Optical and luminescent VUV spectroscopy of La2Be2O5 crystals

Electronic excitations and the processes of their radiative relaxation are studied in pure and Ce³⁺ ion-doped crystals of lanthanum beryllate excited by synchrotron radiation in the x-ray and VUV ranges by methods of optical and luminescent vacuum ultraviolet time-resolved spectroscopy. Manifestations of excitons of the valence band are absent in the reflection spectra. However, a fast (τ=1.7 ns) and a slow (microsecond range) channel corresponding to two possible types of self-trapped excitons (STE) are found in radiative relaxation of intrinsic electronic excitations at T=10 K. The slow channel corresponds to emission of STE formed through recombination, the fast channel corresponds to emission of relaxed metastable excitons from the STE state. In the energy region higher than 14 eV (E>2E _g), the effect of multiplication of electronic excitations due to generation of secondary electron-hole pairs resulting from inelastic scattering of both hot photoelectrons and hot photoholes is exhibited.     

3s- and 3p-core level excitations in 3d-transition metal oxides from electron-energy-loss spectroscopy

3s- and 3p-core level excitations for a large number of 3d-transition metal oxides, with a formal 3d occupation from 3d⁰ to 3d¹⁰, have been measured by electron energy loss spectroscopy in reflection geometry (REELS) with primary energies 200 eV≤E ₀≤1600 eV. Their intensities decrease systematically with the formal 3d-count, classifying them as transitions to empty 3d-states. The structure of the 3s excitations is analysed in detail and is compared to the 3s-XPS photoemission spectra of the samples. This 3s-REELS structure and its change with the 3d occupation can be explained by the assumption that the excitation arises mainly from a 3s²3dⁿ→3s¹3dⁿ⁺¹ quadrupole transition.     

3s- and 3p-core level excitations in 3d-transition metal oxides from electron-energy-loss spectroscopy

3s- and 3p-core level excitations for a large number of 3d-transition metal oxides, with a formal 3d occupation from 3d⁰ to 3d¹⁰, have been measured by electron energy loss spectroscopy in reflection geometry (REELS) with primary energies 200 eV≤E ₀≤1600 eV. Their intensities decrease systematically with the formal 3d-count, classifying them as transitions to empty 3d-states. The structure of the 3s excitations is analysed in detail and is compared to the 3s-XPS photoemission spectra of the samples. This 3s-REELS structure and its change with the 3d occupation can be explained by the assumption that the excitation arises mainly from a 3s²3dⁿ→3s¹3dⁿ⁺¹ quadrupole transition.     

Energy loss spectra,auger spectra and high energy electron diffraction of A-15 superconductors

We have measured the electron energy loss spectra of V₃Si, Nb₃Al, Nb₃Sn, Nb₃Ge (annealed) and Nb₃Ge (sputtered) from 3 to 70 eV. Numerous new structures were discovered in this study which could be related to interband transitions, plasma excitations, and core transitions. We have also measured the high energy electron diffraction patterns and Auger spectra which characterized the lattice structures and compositions of these surfaces.     

Spectroscopic investigation on the electronic structure of a 5d band insulator SrHfO3 in proximity to ferroelectric instability: Comparison with SrTiO3 and SrZrO3

We investigated the high-energy electronic structure of a 5d perovskite SrHfO₃ by using optical spectroscopy and O 1s x-ray absorption spectroscopy. From the combined spectra the values of electronic structure parameters are estimated properly. In particular, the crystal field splitting energy, which is closely associated with the p–d hybridization strength, is as high as ～5 eV, and the Sr 4d bands appear to be strongly mixed with the Hf 5d bands. These findings are discussed in relation to a possible ferroelectric instability in SrHfO₃, and are compared with electronic properties of similar compounds, 3d SrTiO₃ and 4d SrZrO₃.     

Distribution d'énergie des électrons secondaires émis par InP et GaP soumis à un bombardement d'ions Ar de 40 keV

The energy distributions N(E) of secondary electrons emitted from GaP and InP samples bombarded with 40 keV Ar⁺ ions have been studied by a retarding potential method and an electronic derivation. The spectra show beyond an intensive peak developed at 2 eV, a detailed spectrum between 80 and 140 eV. The analysis of this spectrum reveales Auger electrons corresponding to L₂₃(P) VV and L₂₃M_IV–V(Ga) V [or L₂₃(P) N_IV-V(In) V] transitions; moreover, peaks due to plasmon excitations and d band excitations can be distinguished.     

Multisites Cr3+ in GGG and GSGG garnets

The emission and excitation spectra of Cr³⁺ -doped Gd₃Ga₅O₁₂ (GGG) and Gd₃Sc₂Ga₃O₁₂ (GSGG) are explained in the light of multisite effects. The situation is particularly complicated in the case of GSGG, where the different sites have²E energy levels near each other which overlap with the⁴A₂ →⁴T₂ absorption bands. The spectra obtained under selective excitations are interpreted on the multisite assumption.     

Titanium K-edge absorption structure in Ti1−x NbxO2

The fine structure in the titanium x-ray K-edge absorption has been measured in Ti_1−x Nb_xO₂ mixed dioxides (x=0–0.1) with rutile structure in a laboratory-type spectrometer by total electron quantum-yield measurement. The position of the XANES lines is shown to be in good agreement with classical x-ray absorption spectra obtained in transmission. The structure and main features of the XANES spectra, including the effects of impurities and manyelectron excitations, are discussed. It is suggested that the intensity of the B peak characteristic of the titanium K edge depends on the Nb concentration and correlates with the charge state of titanium ions. Fiz. Tverd. Tela (St. Petersburg) 41, 894–896 (May 1999)     

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.     

Quasi-particle excitations and phonon broadening effects in the O ls spectra of Ln(OH)3 (Ln = La,Nd, Sm and Gd)

The O ls photoelectron spectra of Ln(OH)₃ (Ln = La, Nd, Sm and Gd) can be approximated by the convolution of a core hole spectral density function and a response function which is related to the phonon broadening and instrumental broadening effects. A non-linear least-squares analysis on the convolution integral indicates that the lifetime of the O ls (in the energy domain) is strongly broadened by the phonon excitations. The FWHM (full width at half maximum) of the phonon excitations range from 1.3 to 2.6 eV.     

Effect of the chemical state of silver on the luminescence properties of GeO<Subscript>2</Subscript>-Eu<Subscript>2</Subscript>O<Subscript>3</Subscript>-Ag films

A sol-gel method is used to prepare GeO₂-Eu₂O₃-Ag films in which the luminescence efficiency of Eu³⁺ ions during UV excitation is comparable to that in films activated by organic europium complexes. The luminescence spectra of these films are recorded, and the films are also studied using EPR and x-ray diffraction. The main origin of this effect is found to be complex Eu-Ag centers with a high quantum yield of the intracenter transfer of excitations to the rare-earth activator from silver ions and Ag _m ⁿ⁺ oligomer clusters located on the surface of silver nanoparticles.     

Electronic structure of Al2O3 from electron energy loss spectroscopy

The electronic structure of Al₂O₃ has been studied by electron energy loss spectroscopy (ELS), and an energy level model of both filled and empty states has been constructed from the ELS and available optical data. For the high temperature pyrolytic α-polycrystalline Al₂O₃ films, the transitions are assumed to originate at the two principal peaks in the valence band density of states and the O(2s) core state, and to terminate on two peaks within the conduction band density of states. We also report energy loss spectra due to excitations out of the deeper Al(2p), Al(2s), Al(1s), and O(1s) core levels. The excitations originating at the Al(2p), Al(2s), and Al(1s) core levels terminate on levels in the conduction band and on an exciton lying about 1 eV below the conduction-band edge.     

Alkali metal oxides: Occupied,unoccupied and excited states

The occupied and unoccupied electronic states of NaO₂ and KO₂ have been investigated by UV photoemission spectroscopy and inverse photoemission spectroscopy, respectively. In addition single electron excitations from occupied into empty levels have been probed by x-ray absorption and electron energy loss spectroscopy. The experimental data are augmented by LCGTO-X model cluster calculations. A detailed assignment of initial and final states is given. The excitation cross sections and an inversion of the ligand field induced3d level splitting in NaO₂ are discussed.     

Dielectric function of YBa2Cu3O7-δ between 50 meV and 50 eV

Electronic excitations with polarization parallel to the CuO₂-planes in single-crystalline YBa₂Cu₃O₇ and YBa₂Cu₃O₆ have been investigated by optical reflectance and by highenergy electron energy-loss spectroscopy in transmission in the energy ranges from 50 meV to 6 eV and 0.2 eV to 150 eV, respectively. From a combination of these experimental data the dielectric function, the reflectivity and the optical conductivity have been obtained in a wide energy range. The observed spectra are interpreted in terms of optical phonons, free-carrier absorption, transitions across the charge-transfer gap, further interband transitions, low-lying core-level excitations, excitonic transitions and valence conservingd-d transitions within the Cu 3d shell combined with O 2p intraband transitions. The charge carrier plasmon and the 4 eV-excitation in YBa₂Cu₃O₆ show quadratic dispersions in momentum transfer. From the dispersion constant of the plasmon a mean value of the Fermi velocity of the charge carriers parallel to the CuO₂ planes has been derived.     

Comparison of the electron energy loss spectrum of erbium metal with ErO2andErH2

Electron energy loss spectra of metallic erbium, Er under different exposures of oxygen at room temperature, and Er deposited in an atmosphere of H₂ are presented in both N(E) and $\text{dN}\text{dE}$ form for primary energies in the range 100–1000 eV. Resonant excitations associated with the 5p and 4d levels in Er show little environmental dependence, and are largely intraatomic in character. In contrast the main plasmon peak shifts to higher energy on exposure to oxygen or hydrogen, and the spectrum of one electron excitations at low energies alters with a decrease in metal losses around 3.5 eV accompanied by a build up of valence band transitions at 8–9 eV. There is no evidence of a stable chemisorption phase under oxygen exposure, but the results are consistent with rapid oxygen incorporation into subsurface layers and oxide formation.     

Low-temperature time-resolved vacuum UV spectroscopy of potassium pentaborate crystals

For the first time, subnanosecond time resolution is attained in the low-temperature (at 7 K) measurements of the photoluminescence (PL) spectra (2–6 eV), the PL excitation spectra (4–32 eV), the PL kinetics, and the reflection spectra (4–21 eV) of undoped potassium pentaborate KB₅O₈·4H₂O (KB5) crystals under selective photoexcitation by synchrotron radiation. The PL peaks associated with the intrinsic defects of the KB5 lattice are detected. The PL bands resulting from radiative annihilation of the localized and self-localized electron excitations are singled out; these excitations are most efficiently photogenerated at the fundamental absorption edge in the region where the free exciton formation is expected. The difference between the PL spectra of the fast and slow components is revealed. An effective low-temperature energy transport over the KB5 hydrogen sublattice is deduced from a drop in efficiency of PL excitation in the interband-transition region as a result of nonradiative energy loss. Long-term vacuum UV irradiation of a KB5 crystal at 7 K gives rise to defects in the hydrogen sublattice, which facilitate localization of the electron excitations and reduce the effective length of their diffusion. This leads to a decrease in the nonradiative energy loss, thus enhancing the efficiency of the PL photoexcitation in the band-to-band transition region.     

Multisites Cr3+ in GGG and GSGG garnets

The emission and excitation spectra of Cr³⁺ -doped Gd₃Ga₅O₁₂ (GGG) and Gd₃Sc₂Ga₃O₁₂ (GSGG) are explained in the light of multisite effects. The situation is particularly complicated in the case of GSGG, where the different sites have²E energy levels near each other which overlap with the⁴A₂ ⁴T₂ absorption bands. The spectra obtained under selective excitations are interpreted on the multisite assumption.     

Possible interplay between a two phonon mode and high energy magnetic excitations in BiFeO<Subscript>3</Subscript>

We have performed Raman measurements on high energy excitations in BiFeO₃ single crystals as a function of both temperature and laser excitation lines. A strong feature observed at 1250 cm^-1 in the Raman spectra has been previously assigned to two phonon overtone. This peak exhibits an unusual frequency shift with the laser lines and the temperature dependence of its Fano lineshape shows two singularities at 150 K and 200 K which can be related to magnetic excitations. In the same energy range, we have also identified the two-magnon excitation with a temperature dependence very similar to the one measured for the one-magnon modes.     

X-ray Photoelectron Spectroscopy of Sb2S3 Crystals

The paper presents the X-ray photoelectron spectra (XPS) of the valence band and core levels of semiconductor ferroelectric Sb₂S₃ single crystals, which show weak phase transitions and anomalies of various physical properties. The XPS were measured with monochromatized Al K _α radiation in the energy range 0-1450 eV and the temperature range 160-450 K. The valence band is located 0.8-7.5 eV below the Fermi level. Experimental results of the valence band and core levels are compared with the results of theoretical ab initio calculations of the molecular model of Sb₂S₃ crystal. The chemical shifts in Sb₂S₃ crystal for the Sb and S states are obtained. Results revealed that the small structural rearrangements at the phase transition T _c1 = 300 K shift the Fermi level and all electronic spectrum. Also, temperature dependence of a spontaneous polarisation shifts the electronic spectra of the valence band and core levels. Specific temperature-dependent excitations in Sb 3d core levels are also revealed.     

Temperature-dependent plasmon excitations in the surface layer of ordered Cu-22.5 at % Mn alloy

Temperature and energy dependences of the characteristic electron energy losses on plasmon excitations are studied in the surface layer of ordered polycrystalline Cu-22.5 at % Mn alloy. Features in the near-surface distribution of Mn are found from an analysis of plasmon excitations and the data of Auger spectroscopy. The observed temperature dependences of the electron energy loss spectra have features in the range of 650–750 K that include the temperature of atomic disordering (T _k = 675 K) in the bulk of the alloy.