Characteristic energy loss spectra of copper crystals with surfaces described by LEED

Energy distributions of electrons back-scattered from copper (100) and (110) surfaces have been obtained for incident electron energies in the range 30 to 350 eV. The relations between optical measurements and the characteristic energy losses, as well as the effect of interband transitions on the bulk and surface plasmon frequencies in metals which do not have ideally free electron plasmas are discussed. By chemisorbing increasing amounts of oxygen on the clean surface, the surface plasmon loss peak was identified in the copper energy loss spectrum from its intensity dependence on the dielectric constant at the surface. This peak has been identified by previous authors as the bulk plasmon loss of a single s-electron plasma oscillation. Our identification of the surface plasmon loss peak implies that the d-electrons in copper do participate in the plasma oscillation and that the bulk plasmon frequency is shifted from its free electron value because of interband transitions.     

Temperature dependence of aluminum nitride reflectance spectra in vacuum ultraviolet region

Temperature dependence of optical reflectance spectra in vacuum ultraviolet region for aluminum nitride has been measured on high-quality single crystal with synchrotron radiation. The dominant structure due to the interband transition is observed at photon energy around 7.7 eV. With decreasing temperature, the energy position of the dominant structure in the reflectance spectra shifts towards higher energy. The experimental data has been fitted to the Bose-Einstein expression and the obtained parameter related to the strength of the electron-phonon interactions is much smaller than that for the peak at 6.2 eV, suggesting that the higher-lying interband transition energy decreases more slowly with increasing temperature in aluminum nitride (AlN).     

金红石相VO2电子结构与光电性质的第一性原理研究

采用完全势线性缀加平面波方法(FP-LAPW)结合密度泛函+U(DFT+U)模型计算了金红石相VO₂的电子结构和光学性质. 电子态密度计算结果表明所采用的方法可以较好的描述体系的导带电子结构. 计算得到体系为导体,V-O键主要由O原子的2 p轨道与V原子的3 d轨道杂化形成,外加光场垂直和平行于c轴时体系的等离子振荡频率为3.44 eV和2.74 eV,光电导率在0-1 eV之间有一个与带内跃迁有关的德鲁德峰,而大于1 eV的光电导率主要由电子带间跃迁产生,得到并分析了带内跃迁过程和带间跃迁过程各自对反射谱和电子能量损失谱的贡献. 关键词： 光电性质 电子结构 缀加平面波方法 2')" href="#">VO₂     

Characteristic energy loss and Auger electron spectra of GaP (110)

The characteristic energy loss and Auger electron spectra of clean GaP (110) have been measured with a four grid retarding field analyser. A peak in the loss spectrum has been found at 11.2 eV which is probably due to a surface plasma loss. The remaining structure has been assigned to direct interband transitions, to single and double bulk plasma losses and to d-band transitions by analogy with previous optical and electron transmission studies. Suggestions are made as to the origin of the peaks in the Auger spectrum and changes in the spectrum in the presence of oxygen are discussed.     

Bestimmung der optischen Konstanten von Lithium und Natrium durch optisch angeregte Oberflächenplasmaschwingungen

The ATR method to excite surface plasmons by light is used to measure optical constants of Lithium and Natrium in the energy region 2–3.8 eV. For energies smaller than 3.5 eV the real part of the dielectric function can be described by the equations valid for the free electron gas. The imaginary part shows an interband transition (~ 2.5 eV at Li, ~ 2.0 eV at Na), which is rather slow (about 1 eV) indicating an indirect transition.     

Band structure and absorption coefficient in GaN/AlGaN quantum wires

The band structures of rectangular GaN/AlGaN quantum wires are modeled by using a parabolic effective-mass theory.The absorption coefficients are calculated in a contact-density matrix approach based on the band structure.The results obtained indicate that the peak absorption coefficients augment with the increase of the injected carrier density,and the optical gain caused by interband transition is polarization anisotropic.For the photon energy near 1.55 eV,we can obtain relatively large peak gain.The calculations support the previous results published in the recent literature.     

Electron energy loss spectroscopic investigation of palladium metal and palladium(II) oxide

Electron energy loss spectra (ELS) obtained from polycrystalline Pd metal and PdO powder using primary electron energies ranging from 100 to 1150 eV have been obtained and examined in an attempt to gain a better understanding of the origins of the loss features and to assess the utility of ELS in investigations of Pd catalysts. The two sets of ELS spectra differ significantly. The ELS spectra from Pd metal exhibit a predominant peak at 6.5 eV, shown to arise from a surface plasmon excitation, and two broad features at 25.1 and 31.9 eV, which originate from bulk loss processes. The broad features consist of several overlapping losses due mainly to interband transitions from the d-band, though a bulk plasmon excitation is believed to produce a feature near 24 eV. Two distinct peaks are present at 3.7 and 7.6 eV in the ELS spectra obtained from PdO, while a broad region of intensity appears over the range from 20 to 40 eV. The peak at 3.7 eV is attributed to a transition between the top of the valence band and the bottom of the conduction band. The feature at 7.6 eV is broad and arises from several overlapping features that are most likely caused by interband transitions rather than collective excitations. Furthermore, the ELS spectra obtained from PdO and oxidized Pd are also quite different indicating that ELS can provide useful information for determining the bonding states of oxygen on Pd-containing catalysts.     

Spectral properties of RuAl<Subscript>2</Subscript> and RuGa<Subscript>2</Subscript> compounds: Ellipsometric analysis

The optical properties of RuAl₂ and RuGa₂ intermetallic compounds have been investigated in the spectral range of 0.22–14 μm. The nature of interband light absorption has been interpreted based on a comparative analysis of calculated and experimental frequency dependences of optical conductivity. The data obtained confirm the existence of pseudogaps with a width of ~0.8 eV localized at the Fermi level in the electron densities of states of these materials, which was predicted in previous energy-band calculations.     

Angular dependence of the electron energy loss spectra from a clean and adsorbate covered W(001) surface

A dispersion analysis of the EELS from a W(001) surface in the range 1 < ΔE < 35 eV has been performed and compared with recent and complete optical data for tungsten. The non-dispersive (k ～ 0) EELS correlated well with a combination of the surface and bulk loss functions calculated from the optical data. Losses at 1–5 eV and a pair at 32 and 34.5 eV were assigned to interband and N_6,7 core ionization excitations respectively. The principal bulk and surface plasmon losses were identified at 24.0 and 20.3 eV respectively. Two further losses at 14.0 and 9.6 eV were also observed and assigned to subsidiary plasmon losses. All four plasmon losses showed only minimal energy dispersion, never exceeding 1.5 eV. A momentum selectivity for separating bulk and surface interband losses was demonstrated with the non-dispersive losses arising from excitations within the bulk even with incident energies as low as 88 eV, whereas their dispersive counterparts were extremely sensitive to the chemical state of the surface. New adsorbate derived losses which develop during adsorption were associated with excitations from the new deep lying adsorbate levels to final state levels at or near the Fermi level. It was concluded that this final state was also responsible for the N_6,7 ionization losses.     

The influence of resonance IR laser radiation on magnetoabsorption in quantum wires

The coefficient of interband absorption of a weak electromagnetic wave by quantum wires in a transverse magnetic field and an intense laser radiation field is calculated. It is shown that, if the laser radiation frequency is equal either to the size quantization frequency (dimensional infrared resonance) or to a hybrid frequency (magnetoinfrared resonance), laser illumination can determine the shape of absorption oscillations. In particular, it is shown that the second magnetoabsorption peak is split into two peaks, the half-widths of which and the distance between which depend on the intensity of resonance laser radiation. The influence of the polarization of IR radiation on the interband absorption in quantum wires is discussed. The dynamics of the frequency dependence of the optical absorption coefficient with increasing intensity of resonance laser radiation is studied.     

Proprietes optiques et structure electronique de MnAu2

The optical spectrum of the helical antiferromagnetic compound MnAu₂(t_N = 90°C) has been measured, using a scanning ellipsometric method, in UHV, between 0.47 and 5.7 eV, at temperatures ranging from 88 to 700 K. Below 0.6 eV the experimental data can be fitted to a Drude-like intraband model. The maximum of the interband absorption occurs at 5.1 eV, while the onset of interband absorption may be placed at 0?.4eV as is suggested by the rapid rise of $\text{ε}{}_2\text{(ω)}\text{λ}$. below 0.5 eV. In the absence of theoretical work, the analysis of the optical spectrum leads to a preliminary rough model of the electronic structure; the proposed local density of d states is represented. The 5.1eV peak is attributed to d → E_F transitions (parabolic edge at 2.7 eV similar to Au), originating in the lower part of the band, associated mainly with Au sites. To account for the moment $\text{(}\text{3.6μ}{}_{\mathrm{S}}\text{at Mn}\text{)}$, the upper d band (mainly Mn sites) is split: the d↑ band is below E_F (interband edge at 0.4eV), while the d↓ band contains 1.4 electrons. ESCA measurements tend to confirm this model. An important unusual fact is the sharp anomaly of /~ε(ω) in the infrared, around T_N; attempts to correlate this with magnetic (s-d) interactions have been initiated.     

Magnetic field-modulated magnetoreflectance of europium chalcogenides

The optical constants of EuO and EuS single crystals have been determined at 300 K by means of a Kramers-Kronig analysis of the reflectivity for photon energies up to 12 eV. For EuS the optical constants have also been determined above and below the Curie temperature in the energy region from 1.5 to 5.7 eV. A first tentative assignment of optical structure to interband transitions has been attempted on the basis of recent orthogonal plane wave (OPW) and earlier augmented plane wave (APW) band structure calculations. For photon energies from 1.2 to 3.8 eV a low magnetic field-modulated magnetoreflectance has been measured using circularly polarized light. By use of the Kramers-Kronig relation for the differential reflectance spectra in conjunction with our data of the optical constants, a detailed analysis of the magnetoreflectance spectra of EuS was carried out for the first time.     

Optical anisotropy at the W(1 1 0) surface

We report ab initio calculations of the anisotropic dielectric function of tungsten (1 1 0) surface using the linear muffin-tin-orbital method. The calculated anisotropy in the optical spectrum, for polarization of light parallel to the surface, exhibits three dominant broad structures at 3.00, 4.01 and 5.34 eV successively positive, negative and then positive. The first peak is clearly assigned to p → d interband transitions in surface atomic sites whereas the two others have their origin in interband transitions in bulk like atoms. Our results, including the interlayer relaxation effect on the surface optical response, are compared to recent reflectance anisotropy measurements.     

Electronic excitations and luminescence of SrMgF4 single crystals

The electronic and crystal structures of SrMgF₄ single crystals grown by the Bridgman method have been investigated. The undoped SrMgF₄ single crystals have been studied using low-temperature (T = 10 K) time-resolved fluorescence optical and vacuum ultraviolet spectroscopy under selective excitation by synchrotron radiation (3.7–36.0 eV). Based on the measured reflectivity spectra and calculated spectra of the optical constants, the following parameters of the electronic structure have been determined for the first time: the minimum energy of interband transitions E _g = 12.55 eV, the position of the first exciton peak E _n = 1 = 11.37 eV, the position of the maximum of the “exciton” luminescence excitation band at 10.7 eV, and the position of the fundamental absorption edge at 10.3 eV. It has been found that photoluminescence excitation occurs predominantly in the region of the low-energy fundamental absorption edge of the crystal and that, at energies above E _g , the energy transfer from the matrix to luminescence centers is inefficient. The exciton migration is the main excitation channel of photoluminescence bands at 2.6–3.3 and 3.3–4.2 eV. The direct photoexcitation is characteristic of photoluminescence from defects at 1.8–2.6 and 4.2–5.5 eV.     

Manifestation of multiband optical properties of MgB2

The optical conductivity of MgB₂ has been determined on a dense polycrystalline sample in the spectral range 6 meV–4.6 eV using a combination of ellipsometric and normal incidence reflectivity measurements. σ₁(ω) features a narrow Drude peak with anomalously small plasma frequency (1.4 eV) and a very broad ‘dome’ structure, which comprises the bulk of the low-energy spectral weight. This fact can be reconciled with the results of band structure calculations by assuming that charge carriers from the 2D σ-bands and the 3D π-bands have principally different impurity scattering rates and negligible interband scattering. This also explains a surprisingly small correlation between the defect concentration and T_c, expected for a two-gap superconductor. The large 3D carrier scattering rate suggests their proximity to the localization limit.     

Frequency dependence of third harmonic generation in polyacetylene

The frequency dependence of the third harmonic generation in trans-polyacetylene is quantitatively interpreted by considering the bands of finite chain and the damping of excitation. Our theory shows that there are two distinct peaks in the spectrum of the third harmonic generation. The first one around 0.6 eV comes from the three-photon resonant transition between the edges of the valence band and conduction band; the second one around 0.9 eV is produced by the two-photon interband transition. The excitation-dependent damping is essential to determine the position of the second peak. The theoretical results agree well with the experimental data.     

Optical absorption of molybdenum thin films between 0.32 and 5.50 eV: Interpretation in terms of energy band transitions

We report, between 0.32 and 5.50 eV, the optical conductivity of polycrystalline thin films of molybdenum. The films have been deposited in ultra high vacuum and the measurements have been carried out in situ. We observe two maxima at 2.75 and 4.0 eV and two shoulders at 1.85 and 5.1 eV. The structure is understood in terms of interband transitions, the conductivity is compared to a joint density of states histogram deduced from Petroff and Viswanathan energy bands.     

Spectroscopic ellipsometric study of the optical properties of Ag2O film prepared by direct-current magnetron reactive sputtering

The Ag2O film, as-deposited by direct-current magnetron reactive sputtering at a substrate temperature of 150℃, clearly shows a preferential orientation (111), and is capable of lowering the threshold value of the thermal decomposition temperature to about 200℃, which is helpful to its application in optical and magneto-optical storage. This paper fits its optical constants in terms of a general oscillator model by using measured ellipsometric parameters. The fitted oscillator energy 2.487 eV is close to the optical direct interband transition energy value of the Ag2O film determined by Tauc equation; whereas, the fitted oscillator energy 4.249 eV is far from the fitted plasma oscillator energy 4.756 eV by single-oscillator energy. The photoluminescence spectrum centred at about 2.31 eV indicates a direct-energy gap photoluminescence mechanism of the Ag2O film.     

Fundamental optical properties of α-RuCl3

The fundamental optical properties in the paramagnetic phase of α-RuCl₃ are studied at different temperatures in the photon energy interval 0.03 to 10 eV. Infrared reflectivity spectra show a transverse optical frequency at 0.038 eV (32 μm) for an E_u mode (E⊥c, in plane atomic displacements). The absorption spectra in the energy range 0.2 to 1 eV reveal three bands (0.29, 0.51, 0.71 eV) attributed to d-d electronic transitions. Reflectance and thermo-reflectance measurements indicate the onset of the charge-transfer transitions at 1.1 eV and show structure at 1.85, 2.55, 3.05, 4.5 eV. The marked reflectivity peak at 5.2 eV is probably related to p(Cl) → s(Ru) band-to-band transitions.     

The influence of the substrate on the optical constants of Al films

Attenuated total reflection (ATR) spectroscopy employing a prism and a dielectric spacer layer is used to determine the optical constants of vacuum deposited aluminium films. In the energy range of the 1.5 eV interband transition, a strong dependence of the optical constants on the material of the underlying layer is found. This dependence is attributed to structural effects in the Al film.