Soft X-ray Appearance Potential Spectra Research Articles

CORRELATION EFFECTS IN NiO: COMPARISON OF NEAR THRESHOLD EXCITATION SPECTROSCOPIES

At UHV-cleaved single-crystal NiO(100) surfaces core electron energy loss spectra (CEELS) at low primary energy and soft x-ray appearance potential spectra (SXAPS) have been measured at the oxygen K-threshold: the near-edge fine-structures have been investigated up to 40 eV above the threshold. Both spectroscopic methods probe in different ways the structure of the lowest empty states. CEELS is closely related to x-ray absorption spectroscopy (XAS), in both cases the excited final configuration consists of one core hole and one additional electron near the Fermi level E F. In APS we can study correlation effects very directly because the excited system consists of one core hole and two additional interacting electrons in localized resp. delocalized states near E F. The interpretation of the CEELS and SXAPS spectra at the oxygen K-edge is in agreement with a description of NiO as an intermediate valence system, where the charge-transfer gap is smaller than the Hubbard correlation energy. The comparison of our measured spectra with the experimental XAS of NiO shows the dominance of optically allowed channels in both spectroscopies.

Soft X-ray appearance potential spectroscopy of cerium compounds

Soft X-ray appearance potential spectra of CeAl2 and CeO2 and BIS of LaAl2 and CeAl2 are presented. SXAPS is sensitive to valence changes. The SXAP spectra are interpreted within the framework of the Gunnarsson–Schönhammer-model. For a quantitative explanation multiplet splitting and dipole selection rules must be taken into account. In the case of CeO2 the X-ray recombination channel f0 → f3d2 → f1 dominates.

Electronic structure of La—Ni intermetallics: a soft X-ray appearance-potential study

Soft X-ray appearance-potential spectra (SXAPS) of La—Ni intermetallic compounds are presented. SXAPS probes the binding energy (BE) of the core levels and provides information about the local density of conduction band states of the constituents as a function of alloy composition. Charge transfer from electropositive La to electronegative Ni is expected in La—Ni intermetallics. Ni metal has only 0.6 d vacancies and Ni spectra should disappear at high La concentrations as a result of charge transfer. Ni peak intensity decreases with increasing La concentration, however, in no case was a complete disappearance of the Ni spectra observed, which is interpreted as evidence for the d character in unoccupied states. The small BE shifts and narrowing of the SXAPS peaks in alloys indicate filling of the 3 d band of Ni due to hybridization of the wave functions centered on different atom sites instead of the charge transfer.

Mossbauer and soft X-ray appearance potential spectroscopy of iron-nickel-boron metallic glasses

Mossbauer spectra and soft X-ray appearance potential spectra from (Fe0.5Ni0.5)1-xBx metallic glasses for 0.16<or=x<or=0.26 are reported. Mossbauer and magnetic data are determined by the occupation difference between majority and minority subbands. Appearance potential spectra, on the other hand, reveal the total density of unoccupied states. From a combination of the two sets of data for iron the authors derive the change of subband occupation as a function of boron concentration. As a result they find charge transfer not only from boron to empty minority iron states but also from majority to minority states. Unoccupied d states of nickel do not show any change as a function of boron content.

Soft X-ray appearance-potential spectroscopy studies of DyFe 2 and ErFe 2 intermetallics

The soft X-ray appearance-potential spectra (SXAPS) of the M 4,5 levels of Dy and Er and the L 2,3 levels of Fe in pure metals and their intermetallics DyFe 2 and ErFe 2 have been obtained in the energy range of 700–1500 eV. SXAPS probes the binding energies of the core levels and the distribution of local unfilled conduction-band states. Both the constituents of DyFe 2 show negative shifts with respect to the pure metals while those of ErFe 2 show a negative shift for Er and a positive shift for Fe. The widths of the Dy and Er peaks increase and that of the Fe decreases on alloying. The results are interpreted in terms of the Fermi level changes and the charge transfer from the rare earth to Fe. The discrepancy in Fe chemical shifts has been correlated to the electronic specific heat capacity coefficient reported for these intermetallics.

Derivation of the density of unoccupied states in polycrystalline ferromagnetic Fe, Co, and Ni from highly resolved appearance potential spectra

By means of a deconvolution technique, which preserves energy resolution and provides stable results, the densities of unoccupied states in polycrystalline ferromagnetic Fe, Co, and Ni have been derived from highly resolved soft x-ray appearance potential spectra of the respective 2p3/2 core levels. The maximum of the empty d states is found at 0.23 eV for Ni, 0.55 eV for Co, and 0.92 eV for Fe with energies relative to the Fermi level and errors within ±0.1 eV. Although APS essentially probes a localized DOS, there is good agreement with single-particle band structure calculations for Ni and probably for Co, too. The higher lying 4s states exhibit a bandlike character in the APS derived DOS. Their energetic position can be reconciled with results from photoemission within the equal core approximation. Deviations from theory in the APS derived DOS for the empty d states in Fe can be explained by invoking either a surface effect or an orbital symmetry dependent final state effect. The exchange splitting between unoccupied states in Fe is determined as 1.9±0.2 eV at H25.

Evidence for fcc-like short range order in Fe-Ni-B metallic glasses

Soft X-ray appearance potential spectra have been measured for iron and nickel in (Fe 0.5Ni 0.5) 100− x B x metallic glasses with x = 16, 18, 20, 22, 24, 26. For x = 16 both iron and nickel spectra exhibit structural features characteristic for the electronic structure of fcc transition metals. It is concluded that in the amorphous state Ni and Fe are arranged in fcc-like geometry beyond nearest neighbours.

Critical point energies from appearance potential spectra of cobalt, nickel, and copper

Structural features in soft X-ray appearance potential spectra (SXAPS) of cobalt, nickel, and copper previously assigned to collective excitations are shown to be due to corresponding structure in the s-p like density of states of these metals. By reference to theoretical work the critical points in the f.c.c. Brillouin zone responsible for these features are identified and experimental critical point energies are derived and compared to various band structure calculations.

Appearance potential study of the 4f levels of cerium, praseodymium and their oxides

The soft X-ray appearance potential spectra of the M 4,5 levels of cerium, praseodymium and their oxides were obtained in the 800–1000 eV energy range using the modulation potential technique. Rare earths, which have the same valence in the metal as in the oxide, are predicted to have localized 4f electrons which are not affected by oxidation. Contrary to this assertion, an effect of oxygen on the spectra of the rare earths, cerium and praseodymium has been recorded. The spectra are interpreted in terms of the atomic-like transitions involving exchange interaction between the 4f electrons and 3d vacancies and are not considered to be a self-convolution of the density of unoccupied 4f states in the above metals. The data are consistent with the trends in localization of the screening orbitals across the lanthanide series.

Soft X-ray appearance-potential spectra of Ho and Ho/Ni alloy

Soft X-ray appearance-potential spectra of the M 4,5 and N 4,5 levels of metallic Ho and Ho/Ni alloy have been obtained in the 140–1400 eV range. The spectrum represents the differential excitation probability of the core levels and mirrors the self-convolution of the density of unoccupied states of the surface atoms under the influence of alloy composition. The observed binding energies of the M 4,5 and N 4,5 levels of Ho/Ni alloy were determined to be lower than those of metallic Ho. The widths of the Ho peaks also showed a corresponding decrease as a result of alloying. Both measurements correlate well with the changes in the Fermi energy as opposed to charge transfer.

Soft x-ray appearance potential spectroscopy study of pure rare-earth metals with a sealed-off analyzer tube

A comparative study of the soft x-ray appearance potential spectra of various pure rare-earth metals has been carried out with a specially designed sealed-off analyzer tube. It is shown that for lanthanide elements with even number of 4f-electrons (except Eu) no spectrum is found in a region of accelerating potential from 50 to 1400 V. The most sensitive peaks are M5 (3d5/2) and M4 (3d3/2), which can serve as ’’finger prints’’ to identify the rare-earth metals. The ratio of the M5/M4 peak heights might be classified into three groups, according to their various values. Satellite peaks also exist, although no satisfactory explanation has yet been given. The purity of most samples is checked by a Mattauch–Herzog type mass spectrometer or by the proton-induced x-ray emission (PIXE).

Deconvolution of appearance potential spectra II

Soft x-ray appearance potential spectra from solids carry information on the local density of unoccupied states in the surface region of the sample. The intensity of the emitted characteristic radiation is, according to a simple model, proportional to a convolution product of the transition densities of the incoming electron and the ionized core electron. In a first approximation, the two transition densities are proportional to the density of states in the unoccupied part of the conduction band. Attempts to retrieve the density of states from measured spectra must rely on deconvolution procedures. In this paper, we introduce a new simple method. It is essentially a stabilized version of the Hagstrum-Becker method which appears to work quickly and reliably. As an example, the method is applied to an ironLIII appearance potential spectru. Comparison of the results with a theoretical calculation of the density of states and with measured Bremsstrahlungsisochromats obtained by Turtle and Liefeld strongly supports the self-convolution model in this case.

Theory for the 2p-shell sxaps for the 3 d-transition metals

The 2 p-shell Soft X-Ray Appearance Potential Spectra (SXAPS) of the 3 d-transition metals have been described as a scattering process whereby both the impinging and the 2 p-core electrons go into the empty conduction band states above the Fermi level ( E F ) of the metals. It is shown that the resonant contribution to the ordinary bremsstrahlung emission process can be of comparable importance to the usual characteristic one.

Theory for the L2,3soft X-ray appearance potential spectra for the 3 d transition metals

The L2,3 soft X-ray appearance potential spectra (SXAPS) of the 3d transition metals have been described as scattering of both the incident electron and the L shell electron into the conduction band (empty) state above the Fermi level (EF) of these metals. The author uses the part of the conduction band that derives from the d localised states to present a parameter-dependence model to describe the appearance potential spectra of these materials (Sc-Ni). Choosing the appropriate parameters for Mn, a model calculation has been performed which produces the asymmetry apparent in the total X- ray yield of Mn.

Soft X-Ray appearance potential spectroscopy of C, N and O adsorbed on Cr, Mo and W

The 1s level soft x-ray appearance potential spectra of C, N and O chemisorbed on polycrystalline Cr, Mo and W surfaces have been observed. The adsorbate ls spectra show the following characteristic features in the chemisorption regime: (i) Cr: one single peak; (ii) Mo and W: a doublet structure. We tentatively assume that the doublet structure is due to a splitting in the antibonding levels of the surface complex. Oxidation of Mo and W at high temperature causes changes in the metal (Mo 2p 3 2 and W 3d 5 2 ) and in the oxygen 1s spectra as compared with the spectra obtained after room temperature adsorption of oxygen.

Appearance-Potential Study of 4f States of Ytterbium

The soft x-ray appearance potential spectra (SXAPS) of M levels of rare-earth ytterbium has been obtained in 1500-1960 eV range with a nondispersive spectrometer which measures the derivative of the total x-ray fluorescence of an electron-bombarded sample as a function of bombarding-electron energy. The data supplement the information derived from absorption spectroscopy but are applicable to the surface region. The M5 spectrum selectively arising from M5-N7 transition consists of an intense positive peak and an undershoot accompanied by two distinct secondary peaks at 2.0 and 11.0 eV above the threshold. The spectra are representative of the trivalent oxide instead of pure divalent metal which is expected to be void of spectral characteristics due to a full 4f shell. The spectra imply a relatively weak exchange interaction between the 4fN+1 subshell configuration and the M4,5 vacancies.

Structure in the aluminum 1 s soft x-ray appearance potential spectrum

The Al 1 s level soft X-ray appearance potential spectrum for clean and oxidized Al has been observed. In the spectrum of the clean metal, structure related to an edge singularity and a plasmon satellite is distinguished. The plasmon coupling strength in the appearance potential spectrum is found to be almost equal to that in X-ray photoelectron spectroscopy and bremsstrahlung isochromat spectroscopy.

Soft X-Ray Appearance Potential Spectra of the Rare Earths

We have measured the M5, M4 Soft X-Ray Appearance Potential Spectra (SXAPS) of the lanthanide series rare earths. Lutetium and ytterbium do not show the huge signal response characteristic of the other rare earths. This is to be expected since both metals have a closed 4f shell containing 14 electrons. More striking is the lack of significant SXAPS response for ytterbium oxide (Yb2O3) in which a single 4f Yb vacancy is known to exist. The contribution to this spectrum from the local density of states is more than an order of magnitude smaller than the intense peaks seen in the other rare earths. The M5 and M4 spectra of the other rare earths are split into a series of lines. All of the intense SXAPS lineshapes are explained in terms of a resonance in the bremsstrahlung emission. The excitations involve a resonant scattering interaction in which both the incident electron and a d core electron scatter into the atomic 4f level.

Soft X-Ray Appearance-Potential Spectrum of Erbium

The soft X-ray appearance-potential spectra of M4,5 and N4,5 levels of Er have been measured. The spectral features correlate well with the soft X-ray absorption spectra and density of states calculations. The results are interpreted in terms of the exchange interaction between the 4f electrons and d vacancies.

Unfolding appearance potential spectra

The Soft X-ray Appearance Potential Spectra of a number of layer materials have been found to be in excellent agreement with the predictions of a conduction band self convolution model. Crystal field splitting of d orbitals in octahedrally co-ordinated transition metal dichalcogenides gives rise to strong effects in AP spectra which are consistent with a self convolution interpretation. A similar argument is successfully applied to the remarkable graphite spectrum, a detailed analysis of which correlates well with secondary electron emission data and also Photon Appearance Potential Spectroscopy. The deduced form of the conduction band d.o.s. is in qualitative accord with Painter and Ellis's band calculations.