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.

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.

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.

X-ray appearance potential spectra and electronic structure of transition metals

The possibility of determining the density-of-states distribution function in the unoccupied part of the conduction bands of the transition metals Cr, Mo and W from experimental APS spectra is considered. Comparison of the results obtained with data from band-structure calculations for these metals is carried out.

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.

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.

Appearance potential spectroscopy studies with Fe/Ni alloys

Soft X-ray appearance potential spectra were recorded for a series of Fe/Ni alloys whose compositions in the surface region were determined by means of Auger electron spectroscopy. The L3 appearance potentials (correlated with the binding energies of 2p3/2 electrons) of Ni increase by 0.5 eV between 0 and 30% Fe and then at even higher Fe concentrations remain constant. The corresponding values for Fe are practically unaffected by alloying. This effect is tentatively ascribed to a local intra-atomic reorganization process of the Ni valence electrons. A quantity derived from the intensities of the APS peaks which is correlated with the (local) densities of unfilled states above EF varies linearly with the alloy composition.

The Soft X‐Ray Appearance Potential Spectra of Diamond and Graphite

AbstractThe soft X‐ray appearance potential spectroscopic technique has been applied to diamond and graphite. The spectrum obtained from diamond is found to be similar to those obtained from other forms of carbon but to contain important differences. From the differences it is inferred that a graphitic over layer is not present. The similarities in the spectra of diamond and graphite suggest that the multiplicity of peaks observed in spectra from carbon is not due to plasmon effects. Agreement between the observed diamond spectrum and that obtained from a density of electron states model is only possible if an empty surface state band is postulated existing between the valence and lower conduction bands. The spectra of both diamond and graphite are effected by the evaporation of tungsten from the thermionic cathode. The contamination is particularly difficult to observe as the technique is shown to be relatively insensitive to evaporated tungsten films.

Soft X-Ray Appearance Potential Spectra of Titanium Oxide and Alloy

SXAPS(Soft X-Ray Appearance Potential Spectroscopy) of titanium oxides and a Ti–Ni(Ni 3.5 wt. %) alloy were studied. Chances in the O(K) spectrum and Ti(L3,2) spectra after sputtering and annealing were observed. The binding energy of Ti(L3) increases with the oxidation number and is in good agreement with soft X-ray absorption spectra. The changes of the separated L3 spectrum correspond to those of soft X-ray absorption spectra. The two subpeaks in Ti2O3 and TiO2 seem to occur from two narrow d bands. The spectrum expected from the theoretical band structure of TiO roughly agrees with the experimental spectrum.