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Time of flight spectra are calculated within the cascade model of phonon-mediated desorption for helium desorbing from graphite, sapphire and Nichrome. One and three-dimensional theories are compared. Deviations from Knudsen's cosine law are traced to dynamical effects. The spectra are — as a function of time — narrower than a Maxwellian distribution and their maxima are generally shifted to shorter times indicating that the desorbed molecules are hotter than the solid temperature. Relevant experiments are discussed. Experimental geometries are analysed. 相似文献
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Rodney Loudon 《物理学进展》2013,62(66):243-280
The contributions of magnons to the optical properties of antiferromagnets having the rutile structure are discussed. The properties considered are electric-dipole active two-magnon absorption in the infra-red, and magnon sidebands on sharp-line exciton transitions in the visible. The discussion is based on a thorough treatment of the properties of excitons and magnons in the antiferromagnetically ordered state. The site-group and space-group symmetries of the magnetic excitations are derived and the selection rules for electric and magnetic dipole transitions are determined. The occurrence of magnetic Davydov splittings of the excitons is investigated, and their symmetry properties throughout the Brillouin zone are derived. The functional dependences of exciton energy on wave vector are calculated. Applications of the theory are made to experimental results on excitons and magnons in MnF2, FeF2 and CoF2. The possible mechanisms for two-magnon and magnon-sideband absorption are discussed, and the influence of crystal symmetry on these mechanisms is described. The two-magnon state responsible for electric-dipole absorption is identified and selection rules for electric-dipole activity are presented. A spin Hamiltonian for the two-magnon process is set up and used to derive expressions for absorption coefficients for electric vector parallel and perpendicular to the crystal c-axis. Comparison with experiment for MnF2 yields numerical values for the parameters of the basic coupling mechanism. The exciton-magnon states which give rise to magnon-sideband absorption are explicitly constructed and electric-dipole selection rules are derived for all possible types of sideband. Spin Hamiltonians for the various magnonsideband absorption processes are presented and used to derive expressions for sideband shapes. The results are applied to the experimental spectra for MnF2 and FeF2 and the sideband shapes in MnF2 are calculated numerically. The sideband shapes observable in emission spectra are also briefly discussed. 相似文献
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Electronic properties of a class of one-dimensional quasiperiodic systems are studied by the extended Kohmoto-Kadanoff-Tang (KKT) renormalization-group method. The employed models are tight-binding diagonal and off-diagonal models. It is showed that the energy spectra of the quasiperiodic systems are Cantor-like, namely the spectra are self-similar and the energy gaps are every-where dense on the realE-line. 相似文献
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D. V. Kolodko D. N. Sinelnikov I. A. Sorokin N. V. Mamedov 《Physics of Atomic Nuclei》2017,80(11):1687-1691
The results of the studies of the energy distribution and atomic-molecular composition of the ions emitted from two Penning ion sources are presented. The transitions between different discharge modes, when the operating pressure and voltage on the anode are changed, are investigated. The energy spectra and mass-charge spectra are measured and the dependences of the fraction of atomic ions on the discharge parameters are determined. 相似文献
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I. V. Puzynin T. L. Boyadzhiev S. I. Vinitskii E. V. Zemlyanaya T. P. Puzynina O. Chuluunbaatar 《Physics of Particles and Nuclei》2007,38(1):70-116
Methods of computational physics developed at JINR for investigation of models of complex physical processes in different fields of theoretical physics are considered. A general mathematical formulation of equations for the models under study is given, numerical methods are described, and information on developed program packages is presented. Particular models of physical processes are discussed. Results of their numerical study are demonstrated. 相似文献
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Methods for computer simulation of strength testing of crystals are proposed. The methods employed are similar to usual static methods, and they are used to investigate deformation and fracture of perfect fcc crystals having different orientations with respect to the tensile force. A strain-induced phase transition from the fcc to the hcp structure is detected, and the formation and displacement of crystal twins are observed. Plastoelastic deformation and fracture of crystals are investigated. 相似文献
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利用太赫兹时域光谱技术测量了聚乙烯、聚丙烯、聚氯乙烯、聚四氟乙烯和丙烯腈-丁二烯-苯乙烯的太赫兹透射谱,研究其在0.2~2.6 THz频段的光谱特性,得到了在室温氮气环境下这些材料的太赫兹吸收谱和折射率色散特性.发现这五种塑料在太赫兹波段的折射率和吸收系数差异显著,折射率分别在1.35~1.85之间,聚乙烯、聚丙烯、聚四氟乙烯吸收很小,相比之下聚氯乙烯、丙烯腈-丁二烯-苯乙烯吸收很大,这为塑料种类的鉴别及高太赫兹透过率塑料衬底选择提供了依据. 相似文献
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Herbert Pietschmann 《Foundations of Physics》1978,8(11-12):905-919
The rules of scientific discovery as formulated by K. Popper are briefly reviewed. Historical examples such as the prediction of planets and outstanding events in elementary particle physics are used to show how these rules are applied by the working physicist. Thus these rules are shown to be actual tools rather than abstract norms in the development of physics. 相似文献
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傅里叶望远镜外场实验性能改进和结果分析 总被引:4,自引:0,他引:4
为了实现对傅里叶望远镜成像系统更接近实际的仿真,改进了外场实验系统结构。采用反射式目标,利用准直扩束镜替代空间滤波器和准直透镜,使用大靶面电荷耦合器件作为监视器。在无大气和包含200m水平大气两种情况下,分别对2.5mm的4种不同空间频谱分布目标进行实验。实验选用9×9,17×17,33×33和65×65傅里叶分量分别进行重构。最高成像角分辨率为3.5″。结果表明含大气与无大气重构结果的Strehl比值相近,从而证明傅里叶望远镜成像系统能够克服下行链路低阶大气扰动的影响。 相似文献
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The canonical (non-parametric) solutions of the variational problems for integral functionals are considered and the canonical solutions of variational problems of mechanics in Minkowski spaces are derived. By combining the variational principles of least action, flow, and hyperflow canonically invariant equations for the energy-momentum variable are obtained. From these equations the equations for the action and wave functions as a general solution of the combined variational problems of mechanics are derived. These equations are applicable for describing different types of particles and interactions and are summarized within the approach of general relativity. 相似文献
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The probability distributions of the fractional intensities and amplitudes of x-ray reflections from a pair of imperfectly
related structures are derived when both the structures satisfy the requirements of a given type of basic Wilson distribution.
These two distributions are used to obtain theoretical expressions for 2 new fractional type ofR-indices which are expected to be useful in the final stages of refinement. The theoretical distributions are also used to
deduce some theoretical distributions which are useful as tests for centrosymmetry via the random permutation method. The
theoretical values of the relevant semi-cumulative functions are also tabulated. 相似文献
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Use of Pulsed Radiography for Investigation of Equations of State of Substances at Megabar Pressures
N. I. Egorov G. V. Boriskov A. I. Bykov Yu. P. Kuropatkin N. B. Lukyanov V. D. Mironenko V. N. Pavlov 《等离子体物理论文集》2011,51(4):333-338
A method of density measurement for substances that are isentropically compressed up to megabar pressures is described in the paper. Measurement results for condensed hydrogen isotopes and aluminum are presented. These results, afterwards, are used for construction of hydrogen isotopes “cold” isentropes. Experimental x‐ray images of the facilities with the hydrogen isotopes, compressed up to several megabars are presented in the paper (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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A. N. Sinel’nikov V. Ya. Artyukhov Yu. V. Aksenova R. T. Kuznetsova 《Optics and Spectroscopy》2012,113(3):291-297
The spectral and luminescence properties of a number of pyrromethene (PM) dyes have been investigated. The absorption and luminescence spectra of PM molecules in different solvents are measured. The electronic structure and electronic excited states are calculated by the method of intermediate neglect of differential overlap with special spectroscopic parameterization. The rate constants of photophysical processes are estimated, and the fluorescence quantum yields are calculated. The fluorescence quantum yields of PM I and PM II molecules are found to decrease upon their excitation to the second short-wavelength absorption band. This phenomenon is interpreted. 相似文献
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Formulas for calculating Legendre functions and associated Legendre functions of the first kind of complex degree using an Ermakov—Lewis invariant are presented. These formulas are straight-forward to implement numerically and are motivated by the lack of computational routines in standard university tools like those of MatLab and Maple. Angular waves propagating in opposite directions are also obtained. The results are particularly useful in complex angular momentum theories and nearside/farside analysis of spin-dependent angular scattering from central potentials. 相似文献