Soft X-ray Appearance Research Articles

Soft x-ray appearance potential spectroscopy study of MgO (100) and α-Al2O3 (100) single crystals

Soft x-ray appearance potential spectroscopy (SXAPS) measurements was used to measure on MgO (100) and α-Al2O3 (100) single crystals. Mg 1s, Al 1s, and O 1s SXAPS self-deconvoluted (SD) spectra were obtained. The features of the Mg 1s and O1s SD spectra are in fair agreement with those of the near-edge x-ray absorption fine structure spectra for an MgO thin film (3 ML) on Ag (100). This suggests that the SXAPS spectra reflect electronic states of the relaxed MgO (100) surface. The features of the Al 1s and O 1s SD spectra are in qualitative agreement with those of the electron energy-loss spectroscopy. The SXAPS SD spectra are discussed in terms of antibonding states and partial density of empty states obtained by theoretical calculations for MgO and α-Al2O3, respectively. The present result suggests that the “approximate dipole selection rule” is applicable to the SXAPS spectra of MgO and α-Al2O3, as well as 3d transition metal oxides.

Appearance potential spectroscopy with a photon counting detector and multiple scattering spectral interpretation

We describe a soft x-ray appearance potential spectroscopy apparatus, which uses a windowless hyperpure Ge detector operated in the photon counting mode. Direct comparisons of recorded spectra with the self-convolution of x-ray absorption spectra and with ab initio simulations in the multiple scattering framework are reported and discussed.

TWO TYPES OF MAGNETIC RECONNECTION IN CORONAL BRIGHT POINTS AND THE CORRESPONDING MAGNETIC CONFIGURATION

Coronal bright points (CBPs) are long-lived small-scale brightenings in the solar corona. They are generally explained by magnetic reconnection. However, the corresponding magnetic configurations are not well understood. We carry out a detailed multi-wavelength analysis of two neighboring CBPs on 2007 March 16, observed in soft X-ray (SXR) and EUV channels. It is seen that the SXR light curves present quasi-periodic flashes with an interval of {approx}1 hr superposed over the long-lived mild brightenings, suggesting that the SXR brightenings of this type of CBPs might consist of two components: one is the gentle brightenings and the other is the CBP flashes. It is found that the strong flashes of the bigger CBP are always accompanied by SXR jets. The potential field extrapolation indicates that both CBPs are covered by a dome-like separatrix surface, with a magnetic null point above. We propose that the repetitive CBP flashes, as well as the recurrent SXR jets, result from the impulsive null-point reconnection, while the long-lived brightenings are due to the interchange reconnection along the separatrix surface. Although the EUV images at high-temperature lines resemble the SXR appearance, the 171 Angstrom-Sign and 195 Angstrom-Sign channels reveal that the blurry CBP in SXR consists ofmore » a cusp-shaped loop and several separate bright patches, which are explained to be due to the null-point reconnection and the separatrix reconnection, respectively.« less

Soft x-ray appearance potential spectroscopy study of NiO and CoO single crystal surfaces

Soft x-ray appearance potential spectroscopy (SXAPS) spectra of Ni 2p, Co 2p, and O 1s for NiO and CoO single crystal surfaces have been measured. Two peaks and a shoulder along with two core lines due to 2p3/2 and 2p1/2 are found for a Ni 2p raw spectrum. A Ni 2p metallic edge was observed for the NiO surface sputtered by Ar ions. An O 1s spectrum of NiO exhibits a broad peak between about 522 and 550 eV and the self-deconvoluted spectrum shows five peaks. For the Co 2p spectrum, two core lines due to 2p3/2 and 2p1/2 along with a shoulder are found and the self-deconvoluted spectrum exhibits four peaks. The broad peak is also seen between about 525 and 555 eV for the O 1s spectrum of CoO and the self-deconvoluted spectrum shows four peaks. The features of the self-deconvoluted SXAPS spectra of Ni 2p, Co 2p, and O 1s for NiO and CoO are found to resemble those of the corresponding near edge x-ray absorption fine structure (NEXAFS). The origin of the peaks is discussed to compare the present work with the previous theoretical calculations for the NEXAFS spectra.

Soft X-ray appearance potential spectroscopy (SXAPS) study of TiO 2(1 1 0)-1 × 2 and (0 0 1)-1 × 1 surfaces

A soft X-ray appearance potential spectroscopy (SXAPS) apparatus with high sensitivity was built to measure non-derivative spectra. SXAPS spectra (non-derivative) of Ti 2p and O 1s for TiO 2(1 1 0)-1 × 2 and (0 0 1)-1 × 1 surfaces have been measured using low incident currents (about 10 μA/cm 2) and a photon counting mode. Density of empty states on Ti and O sites are deduced by self-deconvoluting the spectra. The self-deconvoluted SXAPS spectra are qualitatively similar to those measured by X-ray absorption spectroscopy (XAS). The Ti 2p 3/2 spectrum shows two strong peaks which correspond to t 2g and e g states. For the O 1s spectrum two strong peaks near the threshold are also found which can be ascribed to O 2pπ and O 2pσ states. These results suggest that the spectra almost obey the dipole selection rule, so-called the “approximate dipole selection rule”. The SXAPS spectra of Ti 2p and O 1s for the (1 1 0) and (0 0 1) surfaces resemble qualitatively, which is consistent with the XAS results. The spectra measured on the (1 1 0)-1 × 2 surface at an incident angle of 45° off normal to the surface and on the (1 1 0) surface sputtered by Ar ions indicate that SXAPS is very sensitive to the surface electronic states.

High-performance x-ray detector for appearance potential spectroscopy

A high-performance detector for soft x-ray appearance potential spectroscopy is described. It basically consists of an alkali halide photon-to-electron converting layer evaporated onto a self-supporting amorphous carbon foil. This combination absorbs the low energy photons thereby increasing the signal-to-noise ratio. Further improvement of the signal-to-noise ratio is achieved by an additional x-ray filter. The choice of filter and converter material depends on the characteristic radiation of interest. The electrons emitted from the converter are detected by means of a microchannel plate. With this detector a signal-to-noise ratio of 150 was obtained at the diamond appearance potential spectroscopic peak for 5 mC charge per point.

Appearance potential study of PrMn2 and SmMn2 intermetallics

Soft X-ray appearance potential spectroscopy (SXAPS) has been employed to study the changes in the electronic structure of PrMn2 and SmMn2 intermetallics as a result of aloying. In this technique the total X-ray emission associated with the thresholds for the excitations of core levels of the atoms in the surface region of the materials is measured. It sensitively provides information regarding the density of unoccupied states at the Fermi level. The SXAPS spectra of the M4,5 levels of Pr and Sm and the L2,3 levels of Mn in these intermetallics are compared with the corresponding elemental spectra. The observed shift in the core levels represents contribution from the shifts due to charge transfer and that due to the hybridization between the Mn 3d and the rare earth 5d bands. The charge transfer shifts have been estimated from the differences in the Fermi levels of the intermetallic and the element using the free electron model. These values have been utilized to obtain the hybridization shifts. The hybridization shifts show a decreasing trend in these intermetallics and is attributed to the decrease in the number of electrons in the 3d–5d hybridized bands. Our results correlating the Mn magnetic moment with the number of electrons are consistent with the reported experimental data. It is concluded that the 3d–5d hybridization plays a significant role in determining the magnetic properties of these intermetallics.

A study of the electronic structure of GdMn 2 by appearance potential spectroscopy (SXAPS)

The GdMn 2 intermetallic has been studied using soft X-ray appearance potential spectroscopy (SXAPS) which exploits the abrupt changes in the total X-ray emission associated with the thresholds for the excitations of core levels of the atoms in the surface region of the materials. This technique is highly sensitive to the density of states at the Fermi level and is therefore particularly suitable for investigating the changes in the unoccupied density of states as a result of alloying rare earths with transition metals. The SXAPS spectra of the M 4,5 levels of Gd and the L 2,3 levels of Mn in the intermetallic are compared with the corresponding elemental spectra. These core levels exhibit a positive chemical shift. The experimentally observed core level shift is a sum of the shifts resulting from charge transfer and hybridization. The charge transfer shift is approximated by the Fermi energy shift. The Fermi energies in the pure elements and in the intermetallic have been estimated from the free electron theory model. Utilizing these Fermi level and core level shifts, the hybridization effects between 3d and 5d bands are obtained in this intermetallic. The 3d–5d hybridization plays a significant role in the magnetism observed in the intermetallic.

Spin-resolved soft x-ray appearance potential spectroscopy on iron and nickel

The spin-dependent densities of empty electronic states in iron and nickel are investigated by spin-resolved soft X-ray appearance potential spectroscopy. Model calculations using spin- and angular-momentum-resolved densities of states with appropriate weighting are in rather good agreement with the experimental data.

Measurements of the bremsstrahlung background in soft X-ray appearance potential spectroscopy

In continuation of our recent report on the design and construction of a versatile soft x-ray appearance potential spectrometer using a Cu-BeO photoelectron multiplier, the results of our measurements of the bremsstrahlung background as a function of spectrometer parameters are discussed.

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.

A versatile soft x-ray appearance potential spectrometer

A versatile soft x-ray appearance potential spectrometer with UHV and several other facilities has been designed and constructed using HAMAMATSU-make Cu-BeO photoelectron multiplier for the detection of soft x rays. Salient features of the design and construction aspects of the instrument and its performance are discussed.

Soft-x-ray appearance potential spectroscopy study of Pr2Fe14−xCoxB

The praseodymium–iron–boron alloy, Pr2Fe14B, is found to possess excellent permanent-magnet properties. In this paper we report the investigation of the electronic structure of the system Pr2Fe14−x Cox B (x=0, 3, 5, 7, and 8.5) by soft x-ray appearance potential spectroscopy (SXAPS). SXAPS exploits the abrupt changes in the total x-ray emission associated with the thresholds for the excitations of core levels of the atoms in the surface region of the materials. This technique is particularly suitable for investigating the changes in the unoccupied states of rare-earth and transition-metal intermetallics which have high density of empty states at the Fermi level.The Pr M5 -level SXAPS peak in the system is found to exhibit negative chemical shift which progressively decreases as a function of Co concentration. However, the L2,3 levels of Fe and Co show increasing positive chemical shift. The chemical shifts data display a maximum at x=7 in these three cases which correlates well with reported behavior in the lattice constants of alloy systems. These chemical shifts have been discussed on the basis of charge transfer among the constituents of the system. The full width at half-maximum of the L3 peaks of Fe and Co indicates a decreasing trend as a function of Co concentration. These spectral characteristics have been interpreted in terms of the d-band narrowing as a result of interaction between 3d bands of Fe and Co.

Soft X-ray appearance potential spectroscopy study of Ni-Fe alloys

Soft X-ray appearance potential spectroscopy has been exploited to study the electronic structure of the Ni x Fe 100− x ( x = 20, 33, 63, 74) alloy system. The binding energies of L 3 levels of Ni and Fe exhibit negative and positive chemical shifts respectively with increasing Ni content. Also the full width at half maximum of the Fe L 3-level peak increases systematically. The changes result from the charge transfer from Fe to Ni and are in agreement with the rigid-band model for the constituents of the alloys.

Appearance potential study of Nd 2Fe 14B

Soft X-ray appearance potential spectroscopy has been exploited to study the electronic structure of Nd 2Fe 14B alloy. The binding energies of the L 3 level of iron and the M 5 level of neodymium in the intermetallic exhibit positive and negative chemical shifts respectively relative to their elemental values. The full width at half maximum of the iron L 3 level peak also increases. The changes are interpreted in terms of the charge transfer from iron to neodymium and are in agreement with the theoretical calculations of the electronic structure of the Nd 2Fe 14B intermetallic.

Soft X-ray appearance potential study of Ni 74Fe 26

Soft X-ray appearance potential spectroscopy (SXAPS) has been exploited to study the electronic structure of the Ni 74Fe 26 alloy. The SXAPS spectra of Ni and Fe in the elemental metals and in the alloy are presented in the L 2,3 energy regions. The L 3 core levels have been used for the measurements. The spectra of Ni and Fe in the alloy are compared with the corresponding elemental spectra. The binding energy of Ni shifts towards lower energy side while that of Fe shifts towards higher energy side in the alloy. The full width at half maximum (FWHM) of the SXAPS positive going peak is a measure of the unoccupied density of states at the Fermi level. The FWHM of Ni decreases and that of Fe increases. Both these results indicate a charge transfer from Fe to Ni in the alloy. The total charge transfer at the Ni site is less than 0.6 electrons per site. The intensity of the L 3 peaks suggests enrichment of the Fe concentration and depletion of the Ni concentration on the surface of the alloy as compared to the bulk composition. A surface state feature is observed for Fe spectra in both the elemental state and the alloy. The negative dip in the alloy spectra increases for Ni but decreases for Fe. This has been interpreted as the shifting of the 4sp band towards E F in the case of Ni and away from E F in case of Fe in the alloy, consistent with the directions of shift observed for the above core levels. The data lend support to the rigid-band model commonly used for 3d transition metals.

A comprehensive study of the X-ray structure and spectrum of IC 443

view Abstract Citations (95) References (61) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Comprehensive Study of the X-Ray Structure and Spectrum of IC 443 Petre, R. ; Szymkowiak, A. E. ; Seward, F. D. ; Willingale, R. Abstract This paper presents the results of a comprehensive study of the X-ray emission from the supernova remnant IC 443, using the solid state spectrometer, IPC, and high-resolution images of the Einstein Observatory, and the medium-energy detector of the HEAO 1 A-2 experiment. A soft X-ray appearance was observed, highly atypical of a supernova remnant in the adiabatic phase, with little correlation between X-ray and optical or radio features. The best-fit models of the low-energy X-ray spectrum of the brightest area of the remnant suggest either that the remnant has not yet attained ionization equilibrium or that the X-rays arise in a multiphase medium. Pronounced soft X-ray spectral differences across the remnant are accounted for by variations in absorption by an intervening molecular cloud. The analysis suggests that, despite the atypical X-ray appearance, the X-ray emission in IC 443 is probably confined to a thin (0.1 pc) shell. Publication: The Astrophysical Journal Pub Date: December 1988 DOI: 10.1086/166922 Bibcode: 1988ApJ...335..215P Keywords: Astronomical Spectroscopy; Supernova Remnants; X Ray Sources; H Ii Regions; Molecular Clouds; Spectrophotometry; Stellar Envelopes; Astrophysics; NEBULAE: INDIVIDUAL ALPHANUMERIC: IC 443; NEBULAE: SUPERNOVA REMNANTS; X-RAYS: SOURCES full text sources ADS | data products SIMBAD (1)

THE OXIDATION OF CERIUM : A SXAPS STUDY

The valence transition of trivalent γ-Ce to tetravalent cerium with oxidation has been studied with Soft X-ray Appearance Potential Spectroscopy (SXAPS). The spectra are discussed within the framework of the Gunnarsson-Schonhammer-model. The inclusion of multiplet effects allows the determination of the 4f-count of cerium in CeO2.

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.

Abstract: Electronic Structure of Al-Mn Metallic Glasses: A Soft X-Ray Appearance Potential Study

Abstract: Electronic Structure of Al-Mn Metallic Glasses: A Soft X-Ray Appearance Potential Study