Photoelectron Energy Loss in Al(002) Revisited: Retrieval of the Single Plasmon Loss Energy Distribution by a Fourier Transform Method

A Fourier transform (FT) algorithm is proposed to retrieve the energy loss function (ELF) of solid surfaces from experimental X-ray photoelectron spectra. The intensity measured over a broad energy range towards lower kinetic energies results from convolution of four spectral distributions: photoemission line shape, multiple plasmon loss probability, X-ray source line structure and Gaussian broadening of the photoelectron analyzer. The FT of the measured XPS spectrum, including the zero-loss peak and all inelastic scattering mechanisms, being a mathematical function of the respective FT of X-ray source, photoemission line shape, multiple plasmon loss function, and Gaussian broadening of the photoelectron analyzer, the proposed algorithm gives straightforward access to the bulk ELF and effective dielectric function of the solid, assuming identical ELF for intrinsic and extrinsic plasmon excitations. This method is applied to aluminum single crystal Al(002) where the photoemission line shape has been computed accurately beyond the Doniach–Sunjic approximation using the Mahan–Wertheim–Citrin approach which takes into account the density of states near the Fermi level; the only adjustable parameters are the singularity index and the broadening energy Г (inverse hole lifetime). After correction for surface plasmon excitations, the q-averaged bulk loss function, <Im[??1 / ε(E, q)]> _q , of Al(002) differs from the optical value Im[??1 / ε(E, q?=?0)] and is well described by the Lindhard–Mermin dispersion relation. A quality criterion of the inversion algorithm is given by the capability of observing weak interband transitions close to the zero-loss peak, namely at 0.65 and 1.65 eV in ε(E, q) as found in optical spectra and ab initio calculations of aluminum.     

Energieverlustmessungen mit 60 keV Elektronen an amorphem und polykristallinem Selen und Tellur und Bestimmung optischer Konstanten

The energy loss functions Im—(1/?) of amorphous and polycrystalline Selenium and Tellurium are determined from energy loss spectra of 60 keV electrons in the energy range up to 30 eV. The optical constants ?₁ and ?₂ are calculated from the energy loss function by Kramers Kronig analysis. The energy difference in the position of the volume plasma loss of amorphous and polycrystalline foils is compared with the calculated change in free electron plasma energy resulting from density change. Characteristic structure dependence of the optical constants are found to be similar for Selenium and Tellurium. They are discussed in terms of the results of band structure calculations.     

Interband transitions in ZnO observed in low energy electron spectroscopy

Using a high resolution two-axis-spectrometer energy loss spectra of electrons specularly reflected from (1¯100) ZnO-surfaces have been investigated. The electron impact energy was varied between 25 and 60 eV. The spectra show a strong dependence on the orientation of thec-axis with respect to the plane of incidence. The dependence of the loss intensities on the angle of incidence and the shape of the spectra can be qualitatively described by the “Dielectric Theory”. Contrary to optical reflection measurements the excitation of excitons could not be observed in agreement with the existence of a surface barrier for excitons.     

Messung der Spinpolarisation langsamer Elektronen nach elastischer Streuung an festen Hg-Targets

SpinpolarizationP(Θ) of slow electrons elastically scattered by a solid Hg-target has been studied in a double-scattering experiment for electron energies between 300 eV and 900 eV. For the same energy range the angular dependence of the scattered intensity has been measured. Maximum degree of polarization found isP=?0.23±0.04 (E=900 eV,Θ=105°).     

Study of the initial stage of yttrium oxidation using characteristic electron-energy-loss spectroscopy

The characteristic electron-energy-loss (EEL) spectra of the pure surface of metallic yttrium and of this surface in the initial stages of oxidation are recorded. The energy of the primary electron beam E _p is 200–1000 eV. The spectra exhibit high-and low-frequency peaks. During oxidation, the positions of the basic peaks in the EEL spectra are significantly shifted. The peaks corresponding to the bulk energy loss shift toward higher energies upon oxidation. The peak corresponding to the low-frequency surface oscillations also shifts, but toward lower energies, and its intensity monotonically decreases with increasing oxygen dose. The differences between the spectra recorded at different E _p are explained as resulting from an increase in the electron escape depth with E _p .     

Multiphoton excitation and thermal activation in indirect bandgap semiconductors

The single crystals of wide- and indirect-bandgap semiconductor CdI₂ were grown and their optical properties as well as the defect-induced excitonic photoluminescence (PL) spectra were studied. The multiphoton excited PL spectra, charactering the emissions from excitons in the visible region, were taken. The PL intensity (I_PL) was found to vary nonlinearly with pumping power (P). The I_PL was also found to decrease with increasing temperature. The temperature dependence of I_PL for each P followed a fashionable relationship, from which the activation energy (?E) of the defect-induced excitonic trapping was calculated. The ?E was found to slightly decrease with increasing P, with an average value of 2.5 meV. The results, however, demonstrates the existence of the self-trapped excitons responsible for the characteristics broad-band emission in the visible range. This study shows that the single crystals of CdI₂ might have potential in applications as thermo-luminescent dosimeters in radiation measurements.     

Vacuum ultraviolet reflectivity and band structure of SrTiO3 and BaTiO3

The room temperature near-normal incidence reflectance spectra for SrTiO₃ and BaTiO₃ have been measured in the energy region 10 to 32 eV. The optical constants for the region 0 to 32eV have been derived from a Kramers-Kronig analysis by including earlier measurements for the low energy region. For SrTiO₃ five peaks in the Ti3d derived conduction band density of states have been found on the basis of experimental data only, neglecting excitonic effects. The energies of these peaks are in excellent agreement with peaks in the density of states calculated by Mattheiss. The results for BaTiO₃ could not be interpreted as unambiguously in such a scheme. Alternatively, some of the transitions could be interpreted as being due to core level excitons.     

Messung der Energieverlustspektren von Aluminium- und Silberfolien mit hoher Auflösung

The energy loss spectra of polycrystalline aluminium and silver foils have been measured with high resolution. For the plasmon energy of aluminium 14.97 eV is obtained. The half width of the plasmon peak is 0.60 eV corresponding to a relaxation time τ=1.1×10^?15 sec in fair agreement with optical data. In silver films of certain thicknesses the surface plasmon peak and the plasmon peak can be separated in the energy loss spectrum. The values of the energy losses are 3.64 and 3.78 eV respectively.     

Über die Winkelabhängigkeit und ihre Unsymmetrie von Energieverlusten an Si und Ge

In Silicon and Germanium one finds three types of characteristic energy losses: the plasma loss at 17 eV in transmission of electrons through thin films, the 10 eV loss in reflection at cleavage surfaces, and the 5 eV loss in transmission through extremely thin films. The present paper is concerned with the 5 eV loss. It is found that the intensity of the 5 eV loss does not depend on variation of thickness of the foils. Thus, it is concluded that the 5 eV loss is due to surface plasma oscillations. In fact, measuring the angular distribution, there is found a dependence of ～? ^?3 in good agreement with that given by the theory. In the case of non-normally incident electrons it is found that the intensity of this loss is not symmetric about the direction of the incident electrons. This is in agreement with the theoretical considerations ofStern andFerrell, and represents a further typical feature of surface plasma losses.     

Emission and excitation spectra of ZnO:Ga and ZnO:Ga,N ceramics

The spectral characteristics of ZnO:Ga and ZnO:Ga,N ceramics prepared by uniaxial hot pressing have been investigated. At room temperature, the edge (exciton) band at 3.12 eV dominates in the luminescence spectra of ZnO:Ga, while a wide luminescence band at 2.37 eV, which is likely to be due to zinc vacancies, is observed in the spectra of ZnO:Ga,N. Upon heating, the edge band maximum shifts to lower energies and the bandwidth increases. The extrapolated position of the edge-band maximum at zero temperature, E _m (0) = 3.367 ± 0.005 eV, is in agreement with the data for thin zinc oxide films. The luminescence excitation spectra in the range from 3 to 6.5 eV are reported and the mechanism of energy transfer to excitons and luminescence centers is considered.     

Ein quantitativer Vergleich der Energieverlustspektren von Elektronen in Aluminium und Kalium mit der dielektrischen Theorie

The energy loss spectra of 34 kV electrons inelastically scattered by thin foils of Aluminium and Potassium were measured and compared with loss intensities, calculated by the dielectric theory from the dielectric constantsε ₁ andε ₂ depending on energy. One obtains a good quantitative agreement of the measured spectra with the calculated energy loss intensities.     

Untersuchung der Anregung innerer Elektronen von Alkalihalogenidmolekülen im Energieverlustspektrum von 25 keV-Elektronen

The energy loss spectra of 25-keV electrons after interaction with alkali halide vapors were measured. For the energy losses in the energy rangeE?6eV the positions of the peaks are consistent with light absorption measurements considering the energy resolution of the loss spectra. At higher energy peaks were observed, which correspond to the excitation of inner electrons belonging to the alkali atoms. From electron diffraction diagramms it follows, that for the lithium halides the concentration of dimers is considerable.     

Study of surface dissociation in KCl and LiF by observation of plasmon losses associated with K and Li

Peaks in the characteristic energy loss spectra of KCl and LiF obtained with an incident electron beam of 500 eV, have been associated with plasmon excitations in the alkali metal layer formed on the surface of these crystals by electron irradiation stimulated dissociation. The irradiation time and temperature dependence of these peaks seem to confirm this interpretation. Auger electron spectroscopy gives good qualitative agreement with previous results.     

First principles study of electronic properties,interband transitions and electron energy loss of <Emphasis Type="Italic">α</Emphasis>-graphyne

The electronic and optical properties of α-graphyne sheet are investigated by using densityfunctional theory. The results confirm that α-graphyne sheet is a zero-gap semimetal. Theoptical properties of the α-graphyne sheet such as dielectric function,refraction index, electron energy loss function, reflectivity, absorption coefficient andextinction index are calculated for both parallel and perpendicular electric fieldpolarizations. The optical spectra are strongly anisotropic along these two polarizations.For (E ∥x), absorption edge is at 0 eV, while there is noabsorption below 8 eV for (E ∥z).     

Structure of the Room-Temperature 2.5 eV <Emphasis Type="Italic">α</Emphasis>-Quartz Single-Crystal Pulsed Cathodoluminescence Band

We study the room-temperature (RT) pulsed cathodoluminescence (PCL) spectrum of a high-purity synthetic α-quartz single crystal. The spectrum consists of two wide bands with intensity maxima at 415 and 490 nm (2.99 and 2.53 eV). The band at 490 nm (2.5 eV) is polarized in the XY crystal plane (perpendicular to the third-order symmetry axis) and possesses a structure with three peaks at 480±2, 487±2, and 493±2 nm (2.58±0.01, 2.55±0.01, and 2.52±0.01 eV). The intensities of the peaks at 480±2 and 493±2 nm increased with increase in the irradiation dose up to 45 kGy. Peaks are equidistant at the energetic scale. The energy separation between the peaks Δ = 0.03 ± 0.01 eV is equal in order of magnitude to energies of Li _x O _y molecular vibrations and to the energy of the optical phonon in α-quartz. We propose an explanation of the experimental data obtained. According to this explanation, the structure observed may be attributed to the amplitude modulation of the quartz 2.5 eV emission band by the crystalline electric fields on frequencies of optical phonons. The nonequilibrium phonons may arise during the electron-beam irradiation.     

Auger- und Coster-Kronig-Übergänge der M-Schale von Krypton

The Auger spectra of theM ₂,M ₃,M ₄,M ₅ subshells of krypton and the Coster-Kronig spectra of theM ₁,M ₂,M ₃ subshells of krypton were measured with an electrostatical spectrometer. The ionization in theM shells was caused by electron impact. The use of a gaseous target made it possible to measure the Auger lines even at energies as low as 25 eV. The absolute energies and relative intensities of a great number of transitions were determined: 22 of theM _{4, 5} spectrum, 14 of theM _{2, 3} spectrum and 2 of theM ₁ spectrum. Only in the case of theM _{2, 3} spectrum a comparison between the relative intensities, determined experimentally, and those calculated byRubenstein forZ=47 was possible. The agreement is only qualitatively. Moreover, from the Auger electron energies measured, the following binding energies were calculated:E(M ₁)=(292,1±1,0) eV,E(M ₂)=(222,1±0,6) eV,E(M ₃)=(214,6±0,6)eV,E(N ₁ N ₁)=(62,81±0,05) eV.     

Effect of surface excitations on the reflection electron energy loss spectrum in silicon

The results of investigating the relative contribution of surface excitations to the reflection electron energy loss spectrum in pure silicon are presented. The primary electron energy is in the range 60–1000eV. Good agreement is obtained between the experimental values of the surface parameter P _S and theoretical calculations. The relative contribution of surface excitations is also determined by decomposing the integral reflection electron energy loss spectra into Gaussian curves.     

Untersuchung der Zustandsdichte und der charakteristischen Energieverluste von Ir,Rh, Pt und Pd mit Isochromatenmessungen

X-ray isochromats of the above metals measured with high resolution are presented. Assuming a rigid band model for the four metals they give — with some usual approximations — the qualitative form of the density of states. It turns out that the densities of statesZ(ε) at the Fermi limits decrease in the succession Ir, Rh, Pt, Pd. This is in contradiction to low temperature measurements of the electronic specific heatc _e which for the same succession give an appreciable raising inZ(ε). It is assumed that this discrepancy is caused by a special coupling of the electrons to the transverse phonons. In consequence of this,Z(ε) of the renormalized electron system measured byc _e can be very different from that of the bare electrons measured by the isochromats. Assuming that the transition temperature of elemental superconductors is related according to BCS-theory to the latter density of states the absence of superconductivity of Rh, Pt and Pd can be understood. In addition the isochromats give values for the characteristic energy losses of the four metals.     

Electron loss spectroscopy of clean and oxygen covered GaAs(110) surfaces

Electron energy loss spectra of clean and oxygen covered GaAs(110) surfaces have been measured with a four grid retarding field analyser. Loss spectra of clean cleaved p- and n-type surfaces are slightly different and different states of adsorption for the oxygen on the two surfaces are found. The loss peaks which are common in the spectra obtained from clean surfaces of both types of material have been interpreted in terms of bulk and surface excitations. The data associated with the bulk excitations are in good agreement with previous optical and electron transmission data while loss peaks at 11.5 and 18.5 eV are interpreted as the surface plasma loss and a surface state transition respectively. For n-type material extra loss peaks were observed. In the case of oxygen adsorption on these surfaces new loss peaks were found at 13.5, 17.2 and 28.1 eV in both spectra and are assumed to be characteristic of the oxygen. Further, for n-type material an extra peak occurs at 8.2 eV.     

Electronic transitions in the VBO<Subscript>3</Subscript> single crystal at high pressures

Optical absorption spectra of single crystals of the ferromagnetic semiconductor VBO₃ are studied at high pressures up to 70 GPa achieved in a diamond-anvil cell. An electronic transition accompanied by sharp changes in the optical parameters and a decrease in the optical gap from E ₀ = 3.02 eV to 2.25 eV is found at the pressure P _C ～ 30 GPa. The gap does not disappear in the high-pressure phase and its value becomes typical of semiconductors. This is indicative of a semiconductor-semiconductor transition. The transition to the metallic state may occur at the critical pressure P _met ≈ 290 GPa.