Core-hole Creation Research Articles

Positron-annihilation-induced Auger electron spectroscopy studies of the (100) and (111) surfaces of silicon

Experimental results are presented which provide a direct comparison of spectra obtained using positron annihilation induced Auger electron spectroscopy (PAES) for two different faces of a semiconductor, Si(100)-(2×1) andSi(111)-(7×7), taken under similar experimental conditions. The factors that affect positron annihilation induced core-hole creation probabilities are discussed and the relationships between parameters that determine the positron annihilation induced Auger intensity and the core-hole annihilation probability by a surface-trapped positron are derived. The experimental annihilation probabilities for Si 2p core-level electrons are found to be significantly different for two studied surfaces: 2.01′0.02% and 0.65 ′0.01% for the Si(100) and Si(111) surfaces, respectively. The large dependence of the core annihilation rate on the crystal face differs significantly from the much smaller face dependence found on metal surfaces. The values of the annihilation probabilities for the two different silicon surfaces provide data for a stringent test of calculations of annihilation characteristics of positrons in surface states. The observed sensitivity of the annihilation probabilities to the crystal face indicates that PAES could serve as an important surface diagnostic tool capable of distinguishing different semiconductor surfaces and defining their state of reconstruction.

Film growth and X-ray induced chemical reactions of thiophene adsorbed on Au(1 1 1)

The adsorption behavior of thiophene on Au(1 1 1) surface has been extensively studied as a function of coverage by two inner-shell spectroscopies; X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Thiophene was adsorbed on Au(1 1 1) at about 130 K with several exposures. It was revealed that the growth of thiophene film on Au surface has three processes: (1) growth of a flat-lying monolayer; (2) growth of a “compressed(inclined)” monolayer; (3) growth of a standing multilayer. Photochemical reactions induced by X-ray irradiation on the thiophene film have also been studied. The yield of the reaction products drastically increases above the coverage at which the compressed monolayer is formed. XPS and NEXAFS results suggest that the photochemical products are oligothiophenes and thiolates. X-ray energy dependence on the yield shows that the yield is significantly enhanced above the C K-edge, which means that the core hole creation of the carbon atom plays an important role for these reactions.

Luminescence of insulating crystals induced by an XUV laser

Results of the study of CsI luminescence excited by 58.5 eV photons of the XUV laser of LULI are presented. The energy of the exciting photons was sufficient for the creation of core holes at the 4d-iodine level. Following a single laser pulse luminescence spectra in the range 200 - 900 nm were detected. Using foil filters the density of excitation was varied from 10 9 to 10 13 photons/cm 2 . The luminescence spectra measured following the laser pulse were substantially different from those measured using synchrotron radiation of the same energy or with X-rays. The following effect observed is discussed: manifestation of a new band attributed to the creation of complex defects (550 nm).

Dichroism in the spin-polarized Auger electron diffraction for the [formula omitted] surface

The magnetic structure of the c(2 × 2) S Fe(001) system was investigated using spin-polarized Auger electron diffraction. We consider the sulfur LMM Auger process following core-hole creation by polarized X-rays. The Auger electron diffraction is described by the scattering path operator including single and multiple scattering processes. The contributions of different sources of the spin polarization, such as spin polarization of the core-hole, the scattering at the surrounding atoms and local magnetic moments, are investigated. The magnetic scattering gives the main contribution to the spin polarization.

Resonant inner-shell photoelectron spectra of ground-state and laser-excited Cr atoms

Photoelectron spectra and partial cross sections of ground-state and laser-excited Cr atoms in the range of the excitations are presented. The experimental spectra are compared with results obtained by calculations within the spin-polarized random-phase approximation with exchange (SP RPAE), in which the dynamic relaxation upon core hole creation is taken into account.

Spin-resolved x-ray photoemission studies of ferromagnetic metals

Recent spin-resolved x-ray photoelectron spectroscopy (SRXPS) studies of ferromagnetic metals are reviewed. SRXPS studies of metallic Fe, Co, Co66Fe4Ni1B14Si15, and Ni demonstrate that core-level photoemission, and the itinerant electron response to core-hole creation, are highly spin-dependent. The exchange splitting of the Fe 2p3/2 level is found to be 0.48±0.05 eV. Lifetime broadening results for the Fe 2p3/2 N↑ (majority spin) and N↓ (minority spin) components indicate conservation of spin in core-hole filling processes involving the valence band. SRXPS study of the Fe 2p3/2 peak asymmetry α reveals a dependence of electron–hole excitation on the spin of the core hole. Spin analysis of the Fe 3s XPS line shape shows it to be a three-component spectrum, rather than the two-component line shape assumed previously. A photon energy dependence of one of the Fe 3s components explains disagreement among previous Fe 3s XPS results. Comparisons of SRXPS from Co metal and Co66Fe4Ni1B14Si15 directly demonstrate the effect of a reduced atomic magnetic moment on the spin dependence of core-level XPS. The behavior of lifetime broadenings for the N↑ and N↓ Co 2p3/2 components show that the reduced Co magnetic moment found in the Co66Fe4Ni1B14Si15 amorphous glass is due to the transfer of ↑-spin valence electron density to the ↓-spin valence band upon glass formation. SRXPS also allows investigation of spin-dependent core-hole screening processes and satellite production, as demonstrated in SRXPS studies of ferromagnetic Ni. Future directions of SRXPS are also explored.

Lattice Relaxation Effect Associated with Core Holes in Ionic Crystals Studied by Time-Resolved Luminescence Spectroscopy

Auger-decay-free core luminescence in RbF, CsF, CsCl, CsBr and BaF 2 has been studied under monochromatic vacuum ultraviolet light excitation using a time-resolved technique. Luminescence spectra associated with core holes are carefully separated from those originating in the valence-band excitation. Based on the spectral shape and energy range of the luminescence bands, a lattice relaxation effect following the core hole creation is discussed.

Crossluminescence in ionic crystals

Crossluminescence (CL) of ionic crystals is regarded with the account for the many-body effects. The crossluminescence yield in the pre-threshold region is considered on the basis of Fano interaction. Relaxation of the doubly-ionised 5p state of the cation ion as manifested in the CL and STE excitation spectra in the region of 4d core-hole creation is cliscussed. The effects of CL quenching are briefly reviewed.

Spin-dependent s-level and valence photoemission from ferromagnetic nickel

Recent spin-resolved x-ray photoelectron spectroscopy studies of Ni core-level and valence-band features are reviewed. Exchange splitting was observed in the main lines of 3s and 2s spectra. These results suggest that the final state Ni 3d electron configuration upon core-hole creation is not purely c3d10 (c=core hole) but rather a mixture of ∼50% c3d9 and ∼50% c3d10. We propose a revised view of core hole screening in Ni in which both unoccupied 3d and unoccupied, unpolarized, and hybridized 3d–4sp states are involved.

Spin-resolved x-ray-photoelectron-spectroscopy study of ferromagnetic iron.

A spin-resolved x-ray-photoelectron-spectroscopy study of the deep core levels of ferromagnetic Fe are reported. The 2p 3/2 , 2p 1/2 , and 2s core levels, as well as the more shallow 3p level, all display interesting spin-dependent structures. These findings indicate the importance of intra-atomic exchange in deep-core-level photoemission from Fe. The electronic response of the metal to core-hole creation is found to be dependent on the spin of the core hole

Desorption of positive and negative ions from SiO2/Si surfaces by electron excitation of core levels.

Electron-stimulated desorption of positive and negative oxygen ions and positive, multiply charged silicon ions from thin ${\mathrm{SiO}}_{2}$/Si films has been studied as a function of impact electron energy. Thresholds observed for ${\mathrm{O}}^{+}$ are correlated with core-level binding energies of substrate atoms. Desorption of negative ions closely follows threshold behavior of positive ions; it is explained in terms of a charge-exchange process in which the outgoing positive ions (or neutrals) capture electrons in the surface region and desorb as negative ions. Desorption of multiply charged silicon ions is also related to creation of core holes, but requires some additional, multielectron excitations.

Relativistic tight-binding calculations of x-ray absorption and magnetic circular dichroism at the L2 and L3 edges of nickel and iron.

The soft-x-ray absorption and magnetic circular dichroism (MCD) cross sections at the ${\mathit{L}}_{2}$ and ${\mathit{L}}_{3}$ core-level edges of Ni and Fe have been calculated using a one-electron tight-binding band-structure approach. The tight-binding scheme is taken from the work of Papaconstantopoulos, supplemented with the inclusion of spin-orbit coupling in the d bands and fully relativistic dipole selection rules. In Ni, a fit to the various ${\mathit{L}}_{2}$ and ${\mathit{L}}_{3}$ intensity ratios can be achieved, but only with values of the d-band spin-orbit parameter \ensuremath{\xi} and exchange parameter ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{ex}}$ at variance with the ground-state band-structure values. For Fe, there is no plausible value of \ensuremath{\xi} capable of explaining the intensity ratios; also, the predicted substructure within the ${\mathit{L}}_{2}$ and ${\mathit{L}}_{3}$ white lines is not seen in experiment. These failures of the one-electron approach are qualitatively consistent with expected many-body electronic rearrangements associated with core-hole creation. Some discussion is offered on sum rules and on orbital versus spin magnetic moments.

Electron rearrangement and energy relaxation due to a core hole creation in molecules

A theoretical calculation has been utilized to study core-ionization and core-excitation states for N 2 O, NO 2 , CO, HCN, H 2 O, and O 3 on the SCF level. The calculation suggests the following three approximate results: (i) For the energy shift rule, the energies of the molecular orbitals (MO) for core hole states (either core ionization or core excitation) are all shifted by approximately the same amount regardless of the MO's character

Spin Resolved Auger-Spectroscopy on Rb-Layers after Core-Hole Creation by Circularly Polarized VUV-Light

Circularly polarized light from BESSY with energies from 15 eV to 20 eV in normal incidence is used to excite oriented 4p3/2-hole states in thick layers of Rb on Pt(111). The CVV-Auger decay of these hole states is studied in a normal-emission set-up. The preferential spin direction of the Auger-electrons has been measured to be parallel to the spin of the exciting photons near the excitation threshold and changes to be antiparallel at somewhat higher photon energies. This behaviour is consistent with an excitation varying with the photon energy and an atom-like model for the decay of the primary hole state. Thereby the two valence electrons participating in the deacy are assumed to be s-like and to be coupled to a singlet state.As the Auger-decay of the excited hole-state does not depend on the excitation, spin dependent Auger-spectroscopy opens a new method of studying excitations and unoccupied electronic states.

Observation of ion desorption from MgO(001) by electron bombardment

Electron ( E + , H 2 + and O + ) from a MgO(001) surface are observed by means of a quadrupole mass analyser (QMA) and by Time-of-Flight (TOF) methods. The desorbed oxygen ions have two different kinetic energies, 7.8 and 4.6 eV. The origin of the ions with the lower energy is identified as adsorbed species which are ionized by electron bombardment (from examining experimental results). The total yield of oxygen ions rises above energies of 80 eV for the primary electron beam. This threshold energy of 80 eV corresponds to the core-hole creation energy in the 2p state of Mg 2+ . The desorption of the faster oxygen ion is explained by the Knotek-Feibelman mechanism of the electronic transitions following core-hole creation in the Mg 2+ ion.

Auger-induced valence processes in alkali halides following alkali core-hole creation.

Ab initio calculated values are presented for the interatomic Auger and radiative decay rates for initial (2p${)}^{2}$ and 2p hole states of the Na ion in NaF. A comparison of the rates shows that these hole states decay overwhelmingly by the nonradiative Auger process. The knowledge of the rates makes possible a clarification of the mechanism of photon-stimulated positive-ion desorption presented by Parks et al. [Phys. Rev. B 28, 4793 (1983)]. Following the creation of a Na(1s) hole in NaF, the ensuing Auger cascade results in the creation of four to six valence holes, or one ${\mathrm{F}}^{+}$ ion and two to four valence holes, on the neighboring negative fluorine ions within an elapsed time of a few times ${10}^{\mathrm{\ensuremath{-}}14}$ s. The majority of the valence holes are produced as hot holes, and are likely to diffuse several tens to hundreds of angstroms in times of the order of picoseconds before self-trapping to form ${V}_{K}$ centers. The fast diffusion of these holes greatly reduces their chance of participating in ion desorption processes. The possibility that a fraction of the holes self-trap immediately and assist in the desorption of positive ions is considered. It is found that positive-ion desorption can occur if enough holes self-trap immediately. However, the required configurations are so special that they may occur with too small a probability to account for the observed positive-ion current. The fast hole diffusion also makes it unlikely that the ${V}_{K}$ centers from the cascade will exhibit the clustering effects which would have been expected had the holes self-trapped in place. An increase in the yield of electron-hole pairs of about 6% is predicted to occur at the Na K edge.

Time-dependent screening in the electron gas.

The transient screening response in real space and time of the electron gas to a suddenly created point charge is calculated in the linear approximation, in a way which allows the decomposition of the response into a part due to plasmons and a part due to electron-hole pairs. The results are expected to be qualitatively correct as a description of core-hole creation in simple metals in the limit of large photoelectron energies. We obtain a clear picture of an electronic ''shock wave'' which propagates outward from the core hole with some dispersion; the group velocities are centered at the Fermi velocity. Behind the shock wave is the static distribution plus a small ringing due to small-q plasmons. The results are examined in the light of experimental evidence for transient effects on core-hole decay.

Polarized Cu K-edge XANES of square planar CuCl42− ion. Experimental and theoretical evidence for shake-down phenomena

To elucidate the origin of the shoulder structure which is known to appear about half-way up the Cu K-edge in the X-ray absorption spectra of Cu(II) compounds, polarized X-ray absorption spectra were measured on the single crystal of (creatinium)2CuCl4 by using synchrotron radiation. The transition responsible for the shoulder structure was found to be exclusively polarized in the direction normal to the molecular plane of the CuCl42− ion. The polarization dependence of other structures in the XANES spectra was also examined and compared with the results of ab initio SCF CI calculation. It is shown that the structure half-way up the Cu K-edge is due to the shake-down accompanying the 1s-4p= excitation, arising from the ligand-to-metal charge transfer induced by the core-hole creation. Structures attributable to the shake-down accompanying the 1s-4pσ and 1s-5p=, σ excitations were also found.

Electron stimulated desorption of excited alkali atoms from alkali halide surfaces

We have measured the alkali resonance photon intensity due to electron stimulated desorption of excited particles from NaCl and KCl targets as a function of incident electron energy (20 to 1000 eV) and sample temperature (room temperature to 300 °C). The energy dependence data suggest a correlation between excited Na and K desorption thresholds and substrate core levels. These are the first experimental correlations of the desorption of neutral atoms, specifically excited neutral atoms, with the creation of core holes. The intensity of emitted radiation is observed to rise sharply and then saturate with increasing temperature.

Ion-stimulated desorption of positive halogen ions

The ejection of ${\mathrm{F}}^{+}$ ions from fluorinated silicon by ${\mathrm{Ar}}^{+}$ or $\mathrm{O}_{2}^{}{}_{}{}^{+}$ impact at 2-10 keV is shown to result from noncollisional processes which follow core-hole creation by the primary ion impact. The ejection mechanism is shown to be closely similar to electron-stimulated desorption, and can be explained using the model of Knotek and Feibelman. ${\mathrm{Cl}}^{+}$ ejection from chlorinated aluminum appears to result from a similar mechanism, whereas ${\mathrm{O}}^{+}$ ejection from oxygen-sputtered silicon appears to be purely collisional. The latter observation is shown to be consistent with the Knotek-Feibelman model.