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     

谱线非均匀展宽条件下原子激发的计算方法

原子吸收谱线展宽对原子光激发有较大的影响.在速率方程理论中人们通常将原子谱线的均匀展宽和非均匀展宽都归于原子谱线的线型函数.本文提出了一种将均匀展宽和非均匀展宽区别处理的原子激发计算方法,并指出当激发光与原子作用时间短于原子热运动平均碰撞时间时,这种区别对待均匀展宽和非均匀展宽的做法是十分必要的.并将文中提出的新方法应用于一个二能级原子跃迁过程的计算,计算结果显示两种方法具有很大差别.     

非单色激光场与同位素原子体系相互作用

本文研究了多模激光场与同位素原子体系相互作用动力学问题。采用混沌模型描述多模激光场，用Ｆｏｋｋｅｒ－Ｐｌａｎｃｋ方程方法，导出了有限带宽混沌场与同位素原子体系相互作用动力学方程。分析了激光线宽对同位素原子激发电离效率和选择性因子的影响。     

厚原子蒸气介质中原子选择性光电离的理论研究

激光在厚原子蒸气介质中传播时会产生脉冲形变和延迟现象,这会直接影响原子多步光电离过程中的电离率和选择性.从原子蒸气激光同位素分离的实际出发,对厚原子蒸气介质中的原子光电离过程进行了研究,利用密度矩阵方法描述原子的光电离过程,利用Maxwell方程描述激光在厚介质中的传播,建立了介质中同时存在两种同位素的激光传播电离方程组,考察了原子蒸气参数和激光参数对厚介质中平均电离率和平均选择性的影响.研究结果表明:对于较厚的原子蒸气介质,激光功率的增加使平均电离率升高,平均选择性下降;对于相对较薄的介质,适当地降低激光功率可以同时提高平均电离率和平均选择性.存在一个正的激光延时使原子蒸气中目标同位素的平均电离率达到最大.尽量延长激光脉冲的宽度不仅可以同时提高目标同位素的平均电离率和平均选择性,还可以降低对激光脉冲之间相对延时的控制精度.     

谱线非均匀展宽条件下原子激发的计算方法

原子吸收谱线展宽对原子光激发有较大的影响.在速率方程理论中人们通常将原子谱线的均匀展宽和非均匀展宽都归于原子谱线的线型函数.本文中提出了一种将均匀展宽和非均匀展宽区别处理的原子激发计算方法,并指出当激发光与原子作用时间短于原子热运动平均碰撞时间时,这种区别对待均匀展宽和非均匀展宽的做法是十分必要的.文中还将本文提出的新方法应用于一个二能级原子跃迁过程的计算,计算结果显示两种方法具有很大差别.     

Cs-Ar混合蒸气中6S1/2→6P3/2激光激发的速度选择粒子数密度和饱和效应

利用一步激发的饱和吸收光谱技术测量了激发态Cs(6P3/2)态的原子密度.室温下的Cs-Ar混合蒸气被852 nm激光激发,在6S1/2→6P3/2跃迁线轮廓接近纯Doppler增宽线型、激光线宽远小于非均匀的Doppler线宽而与均匀Lorentz线宽相比的条件下,可以确定基态Cs原子中可能被激光吸收的具有速度分量υz的粒子数密度N(υz).在激光功率20μW至2.5 mW的范围内,测量了吸收系数,得到了6P3/2态的速度选择布居数密度.利用从Cs空心阴极灯发出的8S1/2→6P3/2窄谱线的吸收测量,也可以测得6P3/2态的原子密度,两种测量方法所得结果符合得很好.Cs-Ar碰撞的谱线增宽增加了有效泵浦率,有5%的基态原子被单模半导体激光器激发到6P3/2态.由测量不同6P3/2态原子密度时的共振852 nm荧光,也证实了饱和吸收测量激发态原子密度方法的可靠性.     

激光共振电离过程中铅同位素歧视效应研究

以速率方程为基础,通过数值模拟方法,对Pb的激光共振电离通道：“6s26p2 3P0—6s26p7s 3P0₁—电离”的激光诱导同位素歧视效应进行了研究,以探讨减小激光诱导歧视效应,准确测定Pb同位素比值的有效途径。通过考虑激光线型、原子吸收谱线线型、同位素位移及同位素超精细结构等因素的影响,对速率方程进行近似。在近似的速率方程基础上,以“1+1”激光共振电离过程为例,进而探讨激光峰值功率密度、带宽和中心波长对Pb的同位素歧视效应的影响。研究结果表明,利用最佳波长法,可基本消除同位素歧视效应的影响;提高激发光光强可使Pb同位素饱和电离,从而消除同位素歧视效应的影响;采用宽带激光激发,可减小同位素歧视效应的影响。     

激光场线宽对四能级原子系统相干效应的影响

采用密度矩阵方程,在引入激光场线宽的条件下,计算了N型四能级原子系统中介质对探测场的吸收。结果表明:耦合场线宽的引入,消弱了四能级原子相干,对抑制吸收有增强作用,对增强吸收有抑制作用,即耦合场线宽的引入导致了退相干。控制场线宽的引入,在电磁感应透明中削弱了四能级原子相干性,降低了共振吸收,增强了光透明;在电磁感应吸收中,控制场线宽的增加使谱线中心两侧的吸收增强,削弱了谱线中心的吸收增强,从而使四能级系统的电磁感应吸收转换为电磁感应透明。     

红外激光载波包络相位对氦原子的极紫外光(XUV)吸收谱的量子调控研究

本文通过数值求解双电子含时薛定谔方程, 研究了利用红外(IR)超短超强激光的载波包络相位(CEP)对氦(He)原子的极紫外光(Extreme Ultra-Violet, XUV)吸收谱进行量子调控的可能性. 当XUV作用到He原子上时, 原子存在两个电离通道: 无明显电子关联的直接电离和带强烈电子关联的间接电离(即通过双激发态自发电离). 两个通道相互干涉可在XUV吸收谱中形成人们所熟知的Fano共振线型, 并且谱线的形状由两个通道间的比例决定. 通过引入另外一束IR激光, 我们发现, 原子的XUV吸收谱将发生明显改变, 即伴随着超短脉冲CEP的改变而 呈现出从Fano线型到Lorentz线型的周期性连续变化. 上述结果表明, 通过合理地控制超短脉冲的CEP可以有效地调控两个电离通道之间的量子干涉, 进而为探测和操控原子中的极端超快电子关联提供可能.     

用于互组跃迁谱测量的窄线宽激光系统

采用Pound-Drerer-Hall稳频技术将689 nm激光锁定在高精细度超稳极低膨胀系数材料腔上,实现用于探测锶原子互组跃迁谱的窄线宽激光.利用光腔衰荡光谱技术,测量了不同阶次多横模情况下腔的精细度,并在理论上分析了平凹型Fabry-Perot腔的损耗与多横模阶次的关系.考虑了光开关延时及探测器响应时间在测量中的影响,对腔衰荡时间的实验测量值进行了修正.利用光纤飞秒光频梳测量了激光器的频率漂移,测出窄线宽激光频率稳定度的秒稳优于2.8×10-13.利用窄线宽激光在锶原子束上观测到具有高信噪比的窄线宽原子跃迁谱线,实验测得谱线的线宽为55 kHz,该窄线宽原子谱线可应用于锶原子二级冷却激光绝对频率的精确测量及锶原子互组跃迁谱的四种同位素位移测量.     

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.     

A study of isotopically selective photoionization of ytterbium atoms for laser isotope separation

Atomic-vapor laser isotope separation (AVLIS) is studied experimentally and theoretically for ytterbium vapors. The optimum ionization scheme and the process dynamics are determined. The photoionization scheme uses the transitions $$6^1 S_0 \mathop \to \limits^{555.648nm} 6^3 P_1 \mathop \to \limits^{581.067nm} (7/2,3/2)_2 \mathop \to \limits^{582.79nm} (52353cm^{ - 1} ).$$ For a numerical study of photoionization dynamics, the mathematical model of the AVLIS process is used, which is based on the density matrix formalism and Maxwell's equations. Selective photoionization of the¹⁶⁸Yb isotope is simulated numerically. The yield and selectivity of the process are determined. It is shown that the length of the photoionization region is limited because the laser radiation is absorbed by atoms of¹⁷¹Yb and¹⁷³Yb isotopes. The advantage of the laser method over the method using an electromagnetic separator is demonstrated.     

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.     

Resonance Ionization Mass Spectrometry (RIMS) with Pulsed and CW-Lasers on Plutonium

The detection of long-lived plutonium isotopes in ultra-trace amounts by resonance ionization mass spectrometry (RIMS) is a well-established routine method. Detection limits of 10⁶ to 10⁷ atoms and precise measurements of the isotopic composition have been achieved. In this work multi-step resonance ionization of plutonium atoms has been performed with tunable lasers having very different output intensities and spectral properties. In order to compare different ways for the resonance ionization of plutonium broadband pulsed dye and titanium:sapphire lasers as well as narrow-band cw-diode and titanium:sapphire lasers have been applied for a number of efficient excitation schemes. It has been shown, that for identical excitation schemes the optical isotope selectivity can be improved by using cw-lasers (bandwidths < 10 MHz) instead of pulsed lasers (bandwidths > 2 GHz). Pulsed and cw-laser systems have been used simultaneously for resonance ionization enabling direct comparisons of pulsed and continuous ionization processes. So far, a three-step, three-color laser excitation scheme has been proven to be most practical in terms of efficiency, selectivity and laser wavelengths. Alternatively a newly discovered three-step, two-color excitation scheme which includes a strong two-photon transition from an excited state into a high-lying autoionizing state yields similar ionization efficiencies. This two-photon transition was characterized with respect to saturation behavior and line width.     

基于原子吸收光谱的碱金属气室内多种混合气体压强测量方法

碱金属气室是基于原子无自旋交换碰撞弛豫的超高灵敏惯性和磁场测量装置的核心敏感器件。碱金属气室内气体的含量会对原子的弛豫以及系统其他参数的选取产生很大的影响,因此精密测量气室内混合气体各自的压强具有重要的意义。当气室内存在气体时谱线会出现压力展宽和频移,且压力展宽远大于自然展宽和多普勒展宽,因此仅考虑压力展宽。利用压力展宽、频移的大小与气体压强存在的函数关系,提出一种基于原子吸收光谱的碱金属气室内多种混合气体压强测量方法。通过扫描碱金属原子的吸收光谱,得到光学深度曲线,并用洛伦兹函数对其拟合,测得多种混合气体引起的单种碱金属原子的混叠压力展宽和频移,再根据已知的单种、单位压强气体引起的单种碱金属原子的压力展宽和频移,联立计算得到多种气体各自的压强。当存在n种碱金属时,最多可以测量4n种混合气体的压强。仿真结果表明,该方法适用于入射激光未被原子完全吸收的情况;激光功率和频率的波动在1%～10%的数量级时,测量精度影响低于0.4%的数量级,而温度波动在1%～10%的数量级时,测量精度影响高达30%的数量级。     

电子束激发原子对金属蒸发的影响

在原子蒸气激光同位素分离工程(AVLIS)中,金属受电子束的加热而熔化,并向真空蒸发,蒸气原子通过电子束的过程中,可能与电子发生非弹性碰撞,被激发到高能级.在膨胀的过程中,高能级的原子通过与原子的非弹性碰撞消激发,将原子内的电子能量转换为蒸气的平动能.为了分析电子束与原子作用对蒸气的密度、速度和温度等物理特性的影响,采样直接模拟蒙特卡罗法(DSMC)模拟钆原子蒸发过程.模拟结果表明,电子束与原子的作用使得原子蒸气的速度增加,密度减小,温度升高 关键词： 金属蒸发 AVLIS DSMC 电子枪     

Evaluation of Limiting Characteristics of Laser Separation of Silicon Isotopes

Isotopically selective photoionization of silicon atoms by laser radiation using tunable dye lasers is considered. The scheme of atomic ionization with two-photon excitation via the resonance level of one of the high states with subsequent excitation of the autoionization state is proposed. The calculations carried out show that the up-to-date development of laser facilities allows one to evaluate the ultimate production of the highly enriched ²⁸Si isotope at a level of 0.1 g/h at a radiating power of 1 W for each wavelength. The excitation selectivity appears to be high (about 100) within the framework of the radiation problem and is actually defined by the conditions of ion extraction, background atom concentration, charge exchange, and so on rather than by the radiation processes.