原子法激光同位素分离光功率优化理论研究

利用速率方程方法研究了光学厚介质中原子多步电离过程的激光功率的配置问题.计算结果表明,在总激光功率一定的情况下,存在使原子电离几率为最大的激光功率分配方式.更重要的一个结果是,在保持激光照射原子数量不变的条件下,当总功率一定时,缩小激光束截面积,增加激光束在原子蒸汽中的传播距离可以大大提高原子的平均电离几率.     

激光线宽和光强对同位素原子选择光电离的影响

本文研究了非单色(有限带宽)激光场与同位素原子体系相互作用的激发光电离过程. 采用混沌场随机模型描述激光场,用密度矩阵理论和Fokker-Planck方程方法首次给出了非单色激光场与多能级原子相互作用的激发动力学方程. 针对三能级同位素原子体系,讨论了激光线宽和激光光强对同位素原子电离概率和激光同位素分离过程中分离选择性的影响. 关键词： 激光同位素分离 激发动力学方程 激光线宽 Rabi频率     

含时电离对飞秒脉冲激光在强双光子吸收介质中传播特性和光限幅行为的影响

以具有强双光子吸收特性的4,4′-二甲氨基二苯乙烯分子体系为研究对象,通过采用时域有限差分法和预估矫正法数值求解Maxwell-Bloch方程,模拟了飞秒脉冲在该分子介质中的传播过程,研究了含时电离对双光子吸收过程和光限幅行为的影响.研究结果表明,光电离使介质与光场的非线性相互作用减弱,同时自发辐射降低, 随着入射电场的增强,光电离对主脉冲演化有明显的影响.光电离使介质的光限幅失效行为减弱.随着光电离截面增加,光限幅的动力学窗口变宽,表明了光电离有利于增强介质的光限幅效应.在不同的传播距离处,脉冲传播呈现 关键词： 含时电离 双光子吸收 光限幅效应 超短脉冲激光     

原子蒸气激光法分离钆同位素的偏振选择性实验

对原子蒸气激光法分离钆同位素中的光电离选择性进行了实验研究.分析了在偏振选择定则基础上分离钆奇数同位素的基本原理;测量了电离路径中总角动量J=2→2→1→0变化时的光电离选择性;观察了光的偏振状态改变时光电离选择性的变化,通过控制光阑的大小观测到杂散光引起的光电离选择性降低;进行了外磁场对光电离选择性的影响实验,基本给出了磁场方向和大小对于光电离选择性的影响程度.在外磁场方向垂直于光束的偏振方向时,很微弱的磁场就能引起光电离选择性的降低,磁场大小在2×10-4T时就看到了明显的选择性变化.当外磁场的方向平行光束的偏振方向时,磁场对于选择性的影响就显得相对很小.     

溴的光电离和辐射复合——平均原子模型速率系数与细致组态速率系数

根据原子自洽场理论(即DCA)，计算了溴原子各种电离度各个壳层的DCA约化光电离截面.与AA模型的平均电子轨道约化光电离截面进行比较，总结出其内在规律.在AA的约化光电离截面的基础上，可以得到电离度分辨的DCA约化光电离截面；进而可以得到细致组态的光电离和辐射复合速率系数.为精密地描述非局域热动平衡(n-LTE)等离子体提供必要的基础. 关键词： 光电离 速率系数 平均原子模型     

激光脉冲延时方案对原子多步共振光致电离的影响

针对宁西京教授提出激光脉冲延时方案,在含时薛定谔方程理论框架下探讨了各种参数对激光共振电离效率的影响;并以镥原子共振电离为例,探讨了该方案用于激光共振电离质谱(LRIMS)同位素分析的可行性.结果表明:由于超精细结构能级激励拉比频率很难满足匹配条件,该方案很难用于宽带激光激励电离LRIMS同位素分析中.在理论分析基础上,给出了基于窄带激光激励电离的LRIMS同位素分析的光致电离方案.     

吸收谱线Doppler展宽对原子多步光电离的影响

原子法激光同位素分离的核心之一是如何高效地将原子激发电离.本文从原子法激光同位素分离的实际情况出发,研究了原子吸收谱线的Doppler展宽对原子电离率的影响.研究中使用的理论工具是原子激发电离的密度矩阵方程,并利用数值计算方法对方程进行求解.研究结果表明:当吸收谱线有Doppler展宽时,在激光参数不变的条件下原子电离率会降低;且当激光功率固定时,存在使原子电离率为最大的线宽值.这与已发表文献中无Doppler展宽时的计算结果有很大不同.为了追求最佳的原子电离效果,在原子法激光同位素分离系统中激光应该尽可能地工作在最佳线宽条件下.如果激光线宽有不可控的随机波动,在技术上让激光线宽略大于最佳线宽更为有利.无论如何控制激光线宽,尽可能地降低原子吸收谱线的Doppler展宽都有利于原子电离率的提高.     

高功率横流CO2激光器脉冲预电离过程研究

本文给出了对高功率横流CO_2激光器脉冲预电离过程的理论和实验研究.表明了预电离过程中光电离的重要作用.实验结果表明,脉冲预电离可增大高功率横流CO_2.激光器的pd值(p为放电气压、d为放电间隔),增大放电区注入功率密度.对于提高此类型激光器的放电稳定性和输出激光功率是一种技术简单而有效的手段.     

脉冲激光场选择性光电离同位素原子

对三个脉冲激光场与两种四能级同位素原子选择性相互作用的动力学过程进行了研究。由旋转波近似下的含时Ｓｃｈｒ?ｄｉｎｇｅｒ方程出发，利用Ｓｙｌｖｅｓｔｅｒ定理分别对于近共振和非共振两种激发过程的动力学方程求解，导出了激发和电离几率的解析表达式。研究结果揭示了窄带强激光激发原子的过程中的相干振荡特性。在近共振激发时，导出了粒子布居在激发态上反转的条件，并由此提供了选择性激发和电离过程中所采用的脉冲激光源之基本参数的选择依据。 关键词：     

Measurement of Photoionization Cross Sections of the Excited States of Titanium

实验研究了钛原子在293～321 nm波段的共振增强多光子电离. 采用激光烧蚀和超声射流相结合的实验技术来制备自由原子,由飞行时间质谱仪实现对钛原子光电离产物的检测. 从离子信号强度对和激光强度的依赖关系导出了钛原子价电子激发态的光电离截面. 实验测量的一些激发态光电离截面在0.2～6.0 Mb. 通过对⁴⁶Ti、⁴⁷Ti、⁴⁸Ti的光电离截面测量研究,没有发现明显的同位素依赖性.     

Ionization yield of two-step photoionization process in an optically thick atomic medium of barium

The kinetics of a two-step photoionization process in optically thick atomic medium of barium (Ba) is studied using the rate equation approach. In the first step, Ba atoms get resonantly excited by laser radiation from their ground state to an intermediate excited state and subsequently are ionized in the second step by another laser radiation. The absorption of exciting radiation is taken into account along its propagation direction (optically thick). However, the medium is assumed to be optically thin for the ionizing radiation. A numerical simulation is done to estimate the ionization yield for time-varying Gaussian shaped laser pulses. The required energy density of the laser pulse to saturate the excitation transition throughout the thick medium is calculated. The effect of optical delay between the laser beams on the ionization yield is simulated. The calculated ionization yield from the simulation is compared with the measured values.     

Numerical studies of atomic three-step photoionization processes with non-monochromatic laser fields

The atomic selective multi-step photoionization process is a critical step in laser isotope separation. In this work, we study three-step photoionization processes with non-monochromatic laser fields theoretically based on the semi-classical theory. Firstly, three bandwidth models, including the chaotic field model, de-correlation model, and phase diffusion model, are introduced into the density matrix equations. The numerical results are compared with each other comprehensively. The phase diffusion model is selected for further simulations in terms of the correspondence degree to physical practice. Subsequently, numerical calculations are carried out to identify the influences of systematic parameters, including laser parameters (Rabi frequency, bandwidth, relative time delay, frequency detuning) and atomic Doppler broadening, on photoionization processes. In order to determine the optimal match among different systematic parameters, the ionization yield of resonant isotope, and selectivity factor are adopted as evaluation indexes to guide the design and optimization process. The results in this work can provide a rewarding reference for laser isotope separation.     

碱金属原子的光激发与光电离

本文研究了碱金属原子在三步激光脉冲作用下的光激发和光电离过程的动力学特性, 重点关注和比较了锂和铯原子的异同. 针对多种激发模式, 本文不但建立了其原子布居数在各个跃迁态的速率方程组, 还给出了各相关态的光激发和光电离过程的解析解. 通过精心设计并选择了特殊情况, 显著简化了解析解的数学表达式, 从而凸显和讨论了其物理内涵. 通过自行编程, 系统地计算和观察了各种激发模式对锂原子的光激发和光电离过程的可能影响, 研究和讨论了电离率随激光参数的变化规律. 在相同激发模式下, 比较和分析了采用两种不同激发路径所导致的各态原子布居率的变化, 凸显了改变原子参数所产生的作用. 探讨了锂和铯原子在类似的激发条件下在电离率方面的差别. 最后, 基于本文的研究结果, 本文指出了优化电离率的多种途径.     

Optimization of feed flow parameters in separation of neodymium isotopes by atomic vapor selective laser photoionization and analysis of results

Experimental data for separation of neodymium isotopes by atomic vapor selective laser photoionization are analyzed. Atom scattering in the working volume and the Doppler shift of the atom absorption line are shown to be the basic deselecting processes in the experimental cell studied. A data processing technique that allows one to determine the effect of either deselecting process on the product enrichment and yield is suggested. The experimental dependence of the target isotope concentration in photoions present in the separating chamber on the angular divergence (collimation) of the atomic vapor flow along the laser beam is found. A method for determining the vapor flow optimal angular collimation is developed.     

Generation of XUV attosecond pulses in the process of atomic ionization by few-cycle laser radiation

Ionization of a model two-electron atom in the presence of a strong field of ultrashort laser pulses is investigated using the numerical integration of the nonstationary Schrödinger equation, which describes the dynamics of a quantum system in the presence of an electromagnetic wave. The features of two-electron ionization in the presence of one-and two-cycle pulses are analyzed. The suppression of double ionization in the presence of ultrashort laser pulses related to a finite-time interelectron energy exchange upon the laser action is demonstrated. The features of the generation of high-order harmonics and single XUV attosecond pulses are studied for the atomic ionization by few-cycle laser pulses. The parameters of the laser pulse are optimized for the effective generation of a single XUV attosecond pulse.     

Distortions of laser pulses in neodymium atomic vapor

An experiment on the simulation of the conditions for propagation of laser pulses in dense neodymium atomic vapor is proposed for the development of a technological setup for the laser separation of the ¹⁵⁰Nd isotope from a kilogram sample of natural mixture. The comparison of waveforms of laser pulses in the presence and in the absence of absorption shows that the absorption predominantly takes place at the leading edge of the pulse. The experimental distortion of the waveform is related to the saturation of absorption. The effect is well described using the calculations based on the formalism of the rate equations.     

Laser pulse sequence effects on selective ionization in three-level systems driven by coherent radiation fields

Using the time-dependent Schrödinger equation we investigate the effect of different pulse sequences of two lasers on selective ionization in three-level systems driven by coherent radiation fields. It is assumed that inhomogeneous broadening as well as the radiative decay can be neglected during the laser pulses. We demonstrate to what an extent the selectivity given by the absorption or relaxation process can be increased by coherent interaction during the first step and different timing of the incoherent interaction during the second step of a two-step photoionization. A comparison is made with the rate-equation results. We could show that utilization of coherent excitation for high ionization selectivity is most efficient if the two laser pulses do not overlap.     

Chirp-dependent ionization of hydrogen atoms in the presence of super-intense laser pulses

We perform a theoretical study on dynamic interference in single photon ionization of ground state hydrogen atoms in the presence of a super-intense ultra-fast chirped laser pulse of different chirp types(equal-power and equal-FWHM laser pulses) by numerically solving the time-dependent Schr ¨odinger equation in one dimension. We investigate the influences of peak intensity and chirp parameters on the instantaneous ionization rate and photoelectron yield, respectively. We also compare the photoelectron energy spectra for the ionization by the laser pulses with different chirp types. We find that the difference between the instantaneous ionization rates for the ionization of hydrogen atom driven by two different chirped laser pulses is originated from the difference in variation of vector potentials with time.