Understanding Tunneling Ionization of Atoms in Laser Fields using the Principle of Multiphoton Absorption

The elaborate energy and momentum spectra of ionized electrons from atoms in laser fields suggest that the ionization dynamics described by tunneling theory should be modified. Although great efforts have been carried out within semiclassical models, there are few discussions describing the multiphoton absorption process within a quantum framework. Comparing the results obtained with the time-dependent Schr?dinger equation(TDSE)and the Keldysh–Faisal–Reiss(KFR) theory, we study the nonperturbative effects of ionization dynamics beyond the KFR theory. The difference in momentum spectra between multiphoton and tunneling regimes is understood in a unified picture with virtual multiphoton absorption processes. For the multiphoton regime, the momentum spectra can be obtained by coherent interference of each periodic contribution. However, the interference of multiphoton absorption peaks will result in a complex structure of virtual multiphoton bands in the tunneling regime. It is shown that the virtual spectra will be almost continuous in the tunneling regime instead of the discrete levels found in the multiphoton regime. Finally, with a model combining the TDSE and the KFR theory,we try to understand the different effects of virtual multiphoton processes on ionization dynamics.     

Multiphoton Ionization of Potassium Atoms in Femtosecond Laser Fields

We study the multiphoton ionization of potassium atoms in 800 nm and 400 nm femtosecond laser fields.In the 800 nm laser field,the potassium atom absorbs three photons and emits one electron via one photon resonance with the 4p intermediate state with the help of the ac-Stark shift.The resonance feature is clearly shown as an Autler-Townes(AT) splitting and is mapped out in the electron kinetic energy spectrum.In a 400 nm laser field,although one photon resonance is possible with the 5p state,no splitting is observed.The different transition amplitudes between 4s-4p and 4s-5p explain the observed results.Due to the AT effect,an unexpected peak in the photoelectron energy spectrum that violates the dipole transition rule is observed.A preliminary explanation involving the spin-orbit interaction in the p state is given to account for this component.The observed ATsplitting in the electron kinetic energy distribution can be used as an effective method to calibrate the intensity of a laser field.     

Ionization of Clusters in Intense Laser Pulses through Collective Electron Dynamics

The motion of electrons and ions in medium-sized rare gas clusters ( approximately 1000 atoms) exposed to intense laser pulses is studied microscopically by means of classical molecular dynamics using a hierarchical tree code. Pulse parameters for optimum ionization are found to be wavelength dependent. This resonant behavior is traced back to a collective electron oscillation inside the charged cluster. It is shown that this dynamics can be well described by a driven and damped harmonic oscillator allowing for a clear discrimination against other energy absorption mechanisms.     

Double Ionization Dynamics of Molecular Hydrogen in Ultrashort Intense Laser Fields

We experimentally investigate the double ionization of molecular hydrogen subjected to ultrashort intense laser pulses.The total kinetic energy release of the two coincident H~+ ions,which provides a diagnosis of different processes to double ionization of H_2,is measured for two different pulse durations,i.e.,25 and 5fs,and various laser intensities.It is found that,for the long pulse duration(i.e.,25 fs),the double ionization occurs mainly via two processes,i.e.,the charge resonance enhanced ionization and recollision-induced double ionization.Moreover,the contributions from these two processes can be significantly modulated by changing the laser intensity.In contrast,for a few-cycle pulse of 5fs,only the recollsion-induced double ionization survives,and in particular,this process could be solely induced by the first-return recollision at appropriate laser intensities,providing an efficient way to probe the sub-laser-cycle molecular dynamics.     

强光场中原子的电离速率

以静态场隧道电离为基础,在准静态近似的条件下,对描述强光场中原子及其各阶离子电离的电离速率进行了较为系统的总结,纠正了早期对隧道电离的电离速率公式进行讨论并且后来的计算中被广泛引用的文献「１０」中公式的错误,地部分稀有气体原子及其各了子在线偏振、圆偏振强光场中的了速度及相应的阈值光强进行了比较。     

纳秒强光场下呋喃的激光电离中高价离子的产生

利用 2 5ns脉冲Nd :YAG 5 32nm的激光 ,在 10 11W/cm2 的光场强度下 ,用飞行时间质谱对不同条件下呋喃的激光电离过程进行了研究 .当利用氩作为载气时 ,首次观察到了较强的Cn + (n =2～ 4 )和Om + (m =2～ 3)离子信号 .通过测定峰分裂和数值模拟计算的方法可以确定其中C2 + 、C3 + 和C4+ 的最可几平动能分别为 2 1、6 3和 10 0eV ,O2 + 和O3 + 的最可几平动能分别为 2 0和 4 0eV .质谱峰型的分裂现象和激光延迟实验结果表明 ,这些高价离子可能来源于呋喃团簇的库仑爆炸过程 .     

二能级三维氢原子模型中强激光场的谐波发射

在强激光场中三维二能级氢原子模型的基础上, 对其含时Schr\"{o}dinger方程进行了非微扰的数值求解, 得到了激光场的高次谐波谱.     

Above-Threshold Ionization of Xenon by Chirped Intense Laser Pulses

We experimentally investigate above-threshold ionization of xenon subject to chirped intense laser pulses. While for few-cycle laser pulses free of chirp, the Freeman resonance narrow peaks in the photoelectron spectra become strongly suppressed, they will re-emerge for laser pulses with a certain chirp. Moreover, the position of the resonant peaks exhibits a strong dependence on the direction of the chirp. The experimental features can be understood by a two-step ionization process; that is, a multi-photon excitation plus subsequent ionization process.     

纳秒强激光中乙腈团簇增强的高价电离研究

利用脉宽为25 ns的脉冲Nd∶YAG 1 064 nm的激光,在1010～1011 W·cm-2的强度下,用飞行时间质谱对乙腈分子束激光电离过程进行了研究。实验中利 用氦气作为载气,乙腈分子束采用扩散和脉冲两种方式进样,当扩散束进样时,乙腈分子的谱峰可用多光子电离来解释,而当脉冲分子束进样时,实验中产生了外层价电子高剥离的Nq+ (q=2～5) 和Cq+(q=1～4)信号谱。实验中同时发现,激光波长增大时,高价离子的谱强度也趋向于增大。激光延迟及谱峰的相关性分析表明,这些高价离子可能来源于乙腈团簇的库仑爆炸 过程。提出一个多光子电离引发,逆韧致吸收加热-电子碰撞电离模型来解释高价离子的产生,对高价离子伴随的波长效应也给出了较为合理的解释。     

Relativistic Derivation of the Ponderomotive Force Produced by Two Intense Laser Fields

The ponderomotive force plays a fundamental role in the absorption of laser light on self-consistent plasma density profiles, in multiple-photon ionization, and in intense field electrodynamics. The relativistic corrections to the ponderomotive force of a transversely polarized electromagnetic wave lead to an approximately 20-percent reduction in the single particle ponderomotive force produced by a 10-?m 1016-W/cm2 laser field. Recent experimental investigations are based on using two intense laser fields to produce desired laser-matter interactions. This paper presents the first derivation of the nonlinear relativistic ponderomotive force produced by two intense laser fields. The results demonstrate that relativistic ponderomotive forces are not additive.     

强激光中原子过程的理论研究

从理论角度结合实际工作,概要地介绍强激光中原子过程的特点、理论研究现状及其在激光核聚变中的可能作用． A most important feature of atomic processes in laser fusion is pointed out. The current situation of theoretical study of these processes is analyzed, then. The possible application of the theoretical study is described by a illustrative example.All of the above mentioned things are based on our practical work.     

Nondipole Effects in Atomic Ionization Induced by an Intense Counter-Rotating Bicircular Laser Field

Nondipole effects occurring in the process of atomic ionization by an intense, mid-infrared, counter-rotating bicircular laser field are investigated using the strong-field approximation with leading-order nondipole corrections. The time integrals appearing in the expression for the differential ionization rate are computed in two ways: numerically, and by applying the saddle-point approximation. The nondipole corrections introduce an asymmetry in the photoelectron momentum distribution along the field propagation direction. The asymmetry is quantified by the partial average value of the propagation-direction momentum component of the photoelectrons and by the normalized difference of the differential ionization rates computed including and excluding the nondipole corrections. Using the saddle-point approximation, it is investigated how the nondipole corrections change the solutions for direct photoelectrons and how this affects the momentum spectra. The impact of nondipole corrections increases with increasing photoelectron energy. Analysis of the complete photoelectron spectra including both direct and rescattered photoelectrons shows that, in the low-energy region, a shift against the propagation direction occurs. The partial average of the propagation–direction momentum component in the rescattering region exhibits a plateau structure and also a local minimum structure that was recently observed in an experiment with a linearly polarized laser field (Lin et al., Phys Rev. Lett. 128, 023201 (2022)).     

Neutral Dissociation of Superexcited Nitric Oxide Induced by Intense Laser Fields

利用波长800 nm、脉宽60 fs、强度0.2 PW/cm²的强激光激发NO分子,随后通过荧光光谱检测到了处于激发态的N和O的中性碎片. 说明强激光导致的多光子激发同样可以引起NO分子的超激发,并且发生中性解离. 采用离子实解离模型,建立了NO分子的超激发态势能曲线. 提出了直接解离和预解离两种解离过程,分别对应生成N^*+O或O^*+N两个通道.     

Simulation of Electron Temperature in Argon Clusters Subjected to Intense Laser Fields

Argon clusters subjected to intense femtosecond laser pulses are ionized by the optical fields and inelastic electron-ion collisions. The cluster plasmas absorb the laser energy through collisional inverse bremsstrahlung, leading the argon cluster plasmas to very hot states. The calculated electron temperature in the clusters indicates that the intense laser-cluster interactions are more energetic than interactions with molecules.     

Simulation of Electron Temperature in Argon Clusters Subjected to Intense Laser Fields

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Resonant Acceleration of Electrons in Combined Self-Consistent Quasistatic Electromagnetic Fields and Intense Laser Fields

Using the single electron model, the acceleration of electrons in combined circularly polarized intense laser fields and the spontaneous quasistatic fields （including axial and azimuthal magnetic fields, the axial and transverse electric fields） produced in intense laser plasma interaction is investigated analytically and numerically by fitting the proper parameters of the quasistatic fields based on the data from the experiment and numerical calculation. A new resonant condition is given. It is found that the resonance acceleration of electron depends not only on laser field, but also on the bounce frequency oscillating in the quasistatic magnetic field and electric field. The net energy gained by electron does not increase monotonously with axial electric field, but there are some optimal axial electric fields.     

Ionization Problem for Hydrogen Atom in Intense Circularly Polarized Laser Field Revisited

Ionization rate for circular polarization laser field is reformulated in length gauge. The Volkov state or the Coulomb-correctsd Volkov one is served at the final wavefunction and would lead to that the ionization rate in length gauge is closer to the experimental data than the corresponding result in velocity gauge. It is also found that the ionization rate as a function of laser intensity has nevertheless the stabilization effect, although in the sense more weakly in length gauge than in velocity gauge, when the field is to be comparable or exceeds to the atomic binding field.     

Identification of Above-Threshold Ionization by Imaging Photoelectrons from Ammonia Molecules in an Intense Femtosecond Laser Field

The above-threshold ionization process of ammonia molecules induced by a femtosecond laser field at 800 nm is studied in the intensity range from 1.6 × 10¹³ to 5.7 × 10¹³ W/cm². Channel switching under different laser intensities is observed and identified in the photoelectron kinetic energy spectra of ammonia. Based on the photoelectron kinetic energy distributions and the photoelectron angular distributions, the characteristic peaks observed are exclusively ass...     

On the Contribution of the Long-Range Scattering Potential in the Process of Multiphoton Above-Threshold Ionization of Atoms in the Laser Field

Journal of Experimental and Theoretical Physics - Using the quantum mechanical analysis of stimulated bremsstrahlung of an electron on an electrostatic potential in the field of a strong...