Core Hole States Research Articles

New electron-ion coincidence setup: Fragmentation of acetonitrile following N 1s core excitation

A new electron-energy-resolved electron-ion coincidence setup was developed based on the Scienta SES-100 hemispherical electron analyzer and a Wiley–McLaren type ion TOF spectrometer. With this setup, an electron-ion coincidence study of gas-phase acetonitrile ( CH 3 CN ) was performed using synchrotron radiation. The fragmentation patterns in coincidence with resonant Auger electrons were recorded, following N 1s core excitations to the molecular orbital of the π CN ∗ character. Also the dissociation of doubly charged molecular ions produced after normal Auger decay of N 1 s − 1 core-hole states was investigated from PEPICO and PEPIPICO maps. Kinetic energy releases in some reactions were determined with the aid of Monte-Carlo modelling.

X-ray absorption spectra from the resonant-convergent first-order polarization propagator approach

The one-photon absorption cross sections of molecular systems have been determined in the high-energy region from the imaginary part of the electric dipole polarizability tensor. In contrast to commonly adopted state-specific methodologies, the complex polarization propagator approach does not require explicit consideration of the excited states and it is open-ended towards multiphoton absorption. It is shown that the electronic relaxation in the core-hole state is well accounted for in the present approach with use of standard density-functional based electronic structure methods. Sample calculations are presented of the $K$-edge x-ray absorption spectra for ${\mathrm{H}}_{2}\mathrm{O}$, CO, ${\mathrm{C}}_{4}{\mathrm{H}}_{4}\mathrm{N}$, and ${\mathrm{C}}_{6}{\mathrm{H}}_{6}$.

Competition between relaxation and decay of a core-level electron ionized state in a charge transfer system

In a weak charge transfer (CT) system, such as an adsorbate–substrate system a core-level electron ionized state in an adsorbate observed by photoelectron spectroscopy (PES) as the smallest binding energy state, may be a metastable state. On time scale of core–hole decay it may relax further to the most stable state by CT core–hole screening via transfer of an electron from the substrate to an unoccupied adsorbate discrete level pulled down below the Fermi level of the substrate by an attractive core–hole potential. The latter state is a CT shakedown state, which is not observable by PES but its presence is visible in a decay spectrum, such as Auger-electron spectroscopy (AES) spectrum. The CT shakedown state resembles a resonantly excited core–hole state lying below the core–hole state of the smallest binding energy. If this is so, the AES spectrum measured at far above the core-level electron ionization threshold has then an additional spectral feature, which resembles the autoionization spectrum by the core–hole decay of the resonantly excited core–hole state. Such an example is the N 1s AES spectrum of N 2 molecule physisorbed on graphite reported by Björneholm et al. [Q. Björneholm, A. Nilsson, A. Sandell, B. Hernnäs, N. Mårtensson, Phys. Rev. Lett. 68 (1992) 1892]. The competition between relaxation and core–hole decay of a core-level electron ionized state in a weak CT system is formulated by a many-body theory and discussed. The present theory provides a theoretical foundation of the interpretation by Björneholm et al. They determined the CT time from the intensity ratio of the AES spectrum by the decay of the CT shakedown state to that by the decay of the core–hole state observed by PES as the smallest binding energy state. However, the CT time cannot be determined, unless the branching ratio of the former decay rate is equal to that of the latter one.

Transition from the radiationless resonant Raman scattering to the normal Auger decay in a charge transfer system

The transition from the radiationless resonant Raman scattering to the normal Auger decay in resonant Auger-electron spectroscopy (RAES) spectra of charge transfer (CT) systems is discussed by treating the relaxation and the core-hole decay of the excited core-hole state on the same footing by a many-body theory. When the resonantly excited electron remains at the excited atomic site during the core-hole decay, the RAES spectrum shows the characteristic feature of the resonant Auger–Raman effect, whereas when the excited electron has been transferred from the atomic site before the core-hole decays, the RAES spectrum shows the normal Auger decay. The present theory supports the interpretation of the variation with photon energy of the intensity ratio of the latter spectrum to the former one in the RAES spectrum by the Ar 2p → 4s resonance of Ar atoms adsorbed on Ru(0 0 1) surface reported by Keller et al. [C. Keller, M. Stichler, G. Comelli, F. Esch, S. Lizzit, D. Menzel, W. Wurth, Phys. Rev. B 57 (1998) 11951]. The transition from the radiationless resonant Raman scattering to the normal Auger decay in the RAES spectrum of CuO reported by Finazzi et al. [M. Finazzi, G. Ghiringhell, O. Tjernberg, Ph. Ohresser, N.B. Brookes, Phys. Rev. B 61 (2000) 4629] is discussed in terms of the relaxation of the resonantly excited core-hole state to the core-electron ionized main-line state by the hole–particle excitations. The merging of the resonant Raman–Auger-electron kinetic energy into the normal one about 2 eV above the absorption maximum in Cu 2O reported by Finazzi et al. [M. Finazzi, G. Ghiringhell, O. Tjernberg, Ph. Ohresser, N.B. Brookes, Phys. Rev. B 61 (2000) 4629] is explained in terms of the change in the characteristics of the screening electron in the two-hole final state. The Ti L 23-M 23V RAES spectra of TiO 2 and TiO 2− x are also analyzed.

Coster–Kronig decay of the 2s hole state in HCl observed by sub-natural linewidth Auger electron spectroscopy

Threshold photoelectron–Auger electron coincidence spectroscopy has been applied to the Coster–Kronig transitions from the Cl 2s core-hole state of HCl. The use of coincidence spectroscopy reveals Auger lines associated with the Cl 2s core-hole state in the sub-natural linewidth regime. The coincidence Auger spectrum reveals two band structures corresponding to the formation of HCl 2+[(Cl 2p) −1 (V) −1] doubly ionized states via Coster–Kronig transitions, which are only weakly observed in the normal Auger spectrum. These structures are tentatively assigned to HCl 2+[(Cl 2p) −1 (V) −1] doubly ionized states with the 2 π or 5 σ valence (V) holes.

Many-electron effect in the Si K-LL resonant Auger-electron spectroscopy spectra of the Si delta layer in GaAs

The Si K-LL resonant Auger-electron spectroscopy (RAES) spectra of silicon delta dopped layers in GaAs with very thin capping layers show both normal Auger decay and resonant Auger decay, when the core-level electron is excited to the conduction band. The resonant Auger peak kinetic energy (KE) shows no dispersion with photon energy, except when excited by the highest energy photons [M.D. Jackson, J.M.C. Thornton, D. Lewis, A. Robinson, M. Fahy, A. Aviary, P. Weightman, Phys. Rev. B71 (2005) 075313]. The RAES spectra are analyzed using a many-body theory. The presence of resonant Auger decay and no dispersion of resonant Auger peak KE with photon energy is explained in terms of the relaxation of a metastable excited core-hole state to a stable one on the time scale of core-hole decay. The excited electron in the conduction band either delocalizes rapidly leaving the ionized Si to decay by a normal Auger decay or drops to a state localized in the Si delta layer before the core-hole decays so that the RAES spectrum has both normal Auger decay and resonant Auger decay. As a result of the relaxation, the resonant Auger peak KE does not show any dispersion with photon energy. The variations with photon energy of the normal or resonant Auger peak intensity, KE, and width are explained in a consistent manner by a many-body theory.

Many-body effect in Auger-electron spectra by participant Auger transitions of a core-hole satellite

A discrete two-hole one-particle (2hlp) shakeup/down state can decay either by spectator Auger transitions or by participant Auger transitions. The 2hlp state created by the participant Auger transition of the 2hlp shakeup/down state can be the same as the one created by the Auger transition of the 1h state. The participant Auger transition by which the electron excited by the shakeup/down fills the core hole, can be dominant. If this is so, when the two discrete core-hole states couple to the same continuum and their separation energy is smaller than or comparable to the core-hole lifetime width, there is a possibility of an interference between the Auger transition of the 1h state and the participant Auger transition of the 2hlp state. The screening of the Auger electron emission of the 1h state via the participant Auger transition of the 2hlp state leads to a pronounced interference in the Auger-electron spectroscopy (AES) spectrum. Because the screening function behaves like a Fano resonance. As the function is critically energy dependent, the photoelectron spectroscopy (PES) line peaks do not coincide with the AES ones, when the latter ones are shifted by the 2h final-state energy. In such a case, the PES line peak measured in coincidence with the AES one is shifted from the noncoincidence PES one. When the photoelectron is collected in coincidence with the Auger electron (or X-ray fluorescence) for a selected decay channel, i.e., when the doubly differential photoionization cross section of the photoelectron and the Auger electron (or X-ray fluorescence) being emitted and collected in coincidence is integrated over the Auger-electron kinetic energies (or X-ray fluorescence energies) of the selected decay channel and summed for the final states of the selected decay channel, the photoelectron spectrum is the noncoincidence (singles) photoelectron spectrum weighted by the branching ratio of the partial decay rate of the selected decay channel so that we can extract the screening function by comparing the coincidence photoelectron spectrum with the singles (noncoincidence) photoelectron one.

Theoretical determination of the surface excitation parameter from X‐ray photoelectron spectroscopy

AbstractThe surface excitation parameter (SEP) is determined by analysis of X‐ray photoelectron spectroscopy (XPS) model developed by Simonsen, Yubero and Tougaard (Phys. Rev. B. 1997; 56: 1612). The model includes excitation due to the static core hole as well as the effect of the surface. The only input in the model is the dielectric function of the medium. More precisely, the SEP is extracted from the theoretical spectrum by calculating the ratio between the surface loss part of the spectrum and the elastic peak intensity. The angular distributions of the SEP are calculated for aluminium and silicon for photoelectrons of 1 and 3.4 keV energy. The SEP for XPS is found to be slightly (5–10%) smaller compared to that for reflection‐electron‐energy‐loss spectroscopy (REELS) calculated within the same model, and this difference is ascribed to the effect of the static core hole. Copyright © 2006 John Wiley & Sons, Ltd.

Measuring the energy of the graphite π + σ plasmon peak

AbstractWe examine the graphite plasmon loss structure in reflection electron energy loss spectroscopy (REELS) and XPS with respect to the issue of measuring the energy, Ep, of the π + σ plasmon peak. The two spectra differ in shape because they are based on different physical processes. In fact, a static core hole—leading to intrinsic (in addition to extrinsic) losses—exists only for XPS. We show that no single energy can be measured on the two spectra for the π + σ peak. Possible procedures to deal with the matter are considered. Copyright © 2006 John Wiley & Sons, Ltd.

Theoretical study of ion desorption from poly-(methyl methacrylate) and poly-(isopropenyl acetate) thin films through core excitation

Site-specific chemical reactions following core excitation of poly-(methyl methacrylate) (PMMA) and poly-(isopropenyl acetate) (PiPAc) thin films were investigated. New x-ray absorption spectra of PMMA and PiPAc at the C and O K edges and theoretical spectra within the framework of density functional theory using model molecules were reported, and some new peak assignments were proposed for these spectra. Core-hole excited state molecular dynamics simulations were performed to discuss dissociation dynamics for the target systems, and some specific reaction mechanisms were discussed and explained theoretically; for example, the amount of CH3 ion fragments for PMMA was enhanced at the C and O K edges through the existence of the repulsive sigma*(O-CH3) excited state.

Many-body effect in Auger-photoelectron coincidence spectroscopy spectra

When the shakeup/down excitations are not negligible in the core-level electron ionization, the photoelectron spectral peak measured in coincidence with a selected singles (noncoincidence) Auger-electron spectral peak does not necessarily coincide with the singles one. We discuss how the interference between the core-hole decay of a fully relaxed core-hole state and that of an incompletely relaxed one via the interaction between the final states created by the respective core-hole decays, affects the kinetic energy shift and asymmetrical lineshape change of the coincidence photoelectron spectrum compared to the singles one. When the final-state interaction is considerable, the interference reduces much the energy shift and the asymmetrical lineshape change. By the Auger-photoelectron coincidence spectroscopy (APECS) we can study the interference effect which does not manifest in the singles photoelectron spectrum. We discuss also the interference effect when the core-hole decay rates of both the fully relaxed core-hole state and the incompletely relaxed one depend critically on the changes in both the Auger-electron kinetic energy and the final-state potential. The effect is fairly independent of the changes.

Shake-off of loosely bound electrons in Auger decays of Kr2pcore hole states

Multicharged Kr ions have been measured using monochromatized undulator radiation combined with a coincidence technique. It has been found that a charge-state distribution of Kr ions being coincident with satellite peaks of Kr $2{p}_{3∕2}$ photoelectron is slightly different from that for the main line. Resonant Auger peaks for $2{p}^{\ensuremath{-}1}\mathrm{nl}\ensuremath{\rightarrow}^{1}G_{4}\phantom{\rule{0.2em}{0ex}}\mathrm{nl}$ transitions generated essentially ${\mathrm{Kr}}^{4+}$ only, which differs from the charge-state distribution for the normal Auger peak. These findings suggest that loosely bound electrons in high Rydberg orbitals are easily shaken-off in electron emission processes.

Study of the competition between relaxation and core-hole decay by Auger-photoelectron coincidence spectroscopy

When an incompletely relaxed core-hole state can relax further to a fully relaxed one on the time scale of core-hole decay, the photoelectron line of the incompletely relaxed state is observed in coincidence with the singles Auger-electron line by the decay of the fully relaxed core-hole state. With an increase in the relaxation rate from the incompletely relaxed core-hole state to the fully relaxed one, the relative intensity of the incompletely relaxed core-hole state which relaxes further to the fully relaxed core-hole state on the time scale of core-hole decay increases in both the coincidence photoelectron main line and the singles one. At the same time, the ratio of the Auger-electron line intensity by the decay of the fully relaxed core-hole state to that by the decay of the incompletely relaxed core-hole state in the Auger-electron spectrum measured in coincidence with the singles photoelectron line of the incompletely relaxed core-hole state increases. When the imaginary part of the initial core-hole self-energy is considerably energy dependent, the photoelectron (or Auger-electron) line measured in coincidence with the singles Auger-electron (or photoelectron) line peak does not necessarily coincide with the singles one. The energy shift between the two line peaks increases with the relaxation rate.

Competition between relaxation and decay of a resonantly excited core-hole state

By a many-body theory it is shown that in general when the time scale of the relaxation from the resonantly excited core-hole state to the fully relaxed core-hole state is much shorter than that of core-hole decay, the autoionization (deexcitation) spectrum is more or less identical to the Auger-electron spectrum by core-hole decay of the fully relaxed core-hole state. Here, the fully relaxed core-hole state is the lowest core-hole state in the X-ray photoelectron spectroscopy spectrum. The present theory explains why the C 1s autoionization spectrum of CO molecule adsorbed on Ni(1 0 0) surface measured at the resonant core-level electron excitation energy is more or less indentical to the Auger-electron spectrum measured at far above the core-level electron ionization limit.

Search for a local effect in multiatom resonant core excitation in a surface species: Photoemission and photon-stimulated desorption fromN2on Ni(111)

We have investigated photoabsorption, N $1s$ photoelectron emission, and photon-stimulated desorption (PSD) of ${\mathrm{N}}^{+}$ ions, from ${\mathrm{N}}_{2}$ molecules perpendicularly chemisorbed on Ni(111) surfaces, in the energy range of the Ni $2{p}_{3∕2,1∕2}$ excitations. For this system, N $1s$ photoemission monitors single-core-hole production, whereas PSD of ${\mathrm{N}}^{+}$ is mainly due to excitation of multiply excited N $1s$ core-hole states. The amplitude variations of these two signals and the kinetic energy distributions (KED's) of the ${\mathrm{N}}^{+}$ ions were recorded as functions of the photon energy. In addition, we measured the amplitude variations of PE and PSD as a function of the photon incidence angle, which was varied from grazing (7\ifmmode^\circ\else\textdegree\fi{} with respect to the surface) to steeper angles (43\ifmmode^\circ\else\textdegree\fi{} and 50\ifmmode^\circ\else\textdegree\fi{} with respect to the surface). For grazing incidence, strong variations of both the photoelectron and the ${\mathrm{N}}^{+}$ signals with photon energy and angle of incidence were found in the Ni $2p$ region which are compatible with x-ray optical (dielectric) effects, one manifestation of multiatom resonant photoemission. The ${\mathrm{N}}^{+}$ KED's, which are known to depend strongly on the nature of the electronic excitation responsible, did not change across the Ni $2{p}_{3∕2}$ threshold, which excludes any type of state selectivity in the interatomic core-coupling effects observed. For N $1s$ photoemission, a first analysis of our data suggests a variation of the N $1s$ signal at the Ni $2p$ edges also for steeper angles of light incidence, of comparable magnitude to that at grazing incidence. However, more careful x-ray photoelectron spectroscopy experiments and the investigation of electronically stimulated desorption of neutral ${\mathrm{N}}_{2}$ molecules and N atoms reveal that these effects are due to a strong increase of beam damage when passing the Ni $2p$ edge; these effects could be reduced by rapidly scanning the sample under the beam. We thus conclude that for high angles of incidence most of the Ni $2p$-related changes in our N $1s$ photoemission signal are due to beam damage.

The L 2:L 3 intensity ratio in soft X-ray emission spectra of 3d-metals

We present L 2,3 X-ray emission spectra of the 3d-metals Ti through Cu. It is found that the ratio of the integrated non-resonant emission intensities of L 2 to L 3 emission, I(L 2)/ I(L 3), is approximately two times smaller than the ratio of the core hole state degeneracies (L 2/L 3 = 1/2). Our atomic calculations show that the L 2L 3M 4,5 Coster–Kronig transition is responsible for the reduction of the I(L 2)/ I(L 3) intensity ratio and explains its gradual decrease with increasing atomic number among the 3d transition metals. For the resonant case we find that accounting for the core–hole interaction and local field effects via TDDFT satisfactorily explains the changes in the I(L 2)/ I(L 3) ratio.

Configuration interaction simulation of the NEXAFS photoabsorption spectrum of naphthalene

The near-edge X-ray absorption fine structure (NEXAFS) spectrum of naphthalene was analysed theoratically using the final state approximation rule. ROHF/SCF calculations were carried out for a localized core-hole cationic state under the Bagus-Schaffer scheme for each channel of inequivalent excitations. CI calculations were performed allowing single and double excitations from carbon 1s electrons to the first ten virtual orbitals. The energies and oscillator strengths were calculated. The final spectrum was generated by convolution of Gaussian peaks with 0.5 eV as FWHM of the statistically-averaged calculated transitions. The theoretical result shows remarkable agreement with the experimental high-resolution NEXAFS data measured at BESSY II. Work is in progress to get better control of the variational wave function collapse problem at CI level through the use of symmetry breaking core-hole excited states (MEG-CI). This methodology might offer a clear and transparent way to take into account the relaxational and correlation energy, the virtual orbital specificity and the required restricted configuration interaction needed to the NEXAFS prediction problem.

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.