Improvement of energy loss near edge structure calculation using Wien2k

The density functional theory (DFT) is a recognised method for the calculation of electronic properties of materials. As such it can also be used for the calculation of energy loss near edge structures. Some care has to be taken since the DFT is intended for ground state calculation. The effect of the core hole left by the excited electron is different in an insulator and in a metal and can be observed in both cases. For an insulator (MgO, Si), a supercell calculation is needed while in the case of copper, extremely good agreement with experiment can be obtained with a partial core hole calculation. In the particular case of the WIEN code (APW method) we show that calculation of low lying edges (Si L at 99eV) where the initial state is not strongly localised can only be done within the dipole approximation and with some care. Random alloys (CuNi) have been calculated previously using a supercell; we show that a particular version of the virtual crystal approximation gives promising results.     

First-principles calculation of the electronic structure and energy loss near edge spectra of chiral carbon nanotubes

We present first principles calculations of the electronic structure of small carbon nanotubes with different chiral angles theta and different diameters (d<1 nm). Results are obtained with a full potential method based on the density functional theory (DFT), with the local density approximation (LDA). We compare the band structure and density of states (DOS) of chiral nanotubes with those of zigzag and armchair tubes with similar diameters. The carbon K-edge energy loss near edge structures (ELNES) have been studied and pi* and sigma* contributions have been evaluated. These contributions give information on the degree of hybridization for the small chiral nanotubes.     

Electron energy loss spectra of a Pd(110) clean surface

Electron energy loss spectra of a Pd(110) clean surface have been measured at primary energies of 40–100 eV. The observed peaks are at the loss energies of ∼ 3, 4.3, 7.5, 11.5, 16, 21.3, 26.5 and 33.8 eV. The 7.5, 26.5, and 33.8 eV peaks are attributed mainly to the bulk plasmon excitations associated with 5s electrons, coupled 5s and a limited number of 4d electrons, and total (4d+5s) electrons, respectively. The rest of the peaks are ascribed mainly to one-electron excitations.     

C 1s near edge X-ray absorption fine structure (NEXAFS) of substituted benzoic acids—A theoretical and experimental study

Ab initio calculations are performed to explain the discrete transitions in experimental C 1s-NEXAFS (near edge X-ray absorption fine structure) spectra of various benzoic acid derivates. Transition energies and oscillator strengths of the contributing C 1s–π^* excitations are computed using the ADC(2) (second-order algebraic-diagrammatic construction) method. This method is demonstrated to be well suited for the finite electronic systems represented by these simple organic acids. There is good agreement between experiment and theory reproducing all the relevant spectral features. Some transitions can only be assigned based on a theoretical foundation. Remaining discrepancies between experimental and computed spectra are discussed.     

Electron energy loss spectra of the system Cu(311)/CO,Xe

The electron energy loss spectrum of a clean Cu(311) surface has been studied by reflected electrons using primary energies between 30 and 300 eV. The losses obtained are compared with literature values. Loss spectra with adsorbed CO and Xe are also reported. Some assignments of the loss mechanisms have been attempted.     

Mobility enhancement of 2DEG in MOVPE-grown AlGaN/AlN/GaN HEMT structure using vicinal (0 0 0 1) sapphire

Al_0.25Ga_0.75N/AlN/GaN high electron mobility transistor (HEMT) structures were grown on (0 0 0 1) sapphire substrates with vicinal angles of 0.0^°,0.25^°,0.5^°$0.0^{°},0.25^{°},0.5^{°}$ and 1.0^°$1.0^{°}$ by metalorganic vapor phase epitaxy (MOVPE). Vicinal sapphire was demonstrated to enhance the step-flow growth to improve morphology, crystal and optical qualities, which eventually suppressed interface scattering and dislocation scattering to enhance the mobility of 2-dimension-electron-gas (2DEG). The optimum vicinal degree was determined to be 0.5^°$0.5^{°}$, and the corresponding 300 K Hall mobility and sheet resistance were 1720 cm²/V s and 301 Ω/sq, respectively. Furthermore, the temperature dependence of Hall measurements proved good high-temperature performance of the 0.5-off sample.     

Electron energy loss and Auger study of epitaxially grown Cu on Ni(100)

Auger and electron energy loss spectra have been measured on films of Cu epitaxially grown on Ni(100). The films were prepared under UHV conditions using a quartz crystal for monitoring the deposition rate. LEED measurements were taken to determine the orientation of the films. The presence of a monolayer of Cu on Ni(100) is enough to suppress the 3p-3d transition on the surface of the sample. The electron energy loss spectra were studied as a function of the primary electron energy (50 to 300 eV). The experimental results were qualitatively analyzed using recent theoretical calculations of Cooper and co-workers. The effect of a small Cu coverage on Ni(100) on the chemisorption of CO and O₂ was also studied. A strong suppression of CO chemisorption at room temperature was observed. In the case of O₂, large exposures are necessary in order to observe a significant amount of oxygen on the surface. The absence of any appreciable chemisorption on the surface of the metal is attributed to the lack of empty d-surface states.     

Electron energy loss spectra and X-ray photoelectron spectra of Ca2V2O7

We have measured the electron energy loss spectra of Ca₂V₂O₇ in the reflexion mode, at incident energies between 200 and 2400 eV, and the X-ray photoelectron spectra excited by Al K α radiation. The abundant loss structures observed can be correlated with the possible interband transitions, collective oscillations, and excitation of O2s and V3p electrons within the V₂O₇^4- ion. The gap width and molecular orbital (MO) spread (or splitting) is about l eV larger in the V₂O₇^4- ion than in its component VO₄^3- ion. Excitation of O2s states, which may occur together with some MO over-gap transitions, displaces the collective oscillations about 7 eV towards lower energies. Deeper V3p electrons are excited with a maximum energy loss some 7 eV above their binding energy. Cross transitions from Ca3p levels into some empty states of the V₂O₇^4- ion, or direct transitions to available states of the Ca²⁺ ion could not be unambiguously identified. The energy dependence of the excitation cross section and of the electron penetration depth results in a significant variation of the relative intensity of various losses over the investigated energy range.     

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.     

The influence of the 1st AlN and the 2nd GaN layers on properties of AlGaN/2nd AlN/2nd GaN/1st AlN/1st GaN structure

Applied Physics A - This is a theoretical study of the 1st AlN interlayer and the 2nd GaN layer on properties of the Al0.3Ga0.7N/2nd AlN/2nd GaN/1st AlN/1st GaN HEMT structure by self-consistently...     

Theoretical studies on the near edge X-ray absorption fine structure spectra of a hexanethiolate monolayer on Ag(1 0 0)

A theoretical study on the S K-edge near edge X-ray absorption fine structure (NEXAFS) of a hexanethiolate monolayer on Ag(1 0 0) has been performed by employing the multiple-scattering cluster (MSC) method. The unoccupied molecular orbitals of the system, which are closely correlated with resonances of the NEXAFS spectra, have been calculated by using the discrete variational (DV)-Xα method. The physical origins of the resonances are elucidated by these theoretical studies. It has been shown that the leading peak at 2470.3 eV is not a π*(S–C), but a resonance corresponding to the transition of 1s electrons into a hybrid orbital of the S(3p) atomic orbital of a hexanethiolate molecule and Ag atomic orbitals. The interaction between the adsorbate and the substrate induces other two weak resonances at 2475.2 and 2478.2 eV in the NEXAFS. The adsorption structure of a hexanethiolate monolayer on Ag(1 0 0) deduced from the theoretical analysis on the NEXAFS is in agreement with that from the SEXAFS of the system.     

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.     

Landscape of s-triazine molecule on Si(100) by a theoretical x-ray photoelectron spectroscopy and x-ray absorption near-edge structure spectra study

The chemisorbed structure for an aromatic molecule on a silicon surface plays an important part in promoting the development of organic semiconductor material science. The carbon K-shell x-ray photoelectron spectroscopy(XPS) and the x-ray absorption near-edge structure(XANES) spectra of the interfacial structure of an s-triazine molecule adsorbed on Si(100) surface have been performed by the first principles, and the landscape of the s-triazine molecule on Si(100) surface has been described in detail. Both the XPS and XANES spectra have shown their dependence on different structures for the pristine s-triazine molecule and its several possible adsorbed configurations. By comparison with the XPS spectra, the XANES spectra display the strongest structural dependency of all of the studied systems and thus could be well applied to identify the chemisorbed s-triazine derivatives. The exploration of spectral components originated from non-equivalent carbons in disparate local environments has also been implemented for both the XPS and XANES spectra of s-triazine adsorbed configurations.     

Electron energy loss spectroscopy of the decomposition of formic acid on Ru(001)

Electron energy loss spectroscopy has demonstrated the existence of both a monodentate and a symmetric bidentate bridging formate as stable intermediates in the decomposition of formic acid on the Ru(001) surface. The monodentate formate converts upon heating to the bidentate formate which decomposes via two pathways: CH bond cleavage to yield CO₂ and adsorbed hydrogen; and CO bond cleavage to yield adsorbed hydrogen, oxygen and CO. Thermal desorption spectra demonstrate the evolution of H₂,H₂O, CO and CO₂ as gaseous products of the decomposition reaction. The observation of this product distribution from Ru(100), Ni(100) and Ni(110) had prompted the proposal of a formic anhydride intermediate, the existence of which is rendered questionable by the spectroscopic results reported here.     

Electron energy loss spectroscopy of ethylene on Ag(110) precovered with oxygen: energy dependence of the cross section

The intensities of the CH out-of-plane bending (v₇) and CH stretch (v₁) vibrations of ethylene adsorbed on Ag(l 10) precovered with oxygen have been measured in EELS as a function of beam energy from 2 to 20 eV. The energy dependence of the v₇ vibration is satisfactorily described by the dipole approximation. For the v₁ vibration, which is purely dipole forbidden, an entirely different energy dependence is found, with a maximum intensity at a beam energy of 2 eV. Comparison of the EEL intensities observed experimentally with those predicted from the dipole moments of the free molecule confirms the conclusion that the molecular plane of chemisorbed ethylene is parallel to the surface.     

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.     

Experimental and theoretical study of the structure and vibrational spectra of valpromide,C7H15CONH2

In this work, we present results of the conformational and vibrational properties of valpromide (Vpd), an amide with antiepileptic activity, studied by IR and Raman spectroscopy at 300 and 77 K, and 300 K, respectively. Experimental data are compared against ab initio calculations performed at B3LYP level with the inclusion of solvatation effects. Experimental results, reinforced by theoretical calculations, point out that Vpd has three conformers on the potential energy surface, with different structures that can be identified in the CO and NH spectral regions. These conformers are defined by different angular arrays of the dihedral angles formed with the CO, C N and C〈 H, C C of the aliphatic chain bonds. The existence of different conformations and structures are discussed on the basis of results derived from electronic localization function (ELF) and natural orbital bond (NBO) analysis. Copyright © 2008 John Wiley & Sons, Ltd.     

大能量损失小动量转移几何条件下氦原子(e, 2e)反应的理论研究

用BBK模型和修正后的BBK模型在共面、大的能量损失和近于最小的动量转移这一特殊几何条件下对氦原子的三重微分散射截面进行了理论计算和研究，对截面的结构进行了分析，并把计算结果与最新实验测量结果进行比较发现：BBK模型和修正后的BBK模型均给出了与实验结果较符合的binary峰和recoil峰的比率以及双峰的角分布. 关键词： binary峰 recoil峰 能量损失 动量转移