Core Hole States Research Articles

Core level ionization dynamics in small molecules studied by x-ray-emission threshold-electron coincidence spectroscopy

X-ray-emission threshold-electron coincidence spectroscopy has been applied to ${\mathrm{N}}_{2}$, ${\mathrm{O}}_{2}$, and ${\mathrm{N}}_{2}\mathrm{O}$. The main features of the spectra can be interpreted as conventional core level threshold electron spectra, free from postcollision interaction effects. The relative cross sections for adiabatic ionization of close-lying core hole states and vibrational substates are presented. The results are compared to theoretical predictions and state-of-the-art photoelectron spectra and are discussed in terms of direct threshold ionization and core vacancy rearrangement processes.

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.

Calculation of XANES/ELNES Spectra of All Edges in Si3N4 and Si2N2O

Using a recently developed first-principles supercell method that includes the electron and core-hole interaction, the XANES/ELNES spectra of Si-L 2,3 , Si-K, and N-K edges in α-Si 3 N 4 , β-Si 3 N 4 , spinel c-Si 3 N 4 , and Si 2 N 2 O were calculated and compared. The difference in total energies between the initial ground state and the final core-hole state provides the transition energy. The calculated spectra are found to be in good agreement with the experimental measurements on β-Si 3 N 4 and c-Si 3 N 4 . The differences in the XANES/ELNES spectra for the same element in different crystals are explained in terms of differences in local bonding. The use of orbital-decomposed local density of states to explain the measured spectra is shown to be inadequate. These results reaffirm the importance of including the core-hole effect in any XANES/ ELNES spectral calculation.

Application of an atomic relaxation model for the interpretation of O1s to Rydberg excited Auger electron spectra of molecular oxygen

Resonant and normal Auger electron spectra associated with an O1s initial vacancy of the O 2 molecule have been recorded using photon energies of 541.80, 541.97 and 650 eV. The resonant Auger spectra obtained at 541.80 and 541.97 eV are interpreted as composed essentially of two characteristic parts, one between 10 and 35 eV binding energy reminiscent to direct photoionisation, and another, above 35 eV, associated primarily with spectator decay of the intermediate core-hole state. The spectra above 35 eV owe some similarity to the ordinary Auger electron spectrum recorded at 650 eV, and an interpretation is made in terms of Rydberg series in the cation converging towards the dicationic ionisation limits. The interpretation is based on a relaxation model introduced earlier for atomic resonant Auger spectra. The assignments of the resonant spectra assume that the relevant intermediate neutral states at 541.80 and 541.97 eV governing the Auger decay are essentially the ν = 0 and 1 vibrational components of the (O1s) −1 ( 4 Σ u - ) 4p σ Rydberg state. Arguments for this interpretation are given.

QUEELS software package for calculation of surface effects in electron spectra

AbstractSurface analysis based on the interpretation of electron spectroscopy such as x‐ray photoelectron spectroscopy, Auger electron spectroscopy, or reflection electron energy loss spectroscopy are influenced by the inelastic scattering processes that take place when an electron moves in the region near a solid surface. The actual energy losses depend on the particular experimental situation. Thus, an electron is attracted by its image charge when it is moving in the vacuum. It will therefore lose energy even after it has left the surface and, similarly, an electron that moves in the vacuum towards the solid surface gains energy. Likewise, the scattering properties for an electron moving close to the surface are different from those of an electron moving in the bulk of the solid. In photoemission and Auger experiments the effect of the static hole will also affect the scattering probabilities of the electron in the first few ångströms as it moves away from the point of excitation. These effects will also cause the ‘effective’ inelastic mean free path to depend on the position of the electron with respect to the surface and with respect to the point of excitation. In the past years, we have developed models to describe these effects within a dielectric response theory for different cases of electrons moving near surfaces in general geometries with or without a static core hole. The formulas are quite involved with many infinite integrals and this is a major barrier for people to apply them in practice. With the purpose of making the calculations easier to perform, we have developed software (QUEELS: QUantitative analysis of Electron Energy Losses at Surfaces) that will do this in a user‐friendly way. It is hoped that with this tool at hand, we will be able to more effectively and quickly get a more complete understanding of the importance of these different effects for quantitative analysis of surface electron energy spectra. Copyright © 2004 John Wiley & Sons, Ltd.

Ligand-field effects for the3pphotoelectron spectra ofCr2O3

A major reason for the departure of core level X-ray photoelectron spectra (XPS) of transition metal cations in oxides from the predictions of atomic models is shown to arise from ligand field splittings in the initial state of photoemission. This splitting often leads to a change in the spatial degeneracy of the initial state but the consequences of this for XPS have not been explicitly identified in prior work. Further changes arise from ligand field splittings in the core-hole final states. Results are reported for non-empirical, cluster model many body wavefunctions for the $3p$ XPS of ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}.$ The agreement of the theoretical cluster model XPS with experiment is considerably improved over the pure atomic model. Furthermore, the treatment allows screening of the core hole through changes in the covalent character of the cluster orbitals. This is quite different from the usual description of screening in oxides within the framework of charge transfer configurations and it offers new insights into the role of charge transfer for satellite structure.

Ab initio calculation of the normal Auger spectrum of C2H2

The normal Auger spectrum of the C2H2 molecule, measured far above the C 1s threshold by Kivimäki et al (1997 J. Phys. B: At. Mol. Opt. Phys. 30 4279), has been calculated by analysing separately the spectral contributions due to the decay processes from the gerade and ungerade carbon 1s core-hole states.

A first-principle approach to X-ray scattering from solids

In this work we have developed a first-principle nonrelativistic many-body theory of X-ray scatterings based on general scattering theory developed by Almbladh and Hedin, which allows renormalization of decaying and inelastic interaction. We separately discuss elastic and inelastic, also non-resonant and resonant X-ray scatterings. In particular the resonant X-ray scatterings are carefully studied beyond the conventional second order perturbation theory to investigate the lifetime of core-hole states on the basis of the sound theoretical background. In the present theory we should classify the stable and the unstable states. The former gives infinitesimal lifetime self-energy whereas the latter gives finite one. We also derive a new formula of the resonant Raman scatterings in terms of two different Green’s functions with different lifetimes where two core levels can contribute, which clearly demonstrate how we can measure lifetime-broadening suppressed X-ray absorption fine structures (XAFS) by use of the resonant X-ray Raman scatterings. Some new aspects of anomalous X-ray scatterings are also discussed by use of the polarization dependence.

Core-hole Hamiltonians and corrected equivalent core model for systems with equivalent atoms

Core ionization in systems with several equivalent atoms gives rise to a set of near-degenerate core-hole states each associated with the removal of an electron from one of the delocalized orbitals. The energy splitting between the core-hole states is the intrinsic feature of core ionized systems and should not be neglected. The conventional equivalent core model (ECM) predicts strictly degenerate core-hole states because the core-hole is thought of as completely localized on one of the equivalent centers. This failure as well as several others inherent to the ECM are successfully removed in the framework of the corrected ECM presented in this paper. Two approaches are available in accordance with the two representations of the core-hole, either delocalized or localized. The Z+1 approximation is an excellent starting point in the localized representation. It must be modified, however, when the delocalized picture is used. To this end we introduce a (Z+Q) system where Q is the magnitude of point charges added to the nuclear charges of equivalent atoms which share the loss of an electron upon core ionization. Systematic corrections improving the conventional ECM are obtained by establishing a connection between the ECM and core-hole Hamiltonians. The core-hole Hamiltonians, being ideally suited for description of core ionization, are derived both in the delocalized and localized representations. Numerical results are presented for N2.

Evidence of ultra-fast dissociation in ammonia observed by resonant Auger electron spectroscopy

We present evidence for ultra-fast dissociation of molecular ammonia when photo-excited to the N1s→4a 1 core-hole state. This finding is based on resonant Auger spectroscopical results as well as qualitative arguments concerning the photon energy dependence of the Auger structures. Calculations of the excited state potential based on the Z+1 approximation were performed. Both the calculations and the measurements indicate that the most likely fragmentation pathway for the core excited ammonia molecules leads to NH 2 * and H fragments.

Angle-resolved Auger-photoelectron coincidence spectroscopy (AR-APECS) of the Ge(100) surface

We have measured the angular distribution of Ge ${L}_{3}{M}_{45}{M}_{45}$ Auger electrons in coincidence with Ge ${2p}_{3/2}$ core photoelectrons along the (001) azimuth of the Ge(100) surface. Intensity modulations arising from diffraction effects are suppressed in the coincidence Auger angular distribution and, when specific emission angles of the photoelectrons are considered, new features appear. We attribute the former effect to enhanced surface specificity of the coincidence technique and the latter to sensitivity of the coincidence measurement to alignment of the core hole state.

Equivalent core model: Extended theory and applications

The ability of the recently developed corrected equivalent core model (cECM) to predict properties of core hole states is examined for the CO molecule. It is shown that systematic corrections derived in this approach significantly improve the results of the conventional equivalent core model (ECM). This opens new possibilities to apply the equivalent core formalism to calculate the energy of core hole states which cannot be usefully done by the ECM itself. On the self-consistent-field level the predictions of geometry changes upon core ionization made by the cECM and those of direct core hole calculations are found to be very similar to each other. There exists, however, an appreciable difference between the total energies of the core hole states obtained in both approaches. A new procedure enabling us to improve the results of the cECM, in particular, to reduce this energy difference is proposed. In contrast to the cECM, where the corrections to the ECM energy are found by deriving the Hamiltonian HZ of the Z system from the Hamiltonian HZ+1 of the (Z+1) system, the corrections are straightforwardly obtained in this new method (c′ECM) by deriving HZ+1 from HZ. The importance of the various systematic corrections to the ECM is discussed.

Theoretical molecular Auger spectra with electron population analysis

An approximation method is proposed to simulate the molecular Auger spectra. Auger transition rates are estimated with atomic populations of valence orbitals on an excited atom by Mulliken and Löwdin population analyses. Normal Auger energies and relative rates for H 2O and NH 3 molecules are evaluated using a full CI wave functions among the valence orbitals constructed from SCF orbitals for the neutral ground state and for the core-hole excited state. Theoretical spectra show fairly good correspondence with the experimental spectra. It is demonstrated that the present approach is applicable for qualitative assignment and analysis of Auger spectra for larger molecules.

High resolution C1s and S2p photoelectron spectra of thiophene

Vibrationally resolved C1s and S2p photoelectron spectra of the thiophene molecule have been recorded using monochromated synchrotron radiation at photon energies of 330 eV and 210 eV, respectively. The photoelectron bands contain complex vibrational structures which are analyzed using ab initio and curve-fitting procedures. The analysis is in good agreement with the experimental spectrum which enables identification of two chemically shifted carbon 1s core hole states. We were also able to determine the molecular-field splitting of the S2p3/2 ionic state to about 99 meV. The molecular-field splitting was moreover calculated using second-order Møller–Plesset perturbation theory, confirming the result from the fitting procedure.

Dissociative multiple photoionization of Br 2, IBr, and I 2 in the VUV and X-ray regions: a comparative study of the inner-shell processes involving Br(3d,3p,3s) and I(4d,4p,4s,3d,3p)

Dissociative multiple photoionization of the bromine, the iodine monobromide, and the iodine molecules in the Br(3d,3p,3s) and I(4d,4p,4s,3d,3p) inner-shell regions has been studied by using time-of-flight (TOF) mass spectrometry coupled to synchrotron radiation in the ranges of 90∼978 eV for Br 2, 60∼133 eV for IBr, and 86∼998 eV for I 2. Total photoion and photoion–photoion coincidence (PIPICO) yields have been recorded as functions of the photon energy. Here, giant shape resonances have been observed beyond the thresholds of the inner-shells owing to the Br(3d 10)→Br(3d 9 ϵf), I(4d 10)→I(4d 9 ϵf), and I(3d 10)→I(3d 9 ϵf) transitions. The dissociation processes of the multiply charged parent ions have also been evaluated from variations of photoelectron–photoion coincidence (PEPICO) and PIPICO spectra with the photon energy. From each Br(3p 3/2) (189.9 eV) and I(4p 3/2) threshold (129.9 eV), quintuple ionization of the molecules begins to play important roles in the photoionization, subsequently yielding ion pairs of X 3+–X 2+ (X=Br, I). From the I(3d 5/2) threshold (627.3 eV), loss of six electrons from iodine molecule additionally begins to play a minor role in the multiple photoionization, giving rise to the formation of ion pairs of either I 3+–I 3+ or I 4+–I 2+. A direct comparison of the strengths and the ranges of the I(4d) and Br(3d) giant resonances was successfully made from dissociative photoionization of IBr. Over the entire energy range examined, 60< E<133 eV, biased charge spread relevant to the specific core-hole states of IBr is observed, presumably reflecting the fact that charge localizes mostly in the excited atoms, which can be accounted for mainly by a two step decay via a fast dissociation followed by autoionization upon the VUV absorption.

Importance of interatomic hole screening in core-level spectroscopy of transition metal oxides: Mn3shole states in MnO

Ab initio theoretical results are reported to determine the role of interatomic screening of the metal core hole in Mn 3s x-ray photoelectron spectra (XPS) of MnO. We present information that finally resolves the controversy about the relative importance of intra-atomic and interatomic effects to the XPS spectra of ionic manganese compounds. We focus on the transitions to high-spin 3s core hole states, using a nonorthogonal configuration interaction method. This method allows for a balanced treatment of configurations that involve different degrees of screening of the core hole. The differences between MnO and NiO are analyzed. Although at first glance the two XPS spectra seem to have similar features, the 3s hole final states have completely different characteristics. In MnO, interatomic screening of the core hole is found to play a minor role. This is in contrast with NiO, where the inclusion of interatomic screening of the metal core hole has been shown to be crucial for a proper explanation of the Ni 3s XPS. The main reason for the difference is an essentially atomic effect, namely the smaller electron affinity of Mn as compared to Ni. We have assigned the weak feature observed in the Mn 3s XPS spectrum of MnO at around 10 eV to be a charge-transfer-type high spin satellite.

Systematic corrections to the equivalent core model

The widely used equivalent core model (ECM) describes core hole states in systems with atomic charge Z by considering corresponding states with fully occupied core in systems with increased charge Z+1. When calculating energies of Z-core hole states, the valence energy of these states often has been assumed to equal the valence energy of the (Z+1) ground state. This approach misses several points: most importantly, the different spin symmetry of the corresponding states. The behavior of core hole states is governed by an effective 2×2 matrix Hamiltonian due to the two possible spin states of the core hole. A recently introduced diagonalization gives rise to a scalar core hole Hamiltonian. Both the ECM and the core hole Hamiltonian act in valence space. This allows establishment of a connection between these two approaches. By expressing the core hole Hamiltonian in the (Z+1) orbital basis, we systematically derive corrections to the ECM. Those corrections, including the one arising because of the different spin symmetry of the corresponding states, are presented in second order of Møller–Plesset perturbation theory (MP2). Hence, they can be implemented very easily so that ground-state calculations in a (Z+1) system may directly provide the core hole state energy in the original Z system.

Charge transfer processes during ion scattering and stimulated desorption of secondary ions from gas-condensed dielectric surfaces

The ion emission mechanism from weakly-interacting solid surfaces has been investigated. The H + ion captures a valence electron via transient chemisorption, so that the ion neutralization probability is related to the nature of bonding of adsorbates. The H + ion is scattered from physisorbed Ar at any coverage whereas the H + yield from solid H 2O decays considerably due to covalency in the hydrogen bond. In electron- and ion-stimulated desorption, the ion ejection probability is correlated intimately with the physisorption/chemisorption of parent atoms or molecules. The emission of F + ions is rather exceptional because they arise from the screened F 2s core-hole state followed by the ionization via the intra-atomic Auger decay after bond breakage. In electron-stimulated desorption of H 2O, hydrated protons are emitted effectively from nanoclusters formed on a solid Ar substrate due to Coulomb repulsion between confined valence holes.

Accurate core electron binding energy calculations using small 6-31G and TZV core hole optimized basis sets

A procedure for optimizing basis sets for core hole binding energies is described. Contracted Gaussian basis sets are optimized for ground state and core hole state atomic configurations, exponents and contraction coefficients being determined by a minimization of the atomic self-consistent field state within a simulated annealing procedure. The basis sets are used in connection with Δself-consistent field, ΔMøller–Plesset and ΔKohn–Sham theory calculations of core electron binding energies and chemical shifts of high accuracy. Whatever the method, the small basis sets optimized in this way give results with an accuracy comparable to that obtained using very extended normal basis sets close to the complete basis set limit. They provide an excellent alternative to treat large molecular systems and push the accuracy of the ΔKohn–Sham technique for binding energy computations even further, exhibiting only small (a few tenths of an electron volt) deviations from experimental data.

AUGER ELECTRON SPECTRA OF Kr2p HOLES USING MONOCHROMATIC SOFT X-RAYS

Normal Auger electron spectra from Kr2p hole states, L 23 M 45 M 45 and L 23 M 23 M 45, have been measured using monochromatized synchrotron radiation and a high resolution electron spectrometer. Measured spectra were reproduced with a fitting calculation, where Voigt functions including an instrumental resolution and natural lifetime widths of related core hole states were used. At most final states estimated energies for spectral peaks agree with those by electron and ion beam techniques. Relative intensities for some peaks are appreciably different from the previous results.