Comparison of simulation methods for electronic structure calculations with experimental electron energy-loss spectra

The electronic structure of hexagonal GaN is studied using two simulation techniques in order to develop a method to interpret the fine-structure of an experimental nitrogen K-edge electron energy loss spectrum obtained using a scanning transmission electron microscope. The application of these simulation methods to the bulk spectrum is a necessary first step in developing a fundamental understanding of the effect of changes in the electronic structure on the properties of defects. It is found here that both of the techniques used, multiple scattering (MS) and density functional theory (DFT), produce excellent agreement with the experimental bulk spectrum. The MS method is limited in accuracy but efficient in time, while the DFT method is more accurate but time consuming. Through the combination of these methods, experimental energy loss spectra can be readily understood, and a means to unravel the complexities of the electronic structure can be determined.     

Comparison of electron spin relaxation times measured by Carr-Purcell-Meiboom-Gill and two-pulse spin-echo sequences

Electron spin relaxation times obtained by two-pulse spin-echo and Carr-Purcell-Meiboom-Gill (CPMG) experiments were compared for samples with: (i) low concentrations of nuclear spins, (ii) higher concentrations of nuclear spins and low concentrations of unpaired electrons, (iii) higher concentrations of nuclear spins and of electron spins, and (iv) dynamic averaging of inequivalent hyperfine couplings on the EPR timescale. In each case, the CPMG time constant decreased as the time between the refocusing pulses increased. For the samples with low concentrations of nuclear spins (the E' center in irradiated amorphous SiO2) the limiting value of the CPMG time constant at short interpulse spacings was similar to the Tm obtained by two-pulse spin echo at small turning angle. For the other samples, the time constants obtained by CPMG at short interpulse spacings were systematically longer than Tm obtained by two-pulse spin echo. For most of the samples, the CPMG time constant decreased with increasing electron spin concentration, which is consistent with the expectation that the CPMG sequence does not refocus dephasing due to electron-electron dipolar interaction between resonant spins. Dynamic processes that average inequivalent hyperfine couplings contributed less to the CPMG time constant than to the spin-echo decay time constant. The impact of nuclear echo envelope modulation on CPMG time constants also was examined. For a Nycomed trityl radical in glassy D2O:glycerol-d8 solution, the CPMG time constant was up to 20 times longer when the time between pulses was approximately equal to integer multiples of the reciprocal of the deuterium Larmor frequency than when the time between pulses was an odd multiple of half the reciprocal of the deuterium Larmor frequency.     

Nonperturbative treatment of ionization with excitation of helium by electron impact

We present cross sections for electron-impact ionization and simultaneous ionization plus excitation of helium by electron impact. The results are obtained from a fully nonperturbative close-coupling formalism using our B-spline R-matrix approach. A large number of pseudostates in the expansion of the wave function represent the coupling to the ionization continuum. We obtain excellent agreement with the directly measured experimental cross section ratios (Bellm et al., Phys. Rev. A 75, 042704 (2007)) for ionization leaving the residual He? ion in either the 1s ground state or the n = 2 (2s + 2p) excited states.     

Raman spectra of single-crystal C60

We have analysed the Raman spectra of C₆₀ single crystals between room temperature and 10K and studied the temperature-induced phase transition in this material. The spectra show crystal field splitting of the internal Raman modes but no evidence for a line shift near the phase transition. The photo-induced transformation of the crystals and its implication on the interpretation of the Raman spectra is discussed. In the low temperature phase we observed two lines at 30 cm^–1 and 41 cm^–1 which we assign to the librational modes of the crystal.     

Joint experimental and computational study of aluminum electron energy loss spectra

Experimental reflection electron energy loss (REEL) spectra are measured from aluminum for primary energies ranging from 130 eV to 2 keV. A Monte Carlo simulation is shortly described and used to calculate the same spectra. The focus is on reproducing the variable weight of surface and bulk losses as the surface sensitivity of spectra changes by changing the primary electron energy. The intensity of surface losses in the simulations is modulated by the thickness of the region where surface excitations occur. Simulations based either on a constant or an energy-dependent thickness for this layer are considered. In both cases, simulated spectra reproduce the experimental trend as a function of energy, though the correct surface-to-bulk intensity ratio for each energy is either underestimated or overestimated.     

Calibration of electron spectra

Energy calibration procedures, which have been used in ESCA, are reviewed. Binding energies of calibration lines suitable for solid and for gaseous samples have been determined. Charging effects of nonconducting samples have been investigated for different X-radiation, sample thickness and material. A calibration relative to lines from some insulating standard compound mixed into the sample was shown to be unreliable, while experiments indicated that in calibrating relative to a surface layer, the carbon line from the hydrocarbon contamination layer can be used as well as a line from a thick layer of a noble metal on a part of the sample.     

Computer simulation of single-crystal and polycrystal sputtering I

Sputtering of Cu single-crystal and polycrystal targets by 27 keV Ar ions has been simulated using the new binary collision cascade computer program OKSANA. The sputtering yield, the sputtering and reflection efficiencies, and the absolute and relative contributions to sputtering from various components have been calculated in a broad range of incidence angles. The obtained angular dependences of the sputtering yield have proved to agree with experimental data. Some features of sputtering due to semichannel focusing of incident particles have been found. The contributions to sputtering from several types of linear collision chains and from the primary knock-on atoms are considered in most detail. It has been shown, in particular, that the pure focused, pure defocused, and mixed focused-defocused collision chains contribute noticeably to sputtering. The contribution from the primary knock-on atoms is angle-dependent and reaches its maximum in the range of glancing angles for both single-crystal and polycrystal targets.     

Free electron laser spectral dynamics with a modulation of electron beam energy

Recent experimental and theoretical works on free electron laser spectral dynamics have pointed out the difficulty to obtain a narrow and stable spectrum operation. This goal can only be achieved by avoiding the sideband generation leading to a broadband and unstable spectrum. Tapered wiggler and two-frequency wiggler are well suited for combining sharp spectrum and high efficiency but are not really compatible with a wide tunability of laser light. Filtering sidebands is a good way for lower power experiments but it seems to be difficult to conceive wideband filters, specially in the far-infrared region. Modulation of electron energy is a new potential soft way for controlling the spectral dynamics of longpulse free electron laser. Spectral dynamics under the modulation is investigated in the linear and non-linear regimes in the far-infrared region. Simulations show that a pulsed and sharp spectrum behavior can be obtained by optimizing the modulation parameters. The interest of such a method for the far-infrared experiments is discussed.     

Pure absorption electron spin echo envelope modulation spectra by using the filter-diagonalization method for harmonic inversion

Harmonic inversion of electron spin echo envelope (ESEEM) time-domain signals by filter diagonalization is investigated as an alternative to Fourier transformation. It is demonstrated that this method features enhanced resolution compared to Fourier-transform magnitude spectra, since it can eliminate dispersive contributions to the line shape, even if no linear phase correction is possible. Furthermore, instrumental artifacts can be easily removed from the spectra if they are narrow either in time or frequency domain. This applies to echo crossings that are only incompletely eliminated by phase cycling and to spurious spectrometer frequencies, respectively. The method is computationally efficient and numerically stable and does not require extensive parameter adjustments or advance knowledge of the number of spectral lines. Experiments on gamma-irradiated methyl-alpha-d-glucopyranoside show that more information can be obtained from typical ESEEM time-domain signals by filter-diagonalization than by Fourier transformation.     

考虑相界效应的Ni基单晶合金纳米压痕模拟

利用分子动力学方法分别模拟金刚石压头压入Ni模型和Ni基单晶合金γ/γ′模型的纳米压痕过程,通过计算得到两种模型[001]晶向的弹性模量及硬度.采用中心对称参数分析不同压入深度时两种模型内部位错形核、长大过程以及Ni基单晶合金γ/γ′(001)相界面错配位错对纳米压痕过程的影响.结果显示:压入深度0.641 nm之前,两种模型的压入载荷-压入深度曲线相似,说明此时相界面处的错配位错对纳米压痕过程的影响很小;压入深度0.995 nm时,在错配位错处发生位错形核,晶体在γ相中沿着{111}面滑移,随即导致Ni基单晶合金γ/γ′模型压入载荷的下降,并在压入深度达到1.487 nm之前低于Ni模型相同压入深度时的压入载荷;压入深度从1.307 nm开始,由于相界面错配位错的阻碍作用,Ni基单晶合金γ/γ′模型压入载荷上升速度较快.     

电子助进化学气相沉积金刚石中发射光谱的蒙特卡罗模拟

采用蒙特卡罗方法，对源料气体为CH4/H2混合气的电子助进化学气相沉积（EACVD）中 的氢原子(H)、碳原子(C)以及CH基团的发射过程进行了模拟.研究了CH4浓度、反应室气压 和衬底偏压等工艺参数对发射光谱及成膜的影响.研究发现，CH基团可能是有利于金刚石薄 膜生长的活性基团，而碳原子不是；偏压的升高可提高电子平均温度及衬底表面附近氢原子 的相对浓度；通过氢原子谱线可测定电子平均温度并找到最佳成膜实验条件.该结果对EACVD 生长金刚石薄膜过程中实时监测电子平均温度，有效控制工艺条件，生长出高质量的金刚石 薄膜具有重要的意义. 关键词： 蒙特卡罗模拟 金刚石薄膜 发射光谱     

Rapid computer simulation of E.S.R. spectra

A fast computer algorithm is presented which permits simulation of the effects of rotational diffusion, electron and nuclear relaxation, microwave power, and modulation frequency upon saturation transfer (passage) E.S.R. spectra. Comparison of theoretical and experimental spectra for nitroxide spin-labelled biomolecules suggests that while the dependence of electron spin-lattice relaxation time upon rotational correlation time is weak, the variation of the ratio of the electron to nuclear spin-lattice relaxation times is significant and consideration of strong nuclear relaxation is necessary for the simulation of spectra characterized by correlation times near the reciprocal of the nitrogen nuclear resonance frequency.     

Comparison between intensity jumps in experimental,simulated and theoretical multielemental spectra

The results obtained by applying the theory developed for the SEICXRF method and applying a Monte Carlo simulation code (MCS) are compared with the experimental data in order to validate this theory. Specifically, an assay is made with the spectra corresponding to a brass sample and a soil sample. The experimental data were corrected considering the chamber efficiency and the dead time of the detection system. The considerations taken into account in order to obtain a correct comparison between theory and experiment are discussed in detail. Good agreement between spectra is observed, taking as reference the given values of the elemental composition. The experimental intensity jumps are in agreement with those predicted by the theory and by the MCS. Copyright © 2003 John Wiley & Sons, Ltd.     

Comparison of silicon LIII,II electron loss spectra and optical absorption

Threshold structure due to L_III,II excitation in the electron loss spectra of silicon at 100.2 ± 0.3 eV is in good agreement with optical absorption measurements of 100.1 ± 0.1 eV.     

Magnetoplasma spectra of two-dimensional electron rings

A hydrodynamical approach is employed to study the collective excitations of a two-dimensional electron gas confined in a ring geometry, under a normal magnetic held. The theory predicts a rich spectrum of magnetoplasma excitations, with several branches of high- and low-frequency resonances. It is shown that the low-frequency resonances can be identified as edge magnetoplasmons localized at the inner and outer boundaries of the ring, exhibiting their characteristic antidot-like and dot-like features. The high-frequency resonances are essentially bulk two-dimensional magnetoplasmons. Our calculated magnetoplasma modes and their associated induced charge densities and dipole moments are compared with a recent experiment, and it is found that most of the experimentally observed features of the spectrum are accounted for with the present theory.