The electronic structure of from high energy spectroscopy

We report the results of a detailed study of the occupied and unoccupied electronic structure of dimers of the new heterofullerene by means of photoemission and electron energy-loss spectroscopy. A close similarity is found between the electronic structures of pristine and with an additional broadening of the spectra in the former due to the distortion of the fullerene cage caused both by dimerization and the chemical substitution. Both the occupied and unoccupied electronic states, as well as the interband transitions between them, attest to the high degree of molecular character retained in the solid state. Comparison of the shake-up structures in the and X-ray photoemission spectra confirm that the highest lying occupied states in the heterofullerene have a strong degree of N character, whereas the lowest lying unoccupied states have mainly C character. We also present the optical conductivity of the heterofullerene (derived from the loss function), which shows an optical gap of 1.4 eV, some 0.4 eV smaller than that of . Received: 25 August 1997 / Revised: 22 September 1997 / Accepted: 16 October 1997     

Optical spectroscopy of electronic surface states

In this paper a number of optical spectroscopic methods for investigating surface electronic structure are discussed, including reflectance techniques, ellipsometry, surface photoconductivity and surface photovoltage spectroscopy. In addition to electron scattering techniques and UV-photoemission, optical spectroscopic methods have contributed much in recent times to the understanding of electronic surface states on solids. A discussion and comparison is given of the nature and significance of information obtained by these methods and exemplary experimental results are presented to illustrate the contribution of the optical techniques to the present knowledge about surface states. The relation between information obtained from optical measurements and electron spectroscopy is considered.     

The electronic structure of KMnO4 investigated by photoemission and electron-energy-loss spectroscopy

From photoemission and electron-energy-loss data the following picture of KMnO₄, with Mn^VII (with a formal charge state Mn⁷⁺ (3d ⁰)) tetrahedrally surrounded by four O^2–-ions, is deduced: strong covalent bonding between Mn^VII and O^2– leads to a considerable occupation of the Mn-3 d shell. The ground state of the (MnO₄)^–1 molecule is an orbital and spin singlet as seen by the absence of any multiplet splitting in the Mn core levels. The valence band shows a four peak structure extending form 4 eV to 8 eV below the Fermi energy. The first peak at 4.2 eV has mainly O-2p character. The remaining peaks are of strongly mixed Mn-3d/O-2p character due to the covalent bonding. This mixing decreases with increasing binding energy. The electron energy loss data show a variety of structures between 2 eV and 10 eV independent of the primary electron energy which defines them as dipole allowed charge-transfer transitions. An additional excitation at 1.8 eV decreases quickly in intensity with increasing electron energy which classifies it as a dipole or spin forbidden transition in the compound. This energy is close to the value of 1.6 eV reported for the activation energy observed in electrical transport data. The results are compared to quantum chemical molecular orbital calculations of the (MnO₄)^–1 molecule.Physics Department, Allahabad University Allahabad 211002, India     

Photoemission spectroscopy and electronic energy levels of CoS

Photoemission (XPS and UPS) and inverse photoemission spectra (BIS) of CoS have been measured. From these data the occupied and empty density of states of this material are deduced. The metallic nature of CoS is not caused by a breakdown of Mott-insulation but rather because of Co(d) - S(p) hybridization.     

Photoelectron spectroscopy of the valence electronic structure of polymers

Photoelectron spectroscopy has been highly developed as an investigative tool of the bulk and surface chemical and electronic structure of condensed matter during the past 2 decades. The use of soft X-ray sources led to the development of electroq spectroscopy for chemical applications (ESCA),^1,2 which is also known to the physicst as X-ray photoelectron spectroscopy (XPS). These developments were followed by the use of ultraviolet sources for the study of gases³ and the bulk and surface electronic structure of solids.^4,5 Recently, the use of the continuous spectral distribution of synchrotron radiation has had a major impact upon the study of solid surfaces^6,7 and has made the largely historical terminology of XPS (or ESCA) and ultraviolet photoelectron spectroscopy (UPS) somewhat less meaningful. Angle-resolved UPS and XPS (ARUPS and ARXPS) can provide additional information about electronic band structure, surface states, and adsorbates, but are typically applied to the study of the surfaces of ideal single crystals, where crystallographic high-symmetry directions as well as the polarization of the light source can be used to high advantage.⁷ Since polymers are almost never single crystals, the angle-resolved issue will not be addressed here.     

The electronic structure of different forms of solid carbon studied by electron momentum spectroscopy

The purpose of this paper is to first give an overview of the information obtained by electron momentum spectroscopy (EMS) on the electronic structure of different forms of solid carbon (single crystal graphite, polycrystalline carbon, different forms of amorphous carbon, and fullerenes). We show for sp² bonded carbon that, with decreasing order, the structures observed change but certain basic features remain. sp³-bonded carbon films and fullerene films both have electronic structures different from these graphitic films. The results of EMS are compared with the results of other spectroscopic and scattering techniques. It is argued that the combination of energy and momentum resolution is crucial in order to get a clear picture of the changes in the electronic structure due to the changes in atomic arangements. Only EMS resolves both energy and momentum for all these solids.     

Nonlinear optical spectroscopy of electronic transitions in hexagonal manganites

The hexagonal rare-earth manganites RMnO₃ (R = Sc, Y, Ho, Er, Tm, Yb, Lu) are a group of materials with an unusual combination of magnetic, electric and optical properties. The electronic structure of these materials was studied by second harmonic (SH) spectroscopy in the range from 1.2 to 3.0 eV. Faraday rotation and absorption spectra were measured in the range from 1.0 to 1.6 eV. Broad bands at ∼1.7 eV and ∼2.7 eV are assigned to electronic transitions between Mn³⁺(3d⁴) levels. The SH spectra are discussed on the basis of a recently developed microscopic theory. Received: 26 April 2001 / Published online: 18 July 2001     

Mössbauer emission spectroscopy of metastable electronic states

In⁵⁷Fe Mössbauer emission spectroscopy, the parent nucleus⁵⁷Co is incorporated in the matrix by diffusion, implantation or co-precipitation. Mössbauer emisssion spectra often differ from the corresponding absorption spectra in several aspects. The “anomalous” features (denoted as “after-effects” of nuclear transformation) comprise mainly (a) anomalous charge states, (b) anomalous spin states, and (c) metastable electronic populations within these charge states. The first two classes of effects were extensively studied during the sixties and seventies. More recently, several investigations have shown that, under certain circumstances, the electronic systems of Fe(II) and Fe(III) have not reached the thermal equilibrium within the Mössbauer time window. These effects have been evidenced (i) within the crystal field ground-state manifolds, but also (ii) as spectral contributions from excited crystal field states. In the present paper, an overview of these experiments will be given, together with their theoretical interpretation and in comparison to results obtained with other methods (e.g. optical spectroscopy). Special emphasis will be given to the systems⁵⁷Co/LiNbO₃ and⁵⁷Co/MgO, where nonequilibrium populations within the⁶A₁ ground manifold of Fe(III) and Γ_5g ground manifold of Fe(II) have been observed in an external magnetic field.     

X-ray spectroscopy of thermally distorted electronic states in crystals

A new type of x-ray spectroscopy is proposed which can detect the thermal-motion-induced distortions of atomic electronic states in crystals. It is shown that those distortions can cause extra Bragg reflections (so-called forbidden reflections) and that their intensity should grow with increasing temperature. The reason is that the thermal displacements, which change the symmetry of atomic environment, can modify the tensor amplitude of x-ray resonant scattering. In the first approximation, the structure factor of extra reflections is proportional to the reflection vector H and to the mean-square thermal displacement for optical phonons. It is demonstrated that the forbidden resonant reflections, observed recently in Ge, could be caused by the thermal motion. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 12, 885–889 (25 June 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

共掺杂金红石TiO2的电子结构和红外光谱研究

本文对大别山双河、碧溪岭地区硬玉石英岩中的金红石进行了Fourier变换红外光谱(FTIR)分析, 结果显示所有金红石颗粒分别在3280 cm^-1和3295 cm^-1 出现强的吸收峰. 基于前人提出H在金红石结构中以孔道中心(CC)和八面体共边(BOE)方式存在的两种模型, 本文采用第一性原理计算方法研究了掺杂(Al, H)和(Fe, H)时金红石相TiO₂的晶体结构和电子结构. 根据O—H键的振动频率和O—H…O键中O—O之间距离的经验关 关键词： 2')" href="#">金红石相TiO₂ 孔道中心 电子结构 第一性原理     

Magnetization-direction-dependent local electronic structure probed by scanning tunneling spectroscopy

Scanning tunneling spectroscopy (STS) of thin Fe films on W(110) shows that the electronic structure of domains and domain walls is different. This experimental result is explained on the basis of first-principles calculations. A detailed analysis reveals that the spin-orbit induced mixing between minority d(xy+xz) and minority d(z(2)) spin states depends on the magnetization direction and changes the local density of states in the vacuum detectable by STS. As a consequence nanometer-scale magnetic structure information is obtained even by using nonmagnetic probe tips.     

Orientation-dependent C60 electronic structures revealed by photoemission spectroscopy

We observe, with angle-resolved photoemission, a dramatic change in the electronic structure of two C60 monolayers, deposited, respectively, on Ag (111) and (100) substrates, and similarly doped with potassium to half filling of the C60 lowest unoccupied molecular orbital. The Fermi surface symmetry, the bandwidth, and the curvature of the dispersion at Gamma point are different. Orientations of the C60 molecules on the two substrates are known to be the main structural difference between the two monolayers, and we present new band-structure calculations for some of these orientations. We conclude that orientations play a key role in the electronic structure of fullerides.     

Analysis of the electronic structure of ZrO2 by Compton spectroscopy

The electronic structure of ZrO₂ is studied using the Compton scattering technique. The first-ever Compton profile measurement on polycrystalline ZrO₂ was obtained using 59.54 keV gamma-rays emanating from the ²⁴¹Am radioisotope. To explain the experimental data, we compute theoretical Compton profile values using the method of linear combination of atomic orbitals in the framework of density functional theory. The correlation scheme proposed by Perdew-Burke-Ernzerhof and the exchange scheme of Becke are considered. The ionic-model-based calculations for a number of configurations, i.e., Zr^+x (O^?x/2)₂ (0 ≤ x ≤ 2), are also performed to estimate the charge transfer on compound formation, and the study supports transfer of 1.5 electrons from Zr to O atoms.     

Capacitance spectroscopy investigation of the spin polarization of two-dimensional electronic systems

It is shown that capacitance spectroscopy can be used to investigate the spin polarization of two-dimensional electronic systems (2DESs). We employed this method to investigate the spin polarization of 2DESs in a GaAs/AlGaAs heterojunction for filling factors of the magneticquantization levels 0.28<ν<0.9. It is proved that in the presence of states of the fractional quantum Hall effect with ν _f =1/3 and 2/3 the ground state of a 2DES with ν>2/3 is incompletely spin-polarized. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 1, 102–107 (10 January 1997)     

An electronic structure study of europium chalcogenides by soft x-ray spectroscopy

The M_V, M_IV and M_III X-ray emission and absorption spectra of EuO and EuS have been studied. The relative positions and the widths of 4f states and valence or conduction distributions have been deduced from these spectra. These data are compared with those obtained by u.v. photoemission and the results of band structure calculations.     

Unoccupied electronic structure of TiOCl studied using x-ray absorption near-edge spectroscopy

We study the unoccupied electronic structure of the spin-1/2 quantum magnet TiOCl using x-ray absorption near-edge spectroscopy (XANES) at the Ti L and O K edges. We acquire data both in total electron and fluorescence yield modes (TEY and FY, respectively). While only the latter allows us to access the unconventional low-temperature spin-Peierls (SP) phase of TiOCl, the signal is found to suffer from significant self-absorption in this case. Nevertheless, we conclude from FY data that effects of the SP distortion on the electronic structure are absent in the incommensurate intermediate phase within experimental accuracy. The similarity of room-temperature FY and TEY data, the latter not being obscured by self-absorption, allows us to use TEY spectra for comparison with simulations. These are performed by means of cluster calculations in D(4h) and D(2h) symmetries using two different codes. We extract values of the crystal-field splitting and parameterize our results using the commonly seen notation of Slater, Racah and Butler. In all cases, good agreement with published values from other studies is found.