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1.
The local, average and electronic structure of the semiconducting materials Si and Ge has been studied using multipole, maximum
entropy method (MEM) and pair distribution function (PDF) analyses, using X-ray powder data. The covalent nature of bonding
and the interaction between the atoms are clearly revealed by the two-dimensional MEM maps plotted on (100) and (110) planes
and one-dimensional density along [100], [110] and [111] directions. The mid-bond electron densities between the atoms are
0.554 e/?3 and 0.187 e/?3 for Si and Ge respectively. In this work, the local structural information has also been obtained by analyzing the atomic
pair distribution function. An attempt has been made in the present work to utilize the X-ray powder data sets to refine the
structure and electron density distribution using the currently available versatile methods, MEM, multipole analysis and determination
of pair distribution function for these two systems.
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Abstract The theoretical background of the maximum entropy method (MEM) when it is applied to restore the electron or nuclear densities from diffraction data is described. In MEM, the concept of “entropy” is introduced to deal with any incompleteness in an observation in a proper way. An incompleteness causes some ambiguities in the results to some extent. The essence of the method is to find a solution which necessarily agrees with the observation, leaving the measure of ambiguities (entropy) maximum. A few results for simple structures with typical types of chemical bonding are also presented. 相似文献
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Total lifetime distribution analysis was employed to obtain fluorescence lifetime profiles of the intrinsic fluorescence ofPseudomonas fluorescens, Escherichia coli, Bacillus subtilis, andStaphylococcus epidermidis. The lifetimes were measured using a multiharmonic Fourier transform phase-modulation fluorometer which can simultaneously
measure the phase shift and demodulation at many modulation frequencies. The 364-nm line from an argon-ion laser and the 325-
and 442-nm lines from a helium-cadmium laser were used for sample excitation. Broad emission windows were used to capture
as much of the bacterial emission as possible for the lifetime measurements. The maximum entropy method was used to recover
lifetime profiles from the multifrequency phasemodulation data. At all three excitation wavelengths, the bacteria exhibited
three lifetime components, in the ranges of 0.5-1, 2–3, and 4–8 ns. Using 325-nm excitation, a fourth component, in the range
of 9–14 ns, was recovered in all of the bacteria; using 364-nm excitation, the fourth component was resolved only in the two
Gram-negative bacteria (P. fluorescens andE. coli). Excitation at 364 nm provided the most reproducible lifetime profiles and showed some differences among the four bacteria. 相似文献
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为了解现代谱分析技术在风廓线雷达中应用的可行性,通过采用实测的和模拟的风廓线雷达回波信号,对比研究了FFT法与最大熵法的谱分析效果。结果表明:(1)当回波信号比较强时,两者都可以得到较好的谱分析效果;但是当回波信号较弱时,最大熵法分析效果优于FFT法,最大熵法对地杂波具有较好的抑制能力。(2)最大熵谱比较光滑,表明最大熵法对随机白噪声也有一定的抑制作用。(3)最大熵法的递推阶数对谱分析结果有一定影响,最终预测误差准则确定的递推阶数一般偏小,采用15阶的递推阶数进行最大熵法分析取得了较好结果。由于风廓线雷达回波通常都较弱,因此研究结果可望用于改善信号处理效果。 相似文献
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High resolution maximum entropy method (MEM) electron density maps have been elucidated for LiF and NaF using reported X-ray structure factors. The ionic nature of the bonding between constituent atoms in both the systems is found to be well pronounced and clearly seen from the electron density maps. The resolution of the present MEM maps is 0.063 Å per pixel for LiF and 0.072 Å per pixel for NaF along the three crystallographic axes. The electron density at the middle of the bond along [111] is found to be 0.0673 e/Å3 for LiF and 0.003 e/Å3 for NaF showing the different ionic strengths of the bonding. The electron density along [100] and [110] has also been drawn and analyzed. The inequality in the ionicity for the individual atoms and the electron content for different ionic radii have also been analyzed and compared with already published results. The wRMEM obtained from MEM analysis is 0.3% for LiF and 0.79% for NaF. 相似文献
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The new quantified version of the maximum entropy method allows one to recover lifetime distributions with a precise statement of the accuracy of position, surface, and broadness of peaks in the distribution. Applications to real data (2,6-ANS in aqueous solutions of sodium dodecyl sulfate micelles of -cyclodextrin) are presented.Died on September 1, 1993. 相似文献
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通过在强磁场条件下,利用环形刀口石墨阴极(刀口尺寸38~39mm)开展电子束轰击收集极内表面铜箔和垂直轰击金属靶片实验,对无箔二极管中电子束的空间密度分布进行了初步研究,并对其产生原因进行了分析。研究结果表明,电子束径向分布在37.2~40.2mm,存在密度较高区域(38.8~39.4mm)和密度最大值点(39.2mm),且均偏向于阴极外侧。无箔二极管环形阴极爆炸发射产生电子束的径向密度分布可用偏态分布近似。 相似文献
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Making use of the operator product expansion, we derive a general class of sum rules for the imaginary part of the single-particle self-energy of the unitary Fermi gas. The sum rules are analyzed numerically with the help of the maximum entropy method, which allows us to extract the single-particle spectral density as a function of both energy and momentum. These spectral densities contain basic information on the properties of the unitary Fermi gas, such as the dispersion relation and the superfluid pairing gap, for which we obtain reasonable agreement with the available results based on quantum Monte-Carlo simulations. 相似文献
10.
本文首先利用悬浮型微波共振探针测量了Ar等离子体的电子密度,并与朗缪尔双探针的测量结果进行了比较,表明了微波共振探针在低密度等离子体测量的可行性.对40.68 MHz单射频容性耦合Ar/SF6和SF6/O2等离子体的测量结果表明:电负性气体SF6掺入Ar等离子体显著降低了等离子体电子密度,但随着增加SF6的流量,电子密度表现为缓慢下降;而O2掺入SF6等离子体中,电子密度则随着O2流量的增加表现为持续的下降.另外,40.68 MHz/13.56 MHz双频激发的SF6/O2容性耦合离子体的电子密度并不随低频功率的变化而变化.本文对上述的实验现象进行了初步的解释. 相似文献
11.
The electron density distribution and the local structure of the high temperature thermoelectric material PbTe has been studied. Powder X-ray data set of PbTe is analyzed in terms of cell parameter, thermal vibration parameters, 1D, 2D and 3-dimensional electron density distributions. The bonding between the atoms using the maximum entropy method (MEM) and bond-length distribution using pair distribution function (PDF) have been analyzed. Both the pictorial and the numerical results of electron density and PDF studies show mixed ionic and covalent characters in PbTe. 相似文献
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K. Matsuo Y. Gosho T. Toyoda Y. Sonoda K. Muraoka M. Akazaki 《International Journal of Infrared and Millimeter Waves》1985,6(10):1051-1059
This paper is concerned with systematic developments of the Fraunhofer-diffraction method for studies of plasma density fluctuations in high-temperature plasmas using infrared lasers, namely (i) development of the general theory of the Fraunhofer-diffraction method, (ii) measurements of fluctuations propagating in an azimuthal direction, (iii) measurements of fluctuation intensities, and (iv) application in measurements on high-temperature plasmas. 相似文献
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P. Bhambhani 《Phase Transitions》2013,86(6):551-560
In this article, the high-pressure structural phase transition and electron momentum density in PbTe have been studied by means of first-principles total energy calculations which are based on the linear combination of atomic orbitals method within generalized gradient approximation. It is observed that the stable phase of PbTe is rocksalt (B1) type and it transforms to an orthorhombic Pnma (B27, FeB type) structure at 6.77?GPa. Further, structural phase transition from the Pnma phase to the cesium chloride (B2) phase has been observed at 13.04?GPa. The anisotropies in theoretical directional Compton profiles indicate larger occupied states along the [100] direction. On the basis of equal-valence-electron-density profiles, it is found that PbTe shows less ionicity as compared to PbS and PbSe. 相似文献
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分别利用电子的漂移速度和等离子体的传播速度计算了大气压下氦等离子体射流的电子密度。 相似文献
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ABSTRACTInteractions of cycloheptatriene derivatives, C7H6X, (X?=?NH, PH, AsH, O, S, Se) with the cations H+, CH3+, Cu+, Al+, Li+, Na+, and K+ are studied using B3LYP functional and 6-311++G(d,p) basis set. The calculated gas-phase cation affinities (CA) and cation basicities (CB) for all molecules decrease as H+ > CH3+ > Cu+ > Al+ > Li+ > Na+ > K+. We used the induced aromaticity in the 7-membered ring of C7H6X upon interaction with the cations, M+, as a measure of C7H6X/M+ interaction. Nucleus-independent chemical shift (NICS) and harmonic oscillator model of aromaticity (HOMA) were used as two indices of aromaticity. The highest and lowest induced aromaticities were observed for interactions of H+ and K+, respectively. Also, the aromaticity induced by interaction with a cation in C7H6AsH and C7H6PH was larger than that in C7H6NH and C7H6O. Hence, the aromaticity was considered as a measure of covalency for the C7H6X/M+ interactions showing a rational dependence on both the molecule and cation. The nature of the interactions was also assessed using electron density, charge distribution analysis and NBO calculations. The results of the aromaticity indices, NICS and HOMA, were compared with the electron density and NBO results. 相似文献
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The symmetric and asymmetric charge distribution of bonding electrons in silicon and gallium arsenide has been studied using experimental powder X-ray data sets. The mid-bond electron densities from the origin are found to be 0.787 and 1.725 e/Å3 at 1.175 and 1.222 Å, respectively, for Si and n-GaAs. The effectiveness of powder data sets in precise bonding studies has been verified. Multipole analysis of the valence charge distribution has also been done. The thermal vibration parameters have been studied and presented for both these systems. The magnitudes of the non-nuclear maxima of both these systems are consistent with those obtained using single-crystal studies. 相似文献
20.
Inequalities of the electron density at the nucleus and radial expectation values of the ground state for the lithium isoelectronic sequence 下载免费PDF全文
The electron density at the nucleus, ρ(0), and the radial expectation values, (-2≤n≤10), of the ground state for the lithium isoelectronic sequence are calculated with a full core plus correlation (FCPC) wavefunctions. By using these obtained expectation values, the accurate inequalities of the electron density at the nucleus and the radial expectation values derived by Gálvez and Porras for these systems are examined and verified. The final results show that FCPC wavefunctions used in this work can give satisfactory results in full configuration space. 相似文献