Electron loss spectra from a W(001)surface

A high resolution energy dispersion analysis of the electron loss spectra (ELS) from a W(001) surface has been done with a specular reflexion geometry. The non-dispersive ELS correlates well with a combination of the bulk and surface loss functions determined from recent optical data whereas the dispersive ELS showed significant differences. Multiple plasmon losses were demonstrated while for the interband losses a momentum selection method has been demonstrated to separate scattering in the surface region from that in the bulk.     

Bulk and surface electron states in ruthenium

Using ab initio pseudopotentials and a mixed basis, we have examined the electronic structure of ruthenium. We have concentrated on the bulk and surface energy bands with specific emphasis on the close packed (0 0 0 1) surface. We present energy band spectra for the surface and bulk bands, and compare these bands to experiment and to other calculations. We find good agreement between our bulk bands and recent photoemission measurements. With respect to our surface bands, the results are in good accord with other calculations both for ruthenium and for “analogous” transition metal surfaces.     

Theory of one-phonon scattering of atoms from metal surfaces: Application to He/Ag(111)

We present explicit expressions for one-phonon scattering of atoms from metal surfaces, within the distorted wave Born approximation by including Van der Waals forces. A comparison is made with the recent high-resolution time of flight spectra for He/Ag(1 1 1). We explain in quantitative terms the position and the intensity of the peaks due to scattering from the Rayleigh wave of silver. Also the bulk phonons contribute to the spectra, but the calculated intensities underestimate the data.     

A self-modeling approach to the resolution of XPS spectra into surface and bulk components

A method of resolving XPS spectra into surface and bulk component spectra for cases of moderate energy resolution is designed based on an analysis of a family of spectra acquired at different polar angles from flat specimens. Assumptions about line shapes are not required, but an analytical model of the angular dependence of the bulk and surface XPS signals is needed when the component spectra overlap in the range of binding energies of interest. Recommendations are made of the error limits of experimental variables for the successful application of the self-modeling method. The method was used successfully in the separation and quantitative analysis of the O(1s) XPS spectra of surface silanols from bulk silicon dioxide of a fully hydrated silicon dioxide surface. The surface (silanol) and bulk (oxide) components were found to be separated by 0.30 eV, and the surface component was found to be broader (1.58 eV) than the bulk component (1.15 eV).     

Tuning the plasmon energy of palladium-hydrogen systems by varying the hydrogen concentration

First-principles calculations are performed to obtain the dielectric function and loss spectra of bulk PdH(x). Hydrogen concentrations between x = 0 and 1 are considered. The calculated spectra are dominated by a broad peak that redshifts in energy with x. The obtained bulk dielectric function is employed to compute the loss spectra of PdH(x) spherical nanoparticles as a function of x. The dominant plasmon peak in the spherical nanoparticle is lowered in energy with respect to the bulk case. However, the dependence of the resonance energy on the hydrogen concentration is roughly similar to that in bulk.     

Surface magnetism of Fe(110) from polarized electron scattering

Exchange- and spin—orbit-induced scattering asymmetry spectra of polarized slow electrons from the ferromagnetic Fe(110) surface have been calculated by dynamical theory and found to agree with recent experimental data taken at room temperature. Comparison of exchange asymmetry spectra, obtained for various interaction and layer-dependent magnetization models, with the data implies firstly an enhancement of the surface magnetization by about 30% with respect to the bulk, and secondly the importance of spin-dependent localized inelastic electron—electron scattering processes.     

Ab initio calculation of Electron Energy Loss spectra of clean and 1ML Fe-covered Ni(111)

The dielectric and the loss functions of Ni(111) are calculated by the Full Potential Linear Muffin Tin Orbitals (FPLMTO) ab initio method whithin the three-layers model (vacuum/surface/bulk). Particular attention is devoted to determine surface and bulk state contributions to the spectra. Good agreement is found with recent experimental EELS data on the Fe-covered Ni(111) surface. Received 10 July 2002 Published online 19 November 2002     

Electronic relaxation of deep bulk trap and interface state in ZnO ceramics

This paper investigates the electronic relaxation of deep bulk trap and interface state in ZnO ceramics based on dielectric spectra measured in a wide range of temperature, frequency and bias, in addition to the steady state response. It discusses the nature of net current flowing over the barrier affected by interface state, and then obtains temperature-dependent barrier height by approximate calculation from steady I--V (current--voltage) characteristics. Additional conductance and capacitance arising from deep bulk trap relaxation are calculated based on the displacement of the cross point between deep bulk trap and Fermi level under small AC signal. From the resonances due to deep bulk trap relaxation on dielectric spectra, the activation energies are obtained as 0.22 eV and 0.35 eV, which are consistent with the electronic levels of the main defect interstitial Zn and vacancy oxygen in the depletion layer. Under moderate bias, another resonance due to interface relaxation is shown on the dielectric spectra. The DC-like conductance is also observed in high temperature region on dielectric spectra, and the activation energy is much smaller than the barrier height in steady state condition, which is attributed to the displacement current coming from the shallow bulk trap relaxation or other factors.     

The spectroscopy of surface vibrations by atom scattering

The recent progress in the production of highly monochromatic atomic beams is opening new perspectives in surface physics, having paved the way for a full determination of the surface vibrational structure. After a discussion on the possible determination of Rayleigh wave dispersion curves from angular distributions exploiting the kinematical focussing effect, a short review is presented on the direct measurement of surface phonon dispersion curves, first achieved by Brusdeylins, Doak and Toennies in alkali halides, from time-of-flight (TOF) spectra of scattered He atoms. A comparison is made with the existing theories of surface phonons in ionic crystals. The state of the art in the theory of inelastic processes is briefly illustrated in order to discuss the theoretical interpretation of TOF spectra. The one-phonon energy loss spectra of He scattering from LiF(001) calculated for a hard corrugated surface model are found to be in general good agreement with the experimental TOF spectra. From such a comparison evidence is obtained that: i) one-phonon processes are predominant, and ii) in addition to Rayleigh waves important contributions to the inelastic scattering come from the surface-projected density of bulk phonons. Important effects due to inelastic resonances with surface bound states are put in evidence and explained by simple kinematical arguments. The possible observation of surface optical modes in NaF(001) is finally discussed.     

Bulk and Surface Effects in X-Ray Magnetic Circular Dichroism of Iron Clusters

General trends in x-ray magnetic circular dichroism (XMCD) spectra of iron clusters of different sizes are theoretically investigated using the fully-relativistic spin-polarized multiple-scattering formalism. Purely geometrical effects of clustering and cluster-size effects are explored separately from effects of various scattering potentials and/or treatment of the surface barrier. We found that a high portion of non-bulk atoms makes the XMCD spectra of iron clusters to appear significantly different from bulk spectrum, even for quite large clusters. If the vacuum surrounding the cluster is taken into account, XMCD spectra of iron clusters change essentially (in comparison with clusters cut out of the bulk or with clusters embedded in a sea of free electrons).     

Secondary anti-stokes emission from the bulk of gallium phosphide at 4.2 K

Secondary radiation (photoluminescence and Raman scattering) emitted by gallium phosphide single crystals at helium temperatures is investigated. It is established for the first time that, in the case when the secondary emission spectra are excited by a cw low-power He-Ne laser, whose linewidth lies in the transparency region of GaP, anti-Stokes photoluminescence from the bulk of the sample occurs due to interband and impurity recombination. The results obtained make it possible to carry out a qualitative and quantitative analysis of impurities which are present in the bulk of a semiconductor by recording the bulk anti-Stokes photoluminescence spectra at low temperatures.     

A study of the orientation dependence of the electron energy loss spectra for single crystal films of copper and silver

Apart from two peaks caused by bulk and surface plasmons, four or five peaks (depending on the crystal type) of electron energy losses due to inter- and intraband electron transitions are observed in the investigation of the electron energy loss spectra for metals (Cu, Ag). A comparative analysis of the spectra for Cu or Ag films reveals a shift of bulk plasmon loss peaks to higher values for polycrystals, as in the case of transition metals and semiconductors. In a study concerning the orientation dependence of the energy loss spectra (ELS) for electrons scattered from the copper and silver surface, the anisotropy of the bulk plasmon peak is found when the incident beam’s polar angle or the sample’s azimuthal angle are altered. The anisotropy of the primary electron energy loss for plasmon excitation is also observed, depending on the sample orientation relative to the direction incident electrons. The energy losses are found to increase with an increasing atomic packing density of planes and crystal transparency relative to the incident beam.     

The importance of the surface density of states in the photoemission spectra of the alkali and alkaline earth metals

Ultraviolet photoelectron spectroscopy results are shown to distinguish between the surface and bulk electronic structure of a simple metal. The spectra from Sr at low photon energies (long escape depth) is described by the bulk electronic structure; the spectra at ≈10 eV (short escape depth) is described by the surface electronic structure.     

Photoconductivity and photoreflectance studies of quantum-wells and superlattices

We have performed room-temperature in-plane photoconductivity and photoreflectance measurements on various types of GaAs/AlAs quantum-wells and superlattices. The photoconductivity spectra of samples with even a few (1 to 7) quantum-wells show sharp excitonic structures superimposed on the bulk GaAs signal, and provide a useful alternative to absorption spectra. The photoreflectance spectra are demonstrated experimentally to follow the third derivative of the photoconductivity spectra, as previously reported for bulk materials, even though the mechanism of photoreflectance must be different in the present two-dimensional systems.     

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.     

Revisiting the valence-band and core-level photoemission spectra of NiO

We have reexamined the valence-band (VB) and core-level electronic structure of NiO by means of hard and soft x-ray photoemission spectroscopies. The spectral weight of the lowest energy state was found to be enhanced in the bulk sensitive Ni 2p core-level spectrum. A configuration-interaction model including a bound state screening has shown agreement with the core-level spectrum and off- and on-resonance VB spectra. These results identify the lowest energy states in the core-level and VB spectra as the Zhang-Rice (ZR) doublet bound states, consistent with the spin-fermion model and recent ab initio calculations within dynamical mean-field theory. The results indicate that the ZR character first ionization (the lowest hole-addition) states are responsible for transport properties in NiO and doped NiO.     

Light scattering from surface acoustic phonons in metals and semiconductors

Several examples of Brillouin scattering from thermally excited surface acoustic phonons in metals and opaque semiconductors are presented. The measured surface velocities are consistently lower by 1–5% than the values calculated from the bulk elastic constants. The spectra also show the continuum of Lamb excitations having some surface-type character with velocities ranging between the bulk transverse velocity and the bulk longitudinal velocity.     

Mutual experimental and theoretical validation of bulk photoemission spectra of Sr1-xCaxVO3

We report high-resolution high-energy photoemission spectra together with parameter-free LDA + DMFT (local density approximation + dynamical mean-field theory) results for Sr1-xCaxVO3, a prototype 3d(1) system. In contrast to earlier investigations the bulk spectra are found to be insensitive to x. The good agreement between experiment and theory confirms the bulk sensitivity of the high-energy photoemission spectra.     

Recent progress on photoluminescence from plasmonic nanostructures: Phenomenon,mechanism, and application

Photoluminescence(PL) from bulk noble metals arises from the interband transition of bound electrons. Plasmonic nanostructures can greatly enhance the quantum yield of noble metals through the localized surface plasmon. In this work,we briefly review recent progress on the phenomenon, mechanism, and application of one-photon PL from plasmonic nanostructures. Particularly, our recent efforts in the study of the PL peak position, partial depolarization, and mode selection from plasmonic nanostructures can bring about a relatively complete and deep understanding of the physical mechanism of one-photon PL from plasmonic nanostructures, paving the way for future applications in plasmonic imaging,plasmonic nanolasing, and surface enhanced fluorescence spectra.     

基于自发瑞利-布里渊散射的氮气体黏滞系数的测量

体黏滞系数是从微观角度认识气体分子黏滞性的重要参数,传统的兆赫兹声频范围的声波吸收方法无法直接应用于声波弛豫效应在千兆赫兹范围的高频领域,而瑞利-布里渊散射则能实现对声波弛豫效应在千兆赫兹的气体体黏滞系数的测量.本文测量了532 nm激光激发的常温下压强分别为1-9 bar的氮气的自发瑞利-布里渊散射光谱,利用已知温度和压强的理论模型对测量光谱进行了比较,获得了准确的散射角.利用该散射角并结合χ~2值最小原理反演得到不同压强(4—9 bar)下氮气的平均体黏滞系数为(1.46±0.14)×10~(-5)kg·m~(-1)·s~(-1),该结果与文献中利用自发瑞利-布里渊散射获得的结果和理论计算结果相近,但与相干瑞利-布里渊散射的测量结果相差明显.利用该平均体黏滞系数对氮气在不同压强下的温度进行了反演,得到各压强下的温度与实际温度的绝对误差小于2.50 K,反演温度的平均值与实际温度误差小于0.15 K,该结果证明了实验测量得到的氮气的体黏滞系数具有较高的准确性,同时也说明利用瑞利-布里渊散射反演气体参数具有较高的准确性和可靠性.