Ionization and Harmonic Generation of Diatomic Molecular Ion

l.IntroductionRecentexPerimentshavcdemonstratedthehigh-orderharmonicgenerationinraregasandmoleculargas[1~'j.TherearemanytheoreticalpaPersdealingwithharmonicgen-erationofatom[6~8j,comparativelysPeaking,theoreticalworksabouttheharmonicgen-erationofmolecuIeareless[9j,esPeciallyabouttheevolutionofionizationandharmonicgenerationwiththeinternuclearseparation,andtherelationbetweenionizationandhar-monicgeneration.InthispaPerwegivesomenumericalresultsofdiatomicmolecularionirradiatedbyalinearlyPOlari…     

激光脉冲在气体中产生的离化波前的演化及其对光脉冲传播的影响

利用带离化子程序的一维粒子模拟程序，对光脉冲与离化波前的相互作用进行了模拟研究，讨论了在原子多度电离的情况下光脉冲与离化波前的相互作用关系.研究表明，由于原子各级离化势的不连续性使得离化波前在空间上出现平台区；离化过程导致激光频率发生蓝移，这个蓝移又对脉冲形状产生调制，使脉冲前沿呈阶跃型增长；阶跃型的光脉冲包络使离化波前的平台区变短，直至整个离化波前分为几个不同梯度的区域.还讨论了不同元素和不同密度的气体中产生的离化波前的特点.同时分析了光脉冲频率随传播距离的关系，指出由于光脉冲宽度的增加，将导致出射的光 关键词： 激光等离子体 光电离 离化波前 激光频率上转换     

脉冲激光诱导金属薄膜电离的分子动力学模拟

采用分子动力学方法模拟研究了激光诱导金属薄膜的电离过程,对激光等离子体形成早期原子的运动轨迹、薄膜表面的温度变化以及原子的电离特性进行了详细分析,并探究了脉冲激光参数对原子电离过程的影响.结果表明,在激光照射过程中,薄膜表面先熔化而后又气化,气化的原子继续吸收激光能量继而电离.激光的峰值功率密度越大,原子电离速率越快,电离数目越多,薄膜表面的温度越高.脉冲宽度越小,原子电离速率越快,薄膜表面的温度越高,但原子的电离数目先增加后减小.     

Atom under an intense laser pulse: Stabilization effect and strong-field approximation

Direct numerical calculations of the single-photon ionization dynamics of the hydrogen atom were compared with the data obtained within the strong-field approximation (SFA). An analysis showed that the SFA model accurately determines the range of electromagnetic field intensities, upon reaching of which the ionization mode deviates from that described within perturbation theory; in particular, the ionization rate decreases with increasing intensity. It was demonstrated that the actual ionization mechanism under an intense pulse differs significantly from the SFA predictions. For example, an analysis of photoelectron angular distributions and energy spectra showed that the strong-field ionization features within the SFA model are primarily controlled by the ionization channel closing effect associated with the ponderomotive shift of the continuum boundary. At the same time, the results of direct numerical calculations of the ionization dynamics suggest that the Kramers-Henneberger atom is formed in a strong field, which is characterized by increased stability to strong-field ionization.     

Comparison of four methods for processing data on exponential decay

Multiphoton resonant ionization in the case of a doubly degenerate intermediate bound state is studied. In the Keldysh-Feisal-Reiss approach, expressions for the energy and angular photoelectron distributions and the quasi-classical formula for the total rate of resonant ionization are obtained. It is shown that the ionization rate may be both higher and lower than the ionization rate in the usual case depending on the relationship between parameters. A situation with a strongly suppressed probability of resonant ionization is possible. In the near-threshold region, the angular dependence of the probability of photoelectron escape is shown to be weaker in comparison with the case of ionization via a nondegenerate level.     

Development of ionization in nonequilibrium inert-gas plasmas in magnetogasdynamic channels

Effects accompanying the interaction of a flow of preionized inert gas with a magnetic field are studied: selective electron heating, the development of nonequilibrium ionization, and the onset of the ionization instability. Local and average densities and temperatures of the electrons are measured and the average ionization rate is determined. It is found that the average electron density increases as the magnetic induction is raised, in both stable and ionization unstable plasmas. The difference in the rates at which ionization develops in these two states is revealed. The mechanism for the coupling between the average ionization rate in an ionization unstable plasma and the spatial-temporal characteristics of the plasma inhomogeneities is established. Zh. Tekh. Fiz. 69, 56–61 (November 1999)     

钨等离子体平均离化度研究

用SHML模型计算了在局域热力学平衡条件下钨等离子体在温度为0.3～10keV、密度为0.001～0.1 g.cm-3范围内的平均离化度,研究了钨等离子体平均离化度随温度、密度的变化规律.结果表明,钨等离子体的平均离化度随着密度的增加而减小,随着温度的升高而增大,并且在增大的过程中出现了三个平台.研究了电离势对离化度的...     

基于光场离化电流机制产生强太赫兹辐射的参数优化研究

基于超短激光脉冲与气体作用通过光场离化电流产生太赫兹(THz)辐射的模型, 研究了用双色激光脉冲的方法产生强THz辐射的优化参数条件. 数值计算表明, 导致THz辐射产生的离化电流主要是由一阶电离过程产生的, 高阶离化对该电流产生的贡献很小. 通过调节基频光与倍频光的配比、相位差都能增大离化电流, 从而可以提高THz辐射振幅. 将激光波长拓展到中红外波段, 也有利于提高离化电流. 此外,改变作用气体的种类也能改变离化电流. 在激光和密度参数相等的情况下, 在氦气中可以产生高于氮气中2倍左右的离化电流.     

超短超强激光与薄膜靶相互作用中不同价态碳离子的来源

研究了超短超强激光与不同厚度薄膜Al靶相互作用中靶背法线方向碳离子的最初来源. 通过对比分析碰撞电离率和场致电离率所起的作用, 发现C⁴⁺ 及更低价态的碳离子主要由场致电离产生, 而高价态的C⁵⁺和C⁶⁺ 离子主要来自于超热电子与靶表面的碰撞电离. 关键词： 超短超强激光与等离子体相互作用 离子加速 场致电离 碰撞电离     

Pulse width effect in ultrafast laser ionization imaging

The effects of different laser pulse widths on laser-induced ionization imaging of microstructures embedded in transparent materials are investigated. It is shown that a femtosecond laser-induced ionization probe can detect the variation of elemental composition of the sample materials with a higher contrast ratio, whereas the ionization probe generated by picosecond laser pulses is more sensitive to the structural change inside optical materials, which can be well explained by the different roles of multiphoton ionization and avalanche ionization involved in material breakdown. These results also suggest that an optimum diagnosis could be obtained if well-selected laser parameters are employed in ultrafast laser ionization imaging.     

一种计算稠密等离子体电导率的半经验模型

为了准确计算稠密等离子体电离平衡,在理想Saha 方程的基础上,加入了压致电离理论和德拜理论相结合的修正。此电离模型可以描述稠密等离子体内电离机制随着密度的增加由热电离转变为压致电离的现象。压致电离理论结合德拜理论修正的Saha 方程的计算结果与其它电离模型结果作了对比和分析。电离气体区域采用完全电离等离子体模型计算电子弛豫时间,而在凝聚态区域采用电子平均自由程计算电子弛豫时间。提出了将两个区域平滑连接起来的公式,从而建立了一种可以计算从理想等离子体态到凝聚态物质的半经验电导率模型,半经验模型计算结果与实验数据作了对比。     

超短强激光脉冲激励甲烷团簇的内电离机理

采用三维粒子动力学模拟方法研究了甲烷团簇在超短强激光脉冲激励下的爆炸动力学行为,重点讨论了几种典型的内电离机理对团簇爆炸过程中离子的价态和动能的影响.研究表明,在激光脉冲强度比较小的情况下,团簇中的原子主要是在光场作用下通过隧道电离的方式发生电离.当激光场进一步增强时,势垒压低电离是电离的主要方式.在相同的较高激光强度下,团簇更容易通过势垒压低电离达到高的电离价态.团簇发生电离后,其内部库仑电场的点火电离效应和内部滞留自由电子的碰撞电离效应也将增强团簇的再次电离过程. 关键词： 超短强激光脉冲 甲烷团簇 内电离     

Electron transfer in the reflection of atoms from metal surfaces

The resonance mechanism for positive and negative ionization of an atom reflected from a metal surface is treated in the time-dependent Hartree-Fock approximation assuming that the atom moves on its classical trajectory. A simple model is parameterized to describe 30–500 eV Na atoms reflected from tungsten, and the ionization probabilities are calculated by numerical integration of the equation of motion of the time evolution operator. Significant ionization can only occur if either the ionization level, or the affinity level of the atom crosses the Fermi level beyond the range of the atom-metal hopping interaction. On clean tungsten, the former situation applies, and the probability of positive ionization is nearly unity, and of negative ionization nearly zero.     

初始电子密度对光学薄膜激光损伤机制的影响

光学元件的激光损伤问题是激光器件向高功率密度方向发展中必须认识和克服的问题.基于Forkker-Planck方程,研究了激光与材料相互作用时的雪崩电离机制、多光子电离机制以及联合两种机制的情况.雪崩电离的产生需要一定密度的初始自由电子存在,该自由电子可以是材料中原本就存在的,也可能是光电离产生的.着重分析了材料中的初始自由电子对材料电离机制的影响.结果表明,雪崩过程在激光作用一段时间后会达到一个稳定的电离阶段（以自由电子平均能量不随时间变化为特征,且此时雪崩电离为材料电离的主导机制）,该时间与光电离速率、材料中初始自由电子密度有关.材料中的初始自由电子可以在一定程度上掩盖光电离的作用效果.
关键词:初始电子;激光损伤;光电离;雪崩电离     

Mass Spectrometry,Review of the Basics: Ionization

Abstract: Mass spectrometry (MS) has become an integral tool in life sciences. The first step in MS analysis is ion formation (ionization). Many ionization methods currently exist; electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) are the most commonly used. ESI relies on the formation of charged droplets releasing ions from the surface (ion evaporation model) or via complete solvent evaporation (charge residual model). MALDI ionization, however, is facilitated via laser energy and the use of a matrix. Despite wide use, ESI cannot efficiently ionize nonpolar compounds. Atmospheric pressure chemical ionization (APCI) and atmospheric pressure photo ionization (APPI) are better suited for such tasks. APPI requires photon energy and a dopant, whereas APCI is similar to chemical ionization. In 2004, ambient MS was introduced in which ionization occurs at the sample in its native form. Desorption electrospray ionization (DESI) and direct analysis in real time (DART) are the most widely used methods. In this mini-review, we provide an overview of the main ionization methods and the mechanisms of ion formation. This article is educational and intended for students/researchers who are not very familiar with MS and would like to learn the basics; it is not for MS experts.     

初始电子密度对光学薄膜激光损伤机制的影响

光学元件的激光损伤问题是激光器件向高功率密度方向发展中必须认识和克服的问题.基于Forkker-Planck方程,研究了激光与材料相互作用时的雪崩电离机制、多光子电离机制以及联合两种机制的情况.雪崩电离的产生需要一定密度的初始自由电子存在,该自由电子可以是材料中原本就存在的,也可能是光电离产生的.着重分析了材料中的初始自由电子对材料电离机制的影响.结果表明,雪崩过程在激光作用一段时间后会达到一个稳定的电离阶段(以自由电子平均能量不随时间变化为特征,且此时雪崩电离为材料电离的主导机制),该时间与光电离速率、材料中初始自由电子密度有关.材料中的初始自由电子可以在一定程度上掩盖光电离的作用效果.     

电场中Eu原子电离阈移动的实验研究

提出了一种新方法, 精确确定了稀土Eu原子第一电离阈的位置. 先用脉冲电场对Eu原子的高激发Rydberg态进行延时场电离探测, 再通过反向静电场排除光电离和自电离等其他路径所产生的离子信号的干扰, 观察并研究了Eu原子第一电离阈随着电场移动的规律. 由此所确定的零场下第一电离阈的数值与采用其他方法所确定的文献值^{[1, 2]}相一致, 从而验证了该方法的可靠性.     

On field adsorption,ionization rates and gas supply

The field ionization of a helium-neon mixture is analyzed to gain information about the imaging process in a field ion microscope. Quantitative measurements are performed on ionization rates, the number of jumps a helium atom performs before ionization and the fraction of helium atoms that escape ionization.     

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为了准确计算稠密等离子体电离平衡,在理想Saha方程的基础上,加入了压致电离理论和德拜理论相结合的修正。此电离模型可以描述稠密等离子体内电离机制随着密度的增加由热电离转变为压致电离的现象。压致电离理论结合德拜理论修正的 Saha 方程的计算结果与其它电离模型结果作了对比和分析。电离气体区域采用完全电离等离子体模型计算电子弛豫时间,而在凝聚态区域采用电子平均自由程计算电子弛豫时间。提出了将两个区域平滑连接起来的公式,从而建立了一种可以计算从理想等离子体态到凝聚态物质的半经验电导率模型,半经验模型计算结果与实验数据作了对比。     

Single and double ionization of neon 2s- and 2p-subshells by proton and electron impact

Absolute single and double ionization cross sections of neon 2s- and 2p-subshells for proton (40–900 keV) and electron impact (0.2–10 keV) have been measured using photon spectroscopy in the spectral range of the vacuum ultraviolet. Cross sections for double ionization decrease more rapidly with increasing impact energy than cross sections for single ionization. No definite asymptotic energy dependence of a Bethe-Fano-plot could be found for double ionization in contrast to single ionization. The experimental results are compared with theoretical predictions of the shake-off model and Gryzinski's classical binary encounter theory. Better agreement is found with the latter, indicating that successive binary collisions have to be considered as a strong mechanism for double ionization by protons or electrons of the investigated energy range. Comparison is made with other experimental results for double ionization by photon impact or capture ionization by proton impact.