Soft X-ray Appearance Potential Spectroscopy 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.

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.

Near-threshold excitation in light rare-earth compounds: A new interpretation of 3d appearance-potential spectra.

A two-step model describing the transitions in electron-excited appearance-potential spectroscopy (APS) is presented for the 3d thresholds of La and Ce compounds. The model permits a consistent interpretation of APS spectra taken with different modes [soft x-ray APS (SXAPS), Auger-electron APS, disappearance-potential spectroscopy]. Within this model an investigation has been performed on La and Ce compounds exhibiting different valencies: ${\mathrm{LaB}}_{6}$, ${\mathrm{CeB}}_{6}$, and ${\mathrm{CeO}}_{2}$. The spectra are discussed in terms of atomiclike 4f states. The fine structure reflects the multiplet structure of the excited configurations modified by state-selective excitation probabilities near the threshold and---in SXAPS---by dipole selection rules. The interpretation of the APS data is supported by comparison with multiplet calculations and core-level electron-energy-loss spectra recorded at low excitation energy.

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.

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.

Core-level spectroscopy of the 4f states of rare earths

Soft X-ray appearance-potential spectroscopy (SXAPS) is a high resolution core-level technique for probing the local density of conduction-band states of solid surfaces. This technique has been exploited to investigate the distribution of 4f states of the rare earth metals. The spectra reflect the transitions of the N 4,5 (4d) electrons to the unoccupied 4f states. These states split into a multiplet as a result of the exchange interaction between the 4f electrons and 4d holes. Multiplicity of this configuration depends upon the number of 4f electrons. A systematic study of the spectral features of rare earth series metals does not reveal any regular or orderly changes, unlike the spectra of 3d transition series metals. These results confirm that the spectra of rare earths are amenable to interpretation in terms of the atomic-like transitions involving exchange interaction rather than in terms of the continuous interband transitions. The experimental results are discussed in the light of the above theoretical considerations.

Appearance potential study of N 4,5-level of light rare earth metals and their oxides

Appearance potential spectroscopy (APS) probes the binding energy of core levels and local conduction band states of solid surfaces. Soft X-ray APS (SXAPS) and Auger electron APS (AEAPS) respectively measure the differential X-ray fluorescence and secondary electron yields as a function of incident electron energy. We have obtained the N 4,5-level SXAPS and AEAPS spectra of La, Ce, Pr, Nd and Sm metals and their oxides. The N 4,5-level AEAPS studies of these metals have not been reported previously. The comparison of the two spectra shows a strong resemblance. Both exhibit multiplet structure below the expected 4d excitation threshold and a broad, 10–20 eV wide peak above threshold followed by small peaks of decreasing intensity. AEAPS peaks are more intense and narrower than SXAPS peaks. The data are used to gain an understanding of the decay mechanism following the excitation of the core levels in these spectroscopies. The spectra recorded from oxidized metals do not show any major changes in the spectral features except for the chemical shift of the threshold. The shifts for all metal oxides are less than 1.0 eV except for Sm which is 2.5 eV. The similarity of the metal and oxide spectra indicates that the APS spectra are dominated by atomic rather than band effects.

Comparison of light rare earths N4,5-level soft X-ray and auger electron appearance-potential spectra

Appearance-potential Spectroscopy (APS) probes the binding energy of core levels and local conduction band states of atoms in the surface region. Soft X-ray APS (SXAPS) and Auger electron APS (AEAPS), respectively, measure the differential X-ray fluorescence and secondary electron yields as a function of incident electron energy. We have obtained the N 4,5-level SXAPS and AEAPS spectra of La, Ce, Pr, Nd and Sm metals. Both spectra exhibit multiplet structure below the expected 4 d excitation threshold and a broad, 10–20 eV wide peak above the threshold followed by small peaks of decreasing intensity. The data are used to gain an understanding of the decay mechanism following the excitation of the core levels in these spectroscopies. The strong similarity observed between the SXAPS and AEAPS indicates that the characteristic emission and not the bremsstrahlung dominates the spectral lineshape in APS.

Soft x-ray appearance potential spectroscopic study of Sm–Co intermetallics

Soft x-ray appearance-potential spectroscopy (SXAPS) exploits the abrupt changes in the total x-ray emission associated with the threshold for the excitation of core levels. The structures corresponding to the core level excitation are enhanced by differentiating the total x-ray yield as a function of incident electron energy. The present work is a study of the M4,5 and N4,5 levels of Sm and L2,3 levels of Co. SXAPS spectra, in the energy range 100–1200 eV, have been obtained for the pure metals and their intermetallics SmCo5 and Sm2Co17. The spectra, representing the derivatives of the self-convolution of the density of unoccupied states above the Fermi level (EF) give information regarding the chemical bonding in these intermetallics. The measured binding energy shifts in SmCo5 and Sm2Co17, relative to the pure metals, are positive for Sm and negative for Co. The Co L3 peak width decreases on alloying, indicating a partial filling of its 3d band as a result of the charge transfer from Sm to Co. These results correlate well with the reported measurements on similar rare earth–cobalt intermetallics.