Elastic and inelastic contributions to the auger electron appearance potential spectrum of titanium

The energy distribution of electrons contributing to the L-shell Auger electron appearance potential spectrum of a polycrystalline titanium surface has been measured. The Auger electron appearance potential spectrum is obtained by differentiating the total secondary electron yield of an electron bombarded sample as a function of incident electron energy. At the threshold for scattering from a core level the secondary yield increases. Most of the electrons contributing to this increase have energies below 30 eV, and result from secondary processes following Auger recombination of the core hole. The elastic yield decreases at the threshold, however, due to opening a new channel for inelastic scattering. A comparison of the elastic yield spectrum (DAPS), the total yield spectrum (AEAPS) and the soft X-ray yield spectrum (SXAPS), shows very similar line shapes, but differences in the relative strengths of the lines.     

Correlation of selected core-level spectroscopies

Spectral characteristics representing the distribution of unoccupied conduction-band states in the 3d transition-series metals Fe, Co, Ni and Cu, obtained using soft X-ray absorption (SXA), bremsstrahlung isochromat (BI) and characteristic isochromat (CI) methods, are related mathematically to those found in soft X-ray appearance-potential Spectroscopy (SXAPS) on the basis of the one-electron band approximation. These mathematical relationships are used to compute SXAPS spectra from reported experimental data obtained by the SXA, BI and CI methods. For the metals considered, the correlations between the computed and experimental SXAPS spectra are found to be good. The observed discrepancies are attributed to differences in the selection rules of the excitation processes, to the effect of the core vacancy on the final state, and to many-body phenomena which are not included in the simplified one-electron band model.     

Soft X-ray appearance-potential spectroscopy studies of DyFe2 and ErFe2 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 DyFe₂ and ErFe₂ 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 DyFe₂ show negative shifts with respect to the pure metals while those of ErFe₂ 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.     

Soft X-ray appearance potential spectroscopy (SXAPS) study of TiO2(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 TiO₂(1 1 0)-1 × 2 and (0 0 1)-1 × 1 surfaces have been measured using low incident currents (about 10 μA/cm²) 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.     

Electron spectroscopic studies of clean and oxidized iron

X-ray photoelectron spectroscopy (XPS) and soft X-ray appearance potential spectroscopy (APS) together with Auger electron spectroscopy (AES) were used to study the electronic properties of clean and oxidized (Fe₃O₄) iron surfaces. The features arising from excitations of electrons from Fe 2p core levels are discussed consistently within the common one-electron picture (i.e. neglecting final state effects). For pure Fe the shape of the APS L₃ peak is evaluated taking into consideration the theoretical density of states above the Fermi level and is found to agree well with that observed. As a consequence it is shown that in this case the appearance potential is about 1 eV larger than the threshold energy for the excitation of a core electron to the Fermi level. Thus for $\text{2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ electrons this quantity results to be 704.8 eV from both XPS and APS techniques. Successive oxidation at 500°C leads to an increase of the appearance potentials of the Fe 2p levels by only 0.5 eV, whereas the positions of the corresponding XPS peaks are shifted by as much as 3.5 eV. However this apparent disagreement can be eliminated by taking into account the above mentioned effect concerning the appearance potentials from pure Fe and the fact that the threshold energies (which determine the appearance potentials) of the XPS signals are shifted only by 1.7 eV. This example demonstrates that considerable care has to be taken in discussing “binding energies” or “chemical shifts” as derived from different electron spectroscopic techniques. The observed splitting of the MVV Auger transition of Fe at 47 eV upon oxidation is interpreted in terms of the qualitative features of the valence band structure of Fe₃O₄ and ascribed to the participation of a cross-transition between O 2p and Fe 3p states.     

Extended ELS fine structures above the M2,3 edges of Cu and Ni

Very detailed electron energy loss spectra of M_2,3 core levels of Cu and Ni in the reflection mode are presented. A careful analysis of the oscillations up to about 300 eV above the edges shows strict analogies to EXAFS and demonstrates that structural information can be obtained with this technique. A comparison with X-ray results above the Cu and Ni K-edges shows that very high accuracy in the radial distribution function can be obtained also with ELS spectra taken above shallow core levels.     

A multispectroscopic investigation of the electronic structure of luminescent,tetrahedrally coordinated vanadium-oxygen compounds: Case of YVO4

Photoelectron (XPS) and core ionization loss (CILS) spectra of YVO₄ have been measured and compared to our earlier appearance potential (APS) data on the same compound, literature data on X-ray emission of YVO₄, X-ray emission and absorption of Na₃VO₄, and the energy level diagram of molecular orbitals of the VO₄^3- cluster. Strong many-body effects, with participation of a 2p core hole on vanadium atoms, are found to lead to a drastic local perturbation of the outer molecular orbitals of the anionic cluster VO₄^3-, at the site of the excited vanadium atoms. Identification of the spectroscopic features in XPS on one hand and the edge spectroscopies CILS and APS on the other hand, enables determination of an empirical energy level diagram of excited configurations, which is different in different spectroscopies used. As a byproduct of our experiment, data on the excitation of oxygen (within the VO₄^3- cluster), yttrium (outside the VO₄^3- cluster), and carbon impurity are presented and discussed.     

Photoluminescence studies of Cd1−xZnxS nanocrystals

Cd_1−xZn_xS (0?x?0.5) nanocrystals have been synthesized using a simple chemical precipitation method. Morphological and crystallographic analyses have been done using transmission electron microscope (TEM) and X-ray diffraction (XRD). Room temperature energy and time resolved photoluminescence spectra of these synthesized nanophosphors have been studied using xenon lamp spectroflourometer and high peak power, pulsed N₂-laser excitation, respectively. Photoluminescence spectra are composed of broad peaks ranging from green to red region of the visible spectrum. Important optical parameters: excited state lifetime, trap-depth and decay constant values have been calculated from recorded luminescence decay curves. These nanophosphors show typical lifetime shortening and high quantum yield with increasing concentration of Zn.     

Vapour phase M4,5N4,5N4,5 Auger electron spectra of antimony and tellurium

High resolution M_4,5N_4,5N_4,5 Auger electron spectra from Sb₄ and Te₂ vapours have been measured using electron impact excitation. The spectra have been decomposed into line components and relative intensities and energies of the components are compared with calculated intensities and energies. The calculations have been done for these molecular samples using the free-atom calculation model involving initially filled shells. The calculations have been treated in the mixed coupling scheme using jj coupling for initial state and intermediate coupling for the final state. The experimental results agree well with the calculated values, indicating that the molecular effects on the relative intensities and energies are very small for these core level transitions. The clear molecular effects are found in the broadening of lines and in the kinetic energy shifts due to extra-atomic relaxation effects.     

Recent development in the appearance potential spectroscopy

The development of the appearance potential spectroscopy (APS) in the last decade is shortly reviewed. It is pointed out in which directions this method can usefully complement the information obtained by other methods of surface analysis. The possibility of determining the density of unoccupied states by deconvolution procedures and nearest neigbours arrangement by the extended APS fine structure is mentioned. Possibilities of qualitative and quantitative analysis are shown. Most interest is devoted to the study of electron excitation and transport processes inside the target. The advantage of the method consists in the fact that it is a threshold technique with excitation caused by monoenergetic primary electrons so that processes connected with excitation of particular sublevels can be distinguished. A model of the electron transport processes for DAPS and AEAPS is presented.     

Electron energy loss spectra and X-ray photoelectron spectra of Ca2V2O7

We have measured the electron energy loss spectra of Ca₂V₂O₇ in the reflexion mode, at incident energies between 200 and 2400 eV, and the X-ray photoelectron spectra excited by Al K α radiation. The abundant loss structures observed can be correlated with the possible interband transitions, collective oscillations, and excitation of O2s and V3p electrons within the V₂O₇^4- ion. The gap width and molecular orbital (MO) spread (or splitting) is about l eV larger in the V₂O₇^4- ion than in its component VO₄^3- ion. Excitation of O2s states, which may occur together with some MO over-gap transitions, displaces the collective oscillations about 7 eV towards lower energies. Deeper V3p electrons are excited with a maximum energy loss some 7 eV above their binding energy. Cross transitions from Ca3p levels into some empty states of the V₂O₇^4- ion, or direct transitions to available states of the Ca²⁺ ion could not be unambiguously identified. The energy dependence of the excitation cross section and of the electron penetration depth results in a significant variation of the relative intensity of various losses over the investigated energy range.     

Electric-field-induced energy spectra and dispersions of ZnO/MgxZn1−xO MQWs

The energy spectra and dispersion relations of carriers in the presence of an electric field applied along the growth direction in ZnO/Mg_xZn_1−xO multiple quantum wells (MQW) are calculated using the asymptotic transfer method (ATM) on the basis of the quasistationary state approximation. The energy spectra of the carriers induce some quasi-bound levels under electric fields. The dispersion relations for the energy of the ground state and lower excitation states still have parabolic shapes for both the electrons and the heavy holes in the presence of a moderate electric field. Our results also reveal that the number of energy levels increases with increasing number of ZnO quantum wells and that the energies increase with both increasing Mg composition x and electric field strength.     

Electron energy loss spectra of the silicon 2p, 2s, carbon 1s and valence shells of tetramethylsilane

Inner shell excitation spectra of tetramethylsilane, (CH₃)₄Si, have been measured in the silicon 2s, 2p (L_I,II,III-shell) and carbon is (K-shell) regions using electron energy-loss spectroscopy at an impact energy of 2.5 keV and a scattering angle of ~1°. The high-resolution valence shell spectrum has also been observed at an impact energy of 3 keV and a zero degree scattering angle. The silicon 2p spectra are compared and contrasted with published photoabsorption spectra of SiF₄, SiH₄, and other related Si-containing molecules with varying ligands.     

The density of unoccupied electronic states in copper

The density of unoccupied electronic states in Cu has been measured employing soft X-ray appearance-potential spectroscopy (SXAPS). The L_III le     

X-ray C K-emission band spectra of graphite fluoride

X-ray C K-emission band spectra of graphite fluoride were measured for the first time by the use of the electron beam excitation. The problem of first importance in obtaining X-ray spectra was the decomposition of graphite fluoride induced by electron beams. The decomposition of the samples during the measurements was controlled by moving the samples at a constant rate. The spectra thus measured showed strong high energy satellite bands. Peak positions of these satellites were 5 eV and 8 eV higher relative to that of the main band. Each satellite was considered to be attributable to the CF and CF₂ bonding, respectively. A similar spectrum was obtained from graphite fluoride, (C₂F)_n. Owing to the zig-zag form of the C-C bond, the polarization of the C K-emission bands of well-ordered flaky graphite fluorides was not observed distinctly.     

The sub-bandgap energy loss satellites in the RIXS spectra of beryllium compounds

Resonant X-ray inelastic scattering spectra have been measured in BeO, phenakite (Be₂SiO₄) and chrysoberyl (BeAl₂O₄) with the excitation energy near the beryllium K edge.The RIXS spectra excited in the vicinity of the Be 1s core resonance show two principal features: the scattering on a valence excitation (which at higher excitation energies verges into the characteristic K_α emission), and a remarkably strong energy loss sideband to the elastic scattering peak. The energy loss shoulder appears to result from lattice relaxation in the absorption site. The comparison of the RIXS spectra of phenakite, chrysoberyl and BeO shows that the strength of the low energy sideband differs greatly; it is strongest in BeO and weakest in phenakite. The Si 2p RIXS spectra of phenakite also display a similar strong sub-bandgap energy loss tail.To gain further insight to this process, transitions in a system with a single vibrational mode have been modelled. The phonon relaxation has been simulated empirically by “smearing” the photoabsortion-populated vibrational levels with lower levels. This simple model is able to qualitatively explain this wide energy loss shoulder.     

Atomic and electronic structures of lutetium oxide Lu2O3

The chemical composition, electronic structure, structure, and physical properties a lutetium oxide Lu₂O₃ film are studied by X-ray photoelectron spectroscopy, ellipsometry, and X-ray absorption spectroscopy. The short-range order in Lu₂O₃ is found to correspond to its cubic modification. The binding energies of the 1s and 2p levels of oxygen and the 4d _5/2 and 4f _7/2 levels of lutetium are 529.2, 5.0 and 7.4, 195.9 eV, respectively. The energy gap determined from the electron energy loss spectrum of the film is 5.9 eV. The electron energy loss spectra have two peaks at 17.4 and 22.0 eV, which can be attributed to the excitation of bulk plasma oscillations. The dispersion of the refractive index is measured by spectral ellipsometry. The refractive index is shown to increase from 1.82 at 1.5 eV to 2.18 at 5.0 eV, and the high-frequency permittivity of Lu₂O₃ is 3.31.     

A complete and self-consistent evaluation of XPS spectra of TiN

The electron configuration in single crystalline (sc-)titanium nitride (TiN) has been quantitatively studied using angle resolved X-ray photoelectron spectroscopy (AR-XPS). All samples were fabricated and transferred in situ so that only minimal surface contaminations were observed. The residual oxygen contamination was separated from the bulk information by extrapolating angle resolved measurements. Special attention is given to the quantitative evaluation of the spectra based on basic principles. Shake-up features are observed on core level lines and appear due two final ionized states. The ratio of this shake-up and the main Ti 2p energy line are investigated in this paper. In order to quantify this shake-up a precise evaluation is required. Here we present an approach to evaluate the XPS spectra of the Ti 2p photoemission line in TiN in a self-consistent manner that accounts for all features observable in an energy window of 80 eV. The evaluation considers the appropriate Tougaard background correction, shake-up features as well as surface and bulk plasmons. The ratio of the Ti 2p_1/2 and Ti 2p_3/2 and the corresponding peaks in the energy loss features fulfill the requirements given by quantum mechanics. The energy loss ΔE due to the shake-up process and the shake-up ratio have been determined quantitatively for oxygen-free bulk titanium nitride. The origin of the shake-up, its intensity and energy difference ΔE are explained by a two electron excitation process.     

Optical properties of Eu:CsGd2F7 downconversion phosphor

Luminescence spectra and excitation spectra in 150-420 nm spectral region have been recorded at room temperature for polycrystalline sample of (0.5%)Eu³⁺:CsGd₂F₇. The relatively intense emission has been observed from ⁵D₃, ⁵D₂ and ⁵D₁ levels. Emission and excitation spectra prove that the excitation energy is efficiently transferred from the ⁶G_J and ⁶I_J levels of Gd³⁺ ions to Eu³⁺ ions. The visible quantum cutting via downconversion has been detected, with efficiency of the cross-relaxation step of ∼50%.