Photoelectron Energy Loss in Al(002) Revisited: Retrieval of the Single Plasmon Loss Energy Distribution by a Fourier Transform Method

A Fourier transform (FT) algorithm is proposed to retrieve the energy loss function (ELF) of solid surfaces from experimental X-ray photoelectron spectra. The intensity measured over a broad energy range towards lower kinetic energies results from convolution of four spectral distributions: photoemission line shape, multiple plasmon loss probability, X-ray source line structure and Gaussian broadening of the photoelectron analyzer. The FT of the measured XPS spectrum, including the zero-loss peak and all inelastic scattering mechanisms, being a mathematical function of the respective FT of X-ray source, photoemission line shape, multiple plasmon loss function, and Gaussian broadening of the photoelectron analyzer, the proposed algorithm gives straightforward access to the bulk ELF and effective dielectric function of the solid, assuming identical ELF for intrinsic and extrinsic plasmon excitations. This method is applied to aluminum single crystal Al(002) where the photoemission line shape has been computed accurately beyond the Doniach–Sunjic approximation using the Mahan–Wertheim–Citrin approach which takes into account the density of states near the Fermi level; the only adjustable parameters are the singularity index and the broadening energy Г (inverse hole lifetime). After correction for surface plasmon excitations, the q-averaged bulk loss function, <Im[??1 / ε(E, q)]> _q , of Al(002) differs from the optical value Im[??1 / ε(E, q?=?0)] and is well described by the Lindhard–Mermin dispersion relation. A quality criterion of the inversion algorithm is given by the capability of observing weak interband transitions close to the zero-loss peak, namely at 0.65 and 1.65 eV in ε(E, q) as found in optical spectra and ab initio calculations of aluminum.     

地基高频加热激励ELF/VLF波对辐射带高能电子的准线性散射

地球内、外辐射带电子通量的变化对于空间飞行器,尤其是中低轨卫星的防护有着非常重要的影响.基于回旋共振波粒相互作用的准线性理论,使用地基高频发射器发射电波调制低电离层背景电流可以人工激励ELF/VLF波,这些波能使辐射带相对论电子发生抛射角散射沉降进入大气层从而降低其生存期.为了定量地分析人工激励ELF/VLF波散射辐射带高能粒子的可行性,针对内、外辐射带,本文选取了两个典型区域：L=4.6和L=1.5.数值计算结果表明,在内、外辐射带由于ELF/VLF波的人工注入而造成的高能电子损失时间尺度很大程度上取决于冷等离子体参量α^*(∝B²/N₀,这里B是背景磁场,N₀是电子数密度)、电波频谱特性和功率,以及与波发生回旋共振的电子能量.一般来讲,在外辐射带人工ELF/VLF哨声波散射相对论电子使之沉降到大气层要容易得多;低能量的高能电子(200keV)要比高能量的相对论电子(500keV)更有效地通过抛射角散射进入大气层.考虑到高频电波加热电离层激励的ELF/VLF波可能会被捕获在磁层空腔中,来回反射从而得到增强,因此在适当的条件下,地基高频加热装置发射足够的电波功率进入电离层诱导大幅度ELF/VLF波注入到内磁层,能够在1至3天的时间尺度内快速散射外辐射带相对论电子使之沉降,也能够在10天量级的时间尺度里散射生存周期一般为100天甚至更长的内辐射带相对论电子. 关键词： 地基高频加热电离层 ELF/VLF波激励 高能电子散射和沉降 共振波粒相互作用     

Useful applications of the electron localization function in high-pressure crystal chemistry

The main features of a new computational code aimed at the topological analysis of the electron localization function (ELF) in crystals are hereby presented. Besides the complete localization of all critical points, the code is able to determine the limiting surfaces that define the chemical regions associated with the so-found maxima (or attractors) of the ELF. Hence, integrations of density operators within these basins provide charges and volumes to cores, bonds and lone pairs. Two illustrative applications have been selected. Firstly, we examine the Z dependence of basin compressibilities in the elements of the first two rows of the periodic table, and secondly, we focus our attention on the chemical changes across the pathway of the zinc blende-rocksalt phase transition in BeO.     

Investigation of the Occurrence of ELF Modulation in HF Signals from the Data of Electromagnetic Monitoring

The results of investigations into the occurrence of extremely low-frequency (ELF) modulation in high-frequency (HF) signals are presented. The seasonal and diurnal behavior of the occurrence of synchronous multichannel (SM) ELF modulation is established. A correlation between the SM ELF modulation occurrence and the presence of sporadic E-layers in the ionosphere is found.     

On the bonding of small group 12 clusters

Characteristic properties as well as possible differences in bonding of small group 12 clusters M_n{\rm M}_n (M = Zn{\rm M} = {\rm Zn}, Cd, Hg; n = 2, ?, 6n = 2, \ldots, 6) have been investigated by quantum chemical ab initio methods, i.e., relativistic large-core pseudopotentials, core-polarization potentials and coupled-cluster correlation treatments. A comparison of cohesive energies and spectroscopic properties like ionization potentials, electron affinities, and vibrational frequencies reveals a close similarity between the clusters of Cd and Hg. For Zn clusters we observed an exceptional increase in stability between Zn₃\rm Zn_3 and Zn₄\rm Zn_4. In order to get a more qualitative picture of the covalent contributions to bonding we have calculated the electron localization function (ELF). The ELF analysis is in accordance with the calculated spectroscopic properties and shows predominant van der Waals interactions with weak covalent contributions for all the cluster sizes considered.     

Carbonyl sulphide under strong laser field: Time-dependent density functional theory

The first 52 fs of a time evolution of the electron density in OCS after an interaction with an intense sub 10 fs laser pulse are studied using the time-dependent density functional theory. The nuclear motion in this linear trimer is simulated by the classical molecular dynamics method. Laser fields of intensity 10¹³ W/cm² and 10¹⁵ W/cm² are used. Details of the laser induced changes of the structure, as well as the ionization rate are sensitive to the applied field intensity and its polarization. It is found that under suitable conditions the OCS molecule bends soon after an interaction with a laser pulse. A deviation from the linear geometry of up to 23.6° and charged ions of up to +3 are observed. The time evolution of electric dipole moments and the time-dependent electron localization function (ELF) are also studied.     

26种材料能量损失函数的计算研究

计算拟合了26种块体材料的能量损失函数,包括18种单质材料（Ag、Al、Au、C、Co、Cs、Cu、Er、Fe、Ge、Mg、Mo、Nb、Ni、Pd、Pt、Si、Te）和8种化合物（AgCl、Al₂O₃、AlAs、CdS、SiO₂、ZnS、ZnSe、ZnTe）.基于Drude-Lindhard模型,将实验测得的能量损失函数拟合为有限个解析函数,通过求和规则验证了拟合结果的准确性.基于拟合结果,模拟了反射电子能量损失谱,模拟结果与实验结果一致.所有材料能量损失函数的拟合参数均在线公开,地址为http：//micro.ustc.edu.cn/ELF/ELF.html.     

Electronic structures of Al–Si clusters and the magic number structure Al8Si4

The low-energy structures of Al₈Si_m (m = 1–6) have been determined by using the genetic algorithm combined with density functional theory and the Second-order Moller-Plesset perturbation theory (MP2) models. The results show that the close-packed structures are preferable in energy for Al–Si clusters and in most cases there exist a few isomers with close energies. The valence molecular orbitals, the orbital level structures and the electron localisation function (ELF) consistently demonstrate that the electronic structures of Al–Si clusters can be described by the jellium model. Al₈Si₄ corresponds to a magic number structure with pronounced stability and large energy gap; the 40 valence electrons form closed 1S²1P⁶1D¹⁰2S²1F¹⁴2P⁶ shells. The ELF attractors also suggest weak covalent Si–Si, Si–Al and Al–Al bonding, and doping Si in aluminium clusters promotes the covalent interaction between Al atoms.     

Simulation of pedestrian evacuation strategies under limited visibility

Evacuation process under limited visibility is different from that under good visibility. To investigate the evacuation efficiency under limited visibility, different evacuation strategies including walking along the wall (S1), following the average movement direction (S2) and following the average position (S3) are proposed in this paper. Performances of these strategies under different visibilities, densities and exit widths are compared based on a revised social force model. Simulation results show that strategy S1 is more effective at low densities while strategy S2 and S3 are more efficient at high densities. It is noted that strategy S2 outperforms S3 under the same condition. In addition, strategy S1 is not sensitive to the change of exit width due to the movement mode of walking along the wall, while the following strategies (S2 and S3) have a better performance under the wide exit condition. The evacuation time for different proportions of pedestrians adopting one strategy in the mixed strategy situation is investigated and the optimal mixture proportion is also discussed. This study provides a new insight into the effect of different evacuation strategies on pedestrian evacuation, which is helpful for evacuees or organizers of public events to make an efficient evacuation plan under limited visibility.     

Physical properties of Ima2-BN under pressure: First principles calculations

A fully orthorhombic boron nitride (BN) polymorph with an orthorhombic symmetry (Ima2-BN, space group: Ima2) was investigated by first-principles calculations. The Ima2-BN under 30 GPa is both mechanically and dynamically stable via elastic constants and phonon spectra. The anisotropic and electronic properties of Ima2-BN under different pressure are investigated in this work. The anisotropic properties calculations show that the Young's modulus of Ima2-BN in (001) plane exhibits the greatest anisotropy under ambient pressure, while in (111) plane it is the greatest when P > 20 GPa, while the (010) plane has always exhibited the minimal anisotropy whether under ambient pressure or high pressure. Ima2-BN is an indirect wider band gap semiconductor material under ambient pressure, and the band gap of Ima2-BN decreases with the increasing pressure. The minimum thermal conductivities κ_min of Ima2-BN is 1.85 W/(cmK), it is slightly higher than of B₄N₄-I and c-BN.     

Ultrasound and microwave assisted synthesis of dihydroxyacetophenone derivatives with or without 1,2-diazine skeleton

A thorough study concerning O-alkylation and α-bromination of dihydroxyacetophenone (DA) and N-alkylation of 1,2-diazine, under ultrasound (US) and microwave (MW) irradiation as well as under conventional thermal heating (TH) is presented. Under US and MW irradiation the yields are higher, the amount of used solvent decreases substantially, the reaction time decreases considerable (from hours or days to minutes) and the consumed energy decreases, consequently the O-alkylation, α-bromination and N-alkylation methods could be considered environmentally friendly. A selective and efficient way to either bis-O-alkylation or mono-O-alkylation of DA has been found, the relative position of the two hydroxyl groups on the phenyl moiety being compulsory. A selective and efficient way for α-bromination in heterogeneous catalysis of DA derivatives under US irradiation is presented. The N-alkylation reaction of DA under US and MW irradiation proved to be the most convenient setup procedure for these types of reactions. Overall, the use of US proved to be more efficient than MW or TH.     

Stochastic resonance of a tri-stable system with α stable noise

A tri-stable system excited by weak periodic signal is taken as a model and the stochastic resonance phenomenon is investigated by additive α stable noise in this paper. The laws for the resonance system parameters q, p, skewness parameter r and intensity amplification factor Q of α stable noise to act on the resonant output are explored under different stability indicies α and asymmetric skewness β of α stable noise. The results indicate that a weak signal can be realized by tuning the system parameters q, p and r under the joint action of additive α stable noise, and the interval of q and p which can induce stochastic resonance does not change with α or β. Moreover, a certain rule is found in which adjusting the intensity amplification factor Q of α stable noise can also realize a synergistic effect when studying the noise-induced stochastic resonance, and the interval of Q does not change with α or β; the best value of the characteristic index is $\alpha =1$ under any system parameter, and the best value of the symmetry parameter is $\beta =1$ under any system parameter. So, the system performance is best when $\alpha =1$ and $\beta =1$. The results will contribute to a reasonable selection of parameter-induced stochastic resonance system parameters and noise intensity of noise-induced stochastic resonance under α stable noise.     

ELF emission generated by the HAARP HF-heater using varying frequency and polarization

We present first observations of the ELF emission generated by the modulated ionospheric HF-heating using the HAARP facility. We focus primarily on the dependence of the polarization and of the amplitude of the ELF wave upon the frequency and polarization of the heating radio wave. A simple 1D computational model of the ELF generation based on the earlier developed model of a horizontal magnetic dipole caused by the HF ionospheric heating [1] is presented. The model develops a qualitative understanding of the changes in the ELF amplitude and polarization due to variations of the heating frequency. The obtained results imply that polarization of the ELF emission generated by the ionospheric HF-heating can be controlled by changing the frequency or polarization of the radio wave. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 8, pp. 728–735, August 1999.     

Couplage Jahn-Teller dans le niveau excite des centres F dans CaO: Effet des contraintes uniaxiales

The zero phonon line associated with the emission band of the F centers in CaO is studied under stress. The absence of any splitting under [111] stress shows that the excited level is mainly coupled to E_g modes of vibration. The effect of the temperature on the intensities of the two components observed under [001] stress shows that the tunneling time is short with respect to the lifetime.     

Ionization of M subshells of Pb atoms by electron impact in an energy range of 5–30 keV

The relative intensities of the M ₅ N and M ₄ N satellites of Pb under electron bombardment of thick targets in the range of accelerating voltages U = 5?30 kV are experimentally investigated. Based on the previously proposed model of M X-ray emission, the relative intensities of these satellites are calculated using the total ionization cross sections of M subshells under electron impact found in different approximations. It is established that, among the models yielding analytical expressions for calculating total ionization cross sections, the model of classical binary collisions provides the best agreement with experimental data in the electron energy range under study. The parameters of the semiempirical Bethe formula for calculating the ionization cross sections of Pb M subshells under electron impact are determined.     

Brillouin scattering investigation of paraelectric KH2PO4 under applied uniaxial stress

The Brillouin scattering spectra of KH₂PO₄ under an applied uniaxial stress have been studied in the neighborhood of the ferroelectric transition. Dependence of shear acoustic phonon on the uniaxial stress is similar to dependence on electric field. The acoustic phonon is overdamped near T_c, but become underdamped under the applied shear stress. In the case of ΔT(= T — T_c) < ～ 0.05 K, application of stress caused a transition not to have any soft acoustic phonon.     

On invariant states and the commutant of a group of quasi-free automorphisms of the CAR algebra. III

It is shown that a state of the CAR algebra is an extreme invariant state under the group of quasi-free automorphisms α_U with unitaries u in a von Neumann algebra M on the one- particle Hilbert space if and only if it is a gauge-invariant quasi-free state Ø _A corresponding to A in M' with 0 ≤ A ≤ 1, under the assumption that M does not contain any finite type I factor direct summands.     

Novel neon-hydrate of cubic ice structure

The stability of ice I cubic (ice I_c) whose voids are occupied by neon particles is investigated using a hybrid type of isobaric grand-canonical Monte Carlo simulation. We show that the resulting neon hydrate is stable under high pressure and temperature where ice I_c alone is unstable, suggesting the existence of a novel neon hydrate of ice I_c. We also show through chemical potential calculation that the neon hydrate of the ice I_c structure is more favorable than the neon hydrate of the ice II structure, whose existence was proven from experiment under high pressure condition.     

Destruction of pressure-induced superconductivity by long-range antiferromagnetic order in Tm2Fe3Si5

Reentrant superconductivity has been detected in a well characterized sample of Tm₂Fe₃Si₅ by ac susceptibility measurements under applied pressure. The superconducting transition temperature displays a pronounced, non-linear behavior under pressure, starting at low pressures with an increase which is among the largest known for any compound (dT_c1/dp = 0.47 K/kbar). The magnetic transition temperature is relatively pressure independent.