散射性介质瞬态矢量辐射传输的蒙特卡洛模拟

瞬态效应和偏振特性对于短脉冲激光在散射性介质中的传输有重要影响。本文采用蒙特卡洛法来求解一维散射性介质内的瞬态偏振辐射传输问题。采用拒绝法确定光束的散射方向。定义了瞬态矢量辐射传输矩阵(TVRTM)来描述瞬态偏振辐射的传输特性,并以此得到了Stokes矢量的角度与时间分布。在蒙特卡洛模型中采用时间平移和叠加原理,可大幅度提高计算精度和计算效率。     

间断有限元法求解一维矢量辐射传输

采用间断有限元法(Discontinuous finite element method,DFEM)求解一维散射性介质内矢量辐射传输问题。推导了散射性介质内矢量辐射传输方程的间断有限元离散格式,空间离散采用间断有限元离散将求解域划分为相互独立的单元,角度离散在传统的均匀分段光滑近似(Piecewise Constant Approximation,PCA)角度离散基础上进行局部加密以得到关键方向上的数值解。采用两个稳态矢量辐射传输算例对间断有限元法求解矢量辐射传输方程的正确性和计算效率进行了验证,在此基础上拓展间断有限元法应用于求解散射性介质内瞬态矢量辐射传输方程,将随着时间推移过程中的瞬态辐射信息和最终的稳态结果相比较验证了本文方法的正确性。     

一种计算中、高能电子被分子散射总截面的修正势方法

在考虑分子内成键原子间的电子云重叠效应的基础上，提出了一种能够在中、高能区准确计算“电子-分子”散射总截面的修正势方法.利用可加性规则及Hartree-Fock波函数，使用这一修正过的复光学势，在30—5000eV内对电子被4个等电子(Z=18)分子(HCl，H₂S，PH₃和SiH₄)散射的总截面进行了计算，并将理论计算值与实验结果及其他理论值进行了比较.结果表明，利用这一修正过的复光学势及可加性规则进行计算，所得理论值与实验结果更为接近. 关键词： 电子散射 总截面 可加性规则 束缚原子     

核子–核子散射过程的研究和矢量介子交换势

在推广的手征SU(3)夸克模型下讨论了核子–核子散射过程和矢量介子交换势.给出了赝标π介子和矢量ρ和ω介子的GCM中心力的交换势和NN散射14个分波的相移.研究表明,矢量介子交换势可以替代单胶子交换势以解释核力的短程排斥.     

中高能正电子被CO，HCl，NH3及SiH4散射的总截面

利用可加性规则，使用Hartree-Fock波函数，采用由束缚原子概念修正过的复光学势(由静电势、极化势及吸收势三部分组成)，在30—3000eV内对正电子被CO，HCl，NH₃和SiH₄散射的总截面进行了计算，且将计算结果与实验结果及其他理论计算结果进行了比较.结果表明，利用被束缚原子概念修正过的复光学势及可加性规则进行计算，所得结果与实验结果的符合程度要比利用未被束缚原子概念修正的复光学势及可加性规则进行计算得到的结果好很多.因此，在复光学势中采用束缚原子概念可提高正电子被分子散射的总截面的计算准确度. 关键词： 正电子散射 可加性规则 束缚原子 总截面     

用直接方法和希耳伯变换计算二维瞬态势及其梯度

本文用直接方法和希耳伯变换给出了计算二维瞬态势及其梯度的新方法.这种方法可以计算复杂度从传统的O(N_t²)降低到O(N_t).在计算势及其梯度时,该方法的直接计算中所用的权重因子不随时间变化,因此它们只需在第一个时间步长时计算一次.所以,当用于二维瞬态散射的时间推进算法中时,该方法可节省大量的CPU时间.数值结果表明该方法的精度非常高.     

中高能电子被甲烷及氯代甲烷散射的总截面

利用可加性规则，使用HartreeFock波函数，采用由束缚原子概念修正过的复光学势（由静 电势、交换势、修正极化势及吸收势这四部分组成），在较大的能量(30—5000eV)范围内对 电子被甲烷及氯代甲烷（CH₄，CCl₄，CHCl₃，CH₂Cl₂和CH ₃Cl）散射的总截面进行了计算，且将计算结果与实验结果及其他理论计算 结果进行 了比较.结果表明，利用被束缚原子概念修正过的复光学势及可加性规则进行计算，所得结 果的精度要比利用未被束缚原子概念修正的复光学势及可加性规则进行计算得到的结果好很 多.因此，在复光学势中采用束缚原子概念可提高电子被分子散射的总截面的计算准确度. 关键词： 电子散射 可加性规则 束缚原子 总截面     

中、高能电子被SO2分子散射的微分截面、动量转移截面及弹性积分截面

在考虑分子内成键原子间的电子云重叠效应的基础上, 提出了一种能够准确计算“中、高能电子-分子”散射的微分截面、动量转移截面及弹性积分截面的修正势方法. 利用可加性规则、使用Hartree-Fock波函数并采用被这一方法修正过的复光学势, 在100—1000eV内对电子被SO₂分子散射的微分截面、动量转移截面及弹性积分截面进行了计算, 并将计算结果与实验及其他理论结果进行比较. 结果表明, 利用这一修正过的复光学势及可加性规则获得的微分截面比利用未修正的复光学势及可加性规则得到的结果准 关键词： 可加性规则 微分截面 动量转移截面 电子散射     

中、高能电子被SO2分子散射的微分截面、动量转移截面及弹性积分截面

在考虑分子内成键原子间的电子云重叠效应的基础上,提出了一种能够准确计算“中、高能电子-分子”散射的微分截面、动量转移截面及弹性积分截面的修正势方法.利用可加性规则、使用Hartree-Fock波函数并采用被这一方法修正过的复光学势,在100—1000eV内对电子被SO2分子散射的微分截面、动量转移截面及弹性积分截面进行了计算,并将计算结果与实验及其他理论结果进行比较.结果表明,利用这一修正过的复光学势及可加性规则获得的微分截面比利用未修正的复光学势及可加性规则得到的结果准确得多,计算得到的动量转移截面及弹性积分截面在入射电子能量不低于200eV时也都比较接近实验值.     

核介质中核子–核子散射截面的BHF微观计算

在Brueckner-Hartree-Fock(BHF)框架下,计算了核介质中核子-核子(N-N)散射总截面.计算中,N-N相互作用势采用Paris势的可分离表示,单粒子谱采用连续选择.计算结果表明,质子-质子散射及质子-中子散射的总截面随核密度的增加而强烈地减小,特别是对低能散射.对结果作了简单的讨论,并与已有的一些计算结果进行了比较.     

Electromagnetic scattering by a time reversal symmetry broken topological insulator sphere

An exact solution of electromagnetic wave scattering by a time reversal symmetry broken topological insulator sphere is researched. According to the constitute relations of topological insulator, we modified magnetic vector potential and electric vector potential of standard Mie theory and derived scattered electromagnetic fields and scattered coefficients. Numerical results show that, when the time reversal symmetry is broken, the extinction efficiencies and the scattering efficiencies are influenced by topological magneto-electric polarizability.     

A Monte Carlo approach for the calculation of polarized light: application to an incident narrow beam

Monte Carlo approaches to compute multiple scattering of polarized light are examined. A Backward Monte Carlo (BMC) method is developed to solve the Stokes vector of the multiple scattered light for an inhomogeneous scattering medium with boundaries. A generalized form of the BMC method in vector notation is proposed. This method can determine the scattered light with sufficient accuracy in both intensity and polarization compared to the same calculation using the doubling-adding method for a plane parallel medium.For application to a narrow incident beam and an inhomogeneous medium, a modified BMC method is developed, borrowing a concept from the Forward Monte Carlo (FMC) method for the first scattering events. Furthermore, a modification of the total scattering matrix, i.e., the combination of the derived scattering matrix with its time inverse, is discussed. This BMC method can be used successfully for model calculations of lidar and other laser measurements of polarized light.     

DRESOR法对瞬态辐射传递问题的研究

用基于蒙特卡洛法(Monte Carlo Method,MCM)的DRESOR法(Distributions of Ratios of Energy Scattered by the medium Or Reflected by the boundary surface)求解入射辐射经过介质散射、壁面反射传递后辐射强度随时间变化的瞬态辐射传递方程(Transient RadiativeTransfer Equation,TRTE)问题。通过在系统内计算一单位瞬态入射辐射对介质的DRESOR数分布,就能计算任意时间内入射辐射在系统内时间响应特性,这样有效提高数值方法处理瞬态辐射问题的通用性。并且能够获得高方向分辨率的辐射强度随时间变化的结果,这是目前大多数数值处理方法比较难做到的,显示出了DRESOR法处理瞬态入射辐射问题的能力．     

CALCULATION OF THE 3-DIMENSIONAL MAGNETIC FIELD BY USE OF THE MODIFIED SCALAR POTENTIAL

A new method for calculating the 3-dimensional magnetic field is described. The modified scalar potential is used to reduce the order of the coupled equations in the comparison with the vector potential under the same node number.     

Time development of a wave packet and the time delay

A one-dimensional scattering problem off a δ-shaped potential is solved analytically and the time development of a wave packet is derived from the time-dependent Schrödinger equation. The exact and explicit expression of the scattered wave packet supplies us with interesting information about the “time delay” by potential scattering in the asymptotic region. It is demonstrated that a wave packet scattered by a spin-flipping potential can give us quite a different value for the delay times from that obtained without spin-degrees of freedom.     

A highly-sensitive automatic transient laser-induced breakdown spectroscopy system with high temporal and spatial resolution

A highly-sensitive automatic transient laser-induced breakdown spectroscopy(LIBS) system is designed and integrated.It successfully avoids the delay time selecting problem in the conventional LIBS system,and realizes the LIBS data acquisition with high spatiotemporal resolution automatically.Multiple transient spectra can be obtained in each measurement,which will provide more information for spectral research.The water-vapour and liquid-water Raman scattering spectra are captured by this system,and the comparison of experimental water-vapour Raman scattering spectrum with theoretical data verifies the reliability of the LIBS system.Based on this system,the air laser induced air breakdown spectra are captured and analysed.The system is also useful for the research on water-vapour Raman Lidar remote sensing.     

High-sensitive automatic transient laser-induced breakdown spectroscopy system with high temporal and spatial resolution

A high-sensitive automatic transient laser-induced breakdown spectroscopy (LIBS) system is designed and integrated. It successfully avoids the delay time selecting problem in conventional LIBS system, and realizes the LIBS data acquisition with high spatiotemporal resolution automatically. Multiple transient spectra can be obtained in each measurement, which will provide more information for spectral research. The water-vapour and liquid-water Raman scattering spectra are captured by this system, and the comparison of experimental water-vapour Raman scattering spectrum with theoretical data verifies the reliability of the LIBS system. Based on this system, the air laser induced air breakdown spectra are captured and analysed. The system is also useful for the research on water-vapour Raman Lidar remote sensing.     

Determination of the temporal response function in femtosecond pump-probe systems

A method of determination of the true temporal response function for pump-probe type of experiments with femtosecond time resolution is presented. An analytical formula allowing calculation of group velocity dispersion (GVD) modified and sample thickness-sensitive pump-probe cross correlation function is provided. The reliability of the formula is further experimentally verified with measurements of ultrafast stimulated Raman scattering and transient absorption signals. Received: 31 October 2000 / Revised version: 6 February 2001 / Published online: 27 April 2001     

频域干涉测量中脉冲延迟时间测定方法的研究

在基于超短激光脉冲频域干涉技术的超快激光与物质相互作用的测量实验中,束一束间的延迟时间对瞬态特性的测量至关重要。利用两束脉冲频谱干涉条纹宽度与脉冲相对延迟时间成反比关系这一特性,阐述了通过对脉冲频谱干涉图进行傅里叶变换,在干涉条纹傅里叶变换面（时间域）上测量脉冲延迟时间的基本原理,并给出了测量误差分析,数值模拟结果显示通过傅里叶变换的方式,在频谱干涉图的时间域上可以有效地提取出两束脉冲间的相对延迟时间。