Time-resolved quantum dynamics of double ionization in strong laser fields

Quantum calculations of a (1+1)-dimensional model for double ionization in strong laser fields are used to trace the time evolution from the ground state through ionization and rescattering to the two-electron escape. The subspace of symmetric escape, a prime characteristic of nonsequential double ionization, remains accessible by a judicious choice of 1D coordinates for the electrons. The time-resolved ionization fluxes show the onset of single and double ionization, the sequence of events during the pulse, and the influences of pulse duration and reveal the relative importance of sequential and nonsequential double ionization, even when ionization takes place during the same field cycle.     

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     

Electron ionization cross sections for neutral fullerene C70

The energy dependent ionization cross sections corresponding to the formation of singly, doubly and triply charged cations in the electron impact ionization of C₇₀ are evaluated employing a semiempirical formulation at the incident electron energies of 100 and 200 eV. The integral ionization cross sections have also been calculated in the energy range varying from ionization thresholds to 1000 eV which revealed a satisfactory agreement with the available experimental and the theoretical data. The ionization rate coefficients corresponding to the various cations have also been evaluated using the presently calculated ionization cross sections and Maxwell-Boltzmann energy distributions.     

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…     

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.     

Electron-impact ionization cross-sections and ionization rate coefficients for atoms and ions from scandium to zinc

Using the empirical formula recently proposed, electron-impact ionization cross-sections for single ionization from the ground state are given for free atoms and for nearly all ionization stages from scandium (Z=21) to zinc (Z=30). For these species ionization rate coefficients are given under the assumption of a Maxwellian distribution of the impacting electrons. Multiple ionization, lowering of ionization potential, or collision limit are not taken into account.     

分子核间距对非时序双电离的影响

利用半经典再散射模型研究了激光强度在一定范围内时分子核间距对非时序双电离过程的影响.分别计算了非时序双电离率、两电子的电离能、两电子的动量相关、Coulomb和激光场的复合势随分子核间距的变化关系.研究表明,分子核间距在1.0—6.0 a.u.范围内时,非时序双电离率和两电子动量和为零的双电离事件数随着分子核间距的增大而增大.当分子核间距继续增大(大于6.0 a.u.)时,非时序双电离率和两电子动量和为零的双电离事件数却减小.     

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.     

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.     

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.     

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     

Comparison of the laser desorption/ionization methods for detecting metal complexes

The results of comparative analysis of laser desorption/ionization, matrix-assisted laser desorption/ ionization, and new laser-induced electron transfer desorption/ionization methods, used to detect chlorophyll A; mercury complex with thiuram; platinum complex with mercaptoquinoline; and lutetium complex with phthalocyanine, modified by crown ether, are presented. The new method is found to have a better ionization efficiency for complex compounds than the conventional laser desorption/ionization methods.     

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)     

Electron-impact ionization cross-sections and ionization rate coefficients for atoms and ions from hydrogen to calcium

Using the empirical formula recently proposed, electron-impact ionization cross-sections for single ionization from the ground state are given for free atoms and for all ionization stages from hydrogen to calcium (Z=20). Ionization rate coefficients are given for these species on the assumption of a Maxwellian distribution of the impacting electrons. Multiple ionization, lowering of ionization potential, or collision limit are not taken into account.     

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.     

裸核离子与中性原子碰撞电离过程的理论研究

基于处理裸核离子与中性原子碰撞电离过程的OBKN和ECPSSR理论模型,系统计算了不同裸核离子与中性原子碰撞K壳层电子俘获截面和直接电离截面,并与其它文献已有的理论和实验结果进行了比较.研究结果表明:碰撞能量较低时,电子俘获截面大于直接电离截面,随着碰撞能量的增加,电子俘获截面和直接电离截面均是先增大后减小且直接电离截面减小地非常缓慢,高能时,直接电离截面大于电子俘获截面.当入射炮弹离子速度接近0.67倍靶原子K壳层电子速度时,电子俘获截面达到最大值,而当入射炮弹离子速度接近靶原子K壳层电子速度时,直接电离截面达到最大值.     

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.     

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。     

Ratiometric comparison of intense field ionization of atoms and diatomic molecules

Intense-laser ionization rates for rare gas atoms and diatomic molecules have been precisely compared by making simultaneous measurements of ionization yield vs laser intensity for mixed atomic and molecular targets. At a given laser intensity, the N (2) and F (2) ionization yields are slightly greater than that of Ar. Conversely, comparison of O (2) and S (2) with Xe indicates significant ionization suppression in these molecules. Recent molecular ionization models that successfully describe ionization suppression in O (2) and its absence in N (2) fail to explain our observations in F (2) and S (2).