealed at 200 °C and 350 °C. The core part is the Ce4+ component with crystalline CaF2-type cubic structures (eight oxygens at 2.343 Å around Ce); the shell part is an amorphous Ce3+ component with different structures. The mechanism of chemical transformation was discussed in this paper.

Interference between extrinsic and intrinsic losses in x-ray absorption fine structure

The interference between extrinsic and intrinsic losses in x-ray absorption fine structure (XAFS) is treated within a Green's-function formalism, without explicit reference to final states. The approach makes use of a quasiboson representation of excitations and perturbation theory in the interaction potential between electrons and quasibosons. These losses lead to an asymmetric broadening of the main quasiparticle peak plus an energy-dependent satellite in the spectral function. The x-ray absorption spectra (XAS) is then given by a convolution of an effective spectral function over a one-electron cross section. It is shown that extrinsic and intrinsic losses tend to cancel near excitation thresholds, and correspondingly, the strength in the main peak increases. At high energies, the theory crosses over to the sudden approximation. These results thus explain the observed weakness of multielectron excitations in XAS. The approach is applied to estimate the many-body corrections to XAFS, beyond the usual mean-free path, using a phasor summation over the spectral function. The asymmetry of the spectral function gives rise to an additional many-body phase shift in the XAFS formula.

Complete spectrum of multielectron excitations at the Br- K edge x-ray absorption spectra

We report the direct measurement by x-ray-absorption spectroscopy of the complete spectrum of multielectron excitations at the ${\mathrm{Br}}^{\ensuremath{-}}$ K edge. Measurements of crystalline tetramethylammonium bromide were made at 30 K and room temperature. The latter spectrum has been found to be largely dominated by the opening of multielectron excitation channels, thus providing a very good model of the ${\mathrm{Br}}^{\ensuremath{-}}$ atomic background which can be used in the analysis of ionic bromine compounds. The complete sequence of multielectron transitions has been clearly identified and compared with theoretical determinations. A direct experimental determination of the $[1s4p]$ double-electron excitation channel has been performed at the ${\mathrm{Br}}^{\ensuremath{-}}$ K edge.

The use of prior information for extracting the post-edge background

Abstract A new method for extracting the post-edge background μ 0 is proposed, the method of Bayesian smoothing. A further evolution of the smoothing spline method is considered as well. Both techniques are capable of taking into account the prior information: about the absorption edge shape, about positions, forms, and strengths of the peculiarities on the μ 0 due to multi-electron excitations. Besides, since the Bayesian approach works in terms of the posterior probability density functions, it is a natural way to determine the errors of the μ 0 construction, which has always been hardly resolvable for any other method. Even with use of the prior information, which narrows the posterior probabilities, the errors of μ 0 are shown to be essentially larger than the experimental noise. Thus, the traditional use of the latter in the definition of χ 2 residual function gives too high a χ 2 and, therefore, too optimistic errors of the fitting parameters.

Cerium oxide nanoparticles coated by surfactant sodiumbis(2-ethylhexyl) sulphosuccinate (AOT): local atomic structures and x-rayabsorption spectroscopic studies

Cerium oxide nanoparticles coated by sodium bis(2-ethylhexyl) sulphosuccinate(AOT) were prepared by using a microemulsion method. Transmission electronmicroscopy revealed an average particle size of 2-3 nm. X-ray diffractionshowed that the cerium oxide nanoparticles retain the CeF2-type cubicstructures like the bulk crystal. The intermediate valence of formallytetravalent compounds had been detected by x-ray-absorption near-edgestructure (XANES) spectra of Ce LIII absorption in bulk CeO2 and the cerium oxide nanoparticles. Two well resolved white lines canbe assigned to the electron configurations of 4f0L and 4f1L, respectively, where L denotes a ligand hole. At the same time,the cerium oxide nanoparticles also showed the structural features oftrivalent compounds, in comparison to the trivalent Ce(NO3)3·6H2O. Four Lorentzian functions and two arctanfunctions were used to fit the normalized XANES spectra. The extendedx-ray-absorption fine-structure (EXAFS) technique was used to probe the localatomic structures around the absorber Ce. The multielectron excitation effecton the EXAFS spectra was eliminated. A core-shell model was used to deducethe near-neighbour structural parameters around cerium. Bulk CeO2with eight oxygen atoms located at 2.343 Å was used as the reference sampleto extract the backscattering amplitude and phase shift of the Ce-O bond. Onehalf of the atoms locate at the core part with the CeF2-type cubicstructures (eight oxygens at 2.343 Å around Ce), the other half of theatoms are amorphous phase located in the shell part (surface of thenanoparticles) with approximately Ce2O3 structuralfeatures (averaged seven oxygens at 2.50 Å around Ce).

Multielectron excitations probed by helicity-modulation XMCD at K-edge in 3d transition metal compounds.

Multielectron excitations (MEE) have been investigated using X-ray magnetic circular dichroism (XMCD) at the K-edge in 3d transition metal compounds. In the range of 50-70 eV above the absorption edge, a dichroic signal that is associated with MEE was identified. This phenomenon is thought to occur due to the super Coster-Kronig transition described as the final states (1s)1(3p)5(3d)n+2 resulting from the 3p --> 3d transitions. In order to verify this interpretation, XMCD measurements using the helicity-modulation method and calculations of the dichroic spectrum were performed based on this super Coster-Kronig transition.

Liquid Rb micrometric droplets confined in paraffin wax: an X-ray absorption spectroscopy study.

We have performed high-quality X-ray absorption measurements on crystalline (c-Rb) and liquid (l-Rb) Rubidium in the range from 15 K to 320 K. Performing a consistent analysis that takes into account the contribution of the medium range structure, we observe that the l-Rb spectrum is compatible with pair correlation function g(r) previously determined by neutron diffraction experiments. Due to the micrometric size of the liquid droplets we were able to observe a slight undercooling down to 290 K. We were also able to study the details of the very strong multielectron excitations channels in terms of resonances, edges and shake-off features at proper theoretical energy values.