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Two- and Three-Photon Partial Photoionization Cross Sections of Li+, Ne8+ and Ar16+ under XUV Radiation
摘要： In this work, we present the photon energy dependence of the two- and three-photon cross sections of the two-electron Li+, Ne8+ and Ar16+ ions, following photoionization from their ground state. The expressions for the cross sections are based on the lowest-order (non-vanishing) perturbation theory for the electric field, while the calculations are made with the use of an ab initio configuration interaction method. The ionization cross section is dominated by pronounced single photon resonances in addition to peaks associated with doubly excited resonances. In the case of two-photon ionization, and in the non-resonant part of the cross section, we find that the 1D ionization channel overwhelms the 1S one. We also observe that, as one moves from the lowest atomic number ion, namely Li+, to the highest atomic number ion, namely Ar16+, the cross sections generally decrease.
关键词： free-electron laser,X-ray radiation,cross sections,lowest-order perturbation theory,multiphoton ionization
更新于2025-09-23 15:23:52
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Full wave analysis of plane wave diffraction by a finite sinusoidal grating: E-polarization case
摘要： Plane wave diffraction studies of a finite sinusoidal grating have previously assumed that the grating length is large in wavelengths and the depth of its corrugations is small compared to a wavelength. This paper introduces a rigorous technique, the Method of Analytical Regularization (MAR), which removes these restrictions. The solution obtained by this method is free from limitations on the parameters of sinusoidal grating and possesses the capability to achieve predetermined accuracy of computations uniformly in a wide frequency band. The results of previous studies, which employed the Wiener-Hopf technique combined with a perturbation method, are compared with those obtained by the MAR; excellent concordance of results in the common parameter regimes of applicability of both methods is found. The different regimes of applicability of each approach are identified; within these, the MAR provides effective and efficient solutions to benchmark problems for testing other approximate techniques.
关键词： efficient computational algorithm in wide frequency band,scattering of E-polarized plane wave,method of analytical regularization,Floquet modes for finite grating,finite sinusoidal grating,Wiener-Hopf technique combined with perturbation method
更新于2025-09-23 15:23:52
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Application of Karpman-Maslov-Solov’ev soliton perturbation theory to systems with distributed gain or losses
摘要： We analyze the most important ideas of the Karpman-Maslov-Solov’ev soliton perturbation theory, and based on it, we solve analytically the long-standing problem of the Schr?dinger soliton interactions in the systems with distributed losses or gain. Our analytical results do give a quite good qualitative and quantitative check of the numerical results known so far. Unexpected and nontrivial result consists in the fact that the relative soliton separation depends on the accumulated (integrated) losses or gain.
关键词： Karpman-Maslov-Solov’ev soliton perturbation theory,Distributed losses or gain,Soliton interactions
更新于2025-09-23 15:23:52
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Vectorial perturbation theory for axisymmetric whispering gallery resonators
摘要： We propose a vectorial perturbation theory for axially symmetric, generally nonspherical whispering gallery resonators made of isotropic and anisotropic optical materials. It is based on analysis of the leading terms in the coupled equations for independent light-field components, as derived from Maxwell's equations, and true boundary conditions. Strong localization of the whispering gallery modes (WGMs) near the resonator rim, controlled by the azimuth modal number m, is the main prerequisite for our analysis. The theory gives high-precision expressions for the WGM frequencies and modal functions, including the evanescent effects. One of important applications of the theory is analysis of anticrossings of the WGM resonances in anisotropic resonators detected in experiments. Simple relations for the frequency avoidance gaps during the anticrossings are derived and compared with experimental data obtained in lithium-niobate-based WGM resonators. We show also that the vectorial effects substantially restrict the field of applicability of the scalar WGM models.
关键词： frequency avoidance gaps,whispering gallery resonators,anisotropic materials,vectorial perturbation theory,anticrossings
更新于2025-09-23 15:23:52
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Diagrammatic approach to nonlinear optical response with application to Weyl semimetals
摘要： Nonlinear optical responses are a crucial probe of physical systems including periodic solids. In the absence of electron-electron interactions, they are calculable with standard perturbation theory starting from the band structure of Bloch electrons, but the resulting formulas are often large and unwieldy, involving many energy denominators from intermediate states. This work gives a Feynman diagram approach to calculating nonlinear responses. This diagrammatic method is a systematic way to perform perturbation theory, which often offers shorter derivations and also provides a natural interpretation of nonlinear responses in terms of physical processes. Applying this method to second-order responses concisely reproduces formulas for the second-order-harmonic shift current. We then apply this method to third-order responses and derive formulas for third-order-harmonic generation and self-focusing of light, which can be directly applied to tight-binding models. Third-order responses in the semiclassical regime include a Berry curvature quadrupole term, whose importance is discussed including symmetry considerations and when the Berry curvature quadrupole becomes the leading contribution. The method is applied to compute third-order optical responses for a model Weyl semimetal, where we find a new topological contribution that diverges in a clean material, as well as resonances with a peculiar linear character.
关键词： Weyl semimetals,perturbation theory,Berry curvature,nonlinear optical responses,Feynman diagrams
更新于2025-09-23 15:23:52
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[IEEE 2018 18th International Symposium on Antenna Technology and Applied Electromagnetics (ANTEM) - Waterloo, ON (2018.8.19-2018.8.22)] 2018 18th International Symposium on Antenna Technology and Applied Electromagnetics (ANTEM) - Cavity Perturbation Technique to Characterize Dielectric Materials at Ka-band
摘要： The characterization of permittivity and dielectric loss of dielectric materials using a perturbation technique is conducted in simulation. The working frequency of this technique has been extended to the Ka-band. Simulation results demonstrate this perturbation technique is suitable for characterization of high permittivity dielectric materials for mm-wave and maintains good accuracy.
关键词： dielectric loss measurement,permittivity measurement,cavity perturbation technique,dielectric materials,Ka-band
更新于2025-09-23 15:23:52
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[IEEE IGARSS 2018 - 2018 IEEE International Geoscience and Remote Sensing Symposium - Valencia (2018.7.22-2018.7.27)] IGARSS 2018 - 2018 IEEE International Geoscience and Remote Sensing Symposium - Focusing of Spaceborne Bistatic SAR Data Based on Time-Domain Perturbation
摘要： Spaceborne bistatic synthetic aperture radar (BiSAR) system with an inclined geosynchronous (GEO) illuminator and a low-earth-orbit (LEO) receiver is capable of providing higher signal-to-noise ratio (SNR), finer resolution and vast area of surveillance. However, spatial-variant characteristics of the echo signal caused by the complex bistatic formation raise technical challenges in data focusing. To address the issue, an improved chirp scaling (CS) algorithm based on time-domain perturbation is proposed. Firstly, the signal model of GEO-LEO bistatic SAR is derived based on the geometry. Then, two-dimension (2-D) perturbation functions are employed to correct the spatial-variant characteristics of the echo signal. Finally, the phase compensation functions are generated to compensate for the residual phase. The proposed algorithm is able to implement wide-swath and high-resolution imaging. Simulation results of point targets show the validity of the presented method.
关键词： chirp scaling,spatial-variant,imaging,bistatic SAR,time-domain perturbation
更新于2025-09-23 15:22:29
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XCT image reconstruction by a modified superiorized iteration and theoretical analysis
摘要： In this paper, we propose an improved iteration superiorization method for X-ray computed tomography image reconstruction. We simplify the classic superiorized iteration by removing two constraints imposed on the perturbation. A novel method is proposed to determine the perturbation amount and direction for the superiorized iteration simultaneously. Some theoretical properties (convergence for instance) of the superiorized iteration sequence with the proposed perturbation are analysed. We present a general proof for the convergence of ART-like iterations with summable perturbations. In addition, we prove the convergence of simultaneous iterations without the summable perturbation assumption. Experiments on simulated and real data not only verify the theoretical result but also show that the proposed algorithm is superior to the classic superiorized iteration and can reconstruct desirable images.
关键词： XCT image reconstruction,superiorization of iteration,proximity perturbation,algebraic method
更新于2025-09-23 15:22:29
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Synthesis, Photophysical and Electrochemical Studies on Peripherally Ruthenated Tetraphenylporphyrin
摘要： Ruthenium(II) complexes of 5-(4-hydroxyphenyl)-10,15,20-tris-(4-methylphenyl)porphyrin and 5,10,15,20-tetrakis(4-hydroxyphenyl)-porphyrin have been synthesized in basic medium and spectroscopically characterized. The complexes show broad bands in the visible region due to overlap of porphyrin transitions with the metal-to-ligand charge transfer (MLCT) transitions arising from cis-Ru(bipy)2Cl2. 1H NMR indicates π-electron interactions between the porphyrin ring and bipyridyl rings. The complexes are emissive both at room temperature and at 77 K with two independent emissions at 77 K, one from the excited state of the porphyrin moiety at 650 and 720 nm and the other from the excited state of the ruthenium moiety at 700 nm. No excited state intramolecular energy transfer reactions have been detected. However the quantum yield of the complexes is reduced due to heavy atom effect. Time resolved fluorescence decay analyses show dual life time at 1.5 and 7.7 ns in dichloromethane. The shorter life time is assigned to singlet-excited state of ruthenium and longer life time to porphyrin singlet excited state. Electrochemical analysis of these species in dichloromethane exhibits three anodic peaks at around 0.68, 0.98 and 1.42 V versus SCE, analogous to the Ru3+/2+ couple and two successive one-electron oxidation processes on the porphyrin ring, respectively. On the other hand, cathodic potentials at around -0.8, -1.4 and -1.54 V versus SCE, with first two peaks corresponding to two successive reduction taking place at the porphyrin ring and the third peak corresponding to reduction of bipyridyl unit. All spectroscopic studies reveal perturbation on the electronic levels of both porphyrin ring and ruthenium chromophore.
关键词： Porphyrin,Ruthenated,Life time,Perturbation,Emission,Electrochemistry
更新于2025-09-23 15:22:29
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Nature of the excited states of layered systems and molecular excimers: Exciplex states and their dependence on structure
摘要： Weakly bound systems, like noble-gas dimers or two-dimensional layered materials (graphite, hexagonal boron nitride, or transition-metal dichalcogenides), exhibit excited electronic states of a particular nature. These so-called exciplex states combine on-site (or intralayer) and charge-transfer (or interlayer) configurations in a well-balanced way. We show by ab initio many-body perturbation theory that the energy and composition of the exciplex states depend sensitively on the bond length or interlayer distance of the material. When the constituents approach each other, the charge-transfer contribution increases and the excitation is redshifted to lower energy. If the system is excited into the exciplex state, then a covalent-like bond results. In consequence, noble-gas dimers form excimer complexes, while layered materials exhibit interlayer contraction.
关键词： charge-transfer excitations,many-body perturbation theory,interlayer contraction,exciplex states,layered systems
更新于2025-09-23 15:22:29