Theoretical study on non-sequential double ionization of carbon disulfide with different bond lengths in linearly polarizedlaser fields

By using a two-dimensional Monte-Carlo classical ensemble method, we investigate the double ionization(DI) process of the CS_2 molecule with different bond lengths in an 800-nm intense laser field. The double ionization probability presents a "knee" structure with equilibrium internuclear distance R = 2.9245 a.u.(a.u. is short for atomic unit). As the bond length of CS increases, the DI probability is enhanced and the "knee" structure becomes less obvious. In addition,the momentum distribution of double ionized electrons is also investigated, which shows the momentum mostly distributed in the first and third quadrants with equilibrium internuclear distance R = 2.9245 a.u. As the bond length of CS increases,the electron momentum becomes evenly distributed in the four quadrants. Furthermore, the energy distributions and the corresponding trajectories of the double-ionized electrons versus time are also demonstrated, which show that the bond length of CS in the CS_2 molecule plays a key role in the DI process.     

Single and double ionization of the hydrogen molecule in an intense few-cycle laser pulse

In this paper, we present ab initio two-electron model calculations of laser-induced single and double ionization of the hydrogen molecule in a linearly polarized laser field with static nuclei located along the polarization axis. Within the model, the center-of-mass motion of the two electrons is restricted along the polarization axis of the field, while the relative electron motion is unrestricted. The results of numerical simulations allow us to identify and characterize the mechanisms leading to single and double ionization in an intense few-cycle laser pulse. The role of the rescattering mechanism on the ionization processes is analyzed in particular.     

强激光场下原子超快动力学过程中的能量交换

利用三维经典系综模型,研究了整个系综两电子(Ar原子为例)从激光场吸收的能量对激光参数(波长、激光强度和椭偏率)的依赖关系.结果显示,当激光强度固定,波长增加时,整个系综两电子从激光场吸收的能量整体呈上升趋势,但不同强度下趋势略有差异.在较低强度时整个系综两电子从激光场吸收的能量对波长的依赖关系呈现持续平稳增加的趋势,在较高强度时呈现先缓慢减小再快速增大的趋势.对强度的依赖关系在不同波长时呈现两个有趣的交叉点.对椭偏率的依赖关系在较低强度时呈现先逐渐减小再缓慢增大的趋势;在中等强度时呈现一个“阶梯型”即先缓慢增大再逐渐减小最后缓慢增大;在更高强度时呈现先逐渐增大再逐渐减小的趋势.为了解释整个系综两电子从激光场吸收的能量对激光参数的依赖关系,把整个系综的动力学过程分为双电离、单电离、受挫单电离和受挫双电离4种通道.然后分析各个通道的特征及其如何主导整个系综两电子从激光场吸收的能量的变化趋势.分析结果表明,整个系综两电子从激光场吸收的能量对波长、激光强度和椭偏率的依赖均是由于某种通道主导整个系综两电子从激光场吸收的能量的结果.     

Influence of Coulomb force between two electrons on double ionization of He-like atoms

In strong-field double ionization,two electrons are ionized by intense laser field.These two electrons move in the laser field and the state is described by a Coulomb-Volkov state,where the repulsive Coulomb state describes the relative motion of the two electrons and the Volkov state describes the center-of-mass motion of the two electrons in the laser field.In the frame of scattering theory,we derive a simple analytical formula of the double ionization of He-like atoms.The effect of the Coulomb force between two electrons on the double ionization process is discussed.Numerical studies disclose that the Coulomb force enhances the ionization rate of high-energy electrons but suppresses the ionization rate of the lowest-energy electrons.     

Multiorbital tunneling ionization of the CO molecule

We coincidently measure the molecular-frame photoelectron angular distribution and the ion sum-momentum distribution of single and double ionization of CO molecules by using circularly and elliptically polarized femtosecond laser pulses, respectively. The orientation dependent ionization rates for various kinetic energy releases allow us to individually identify the ionizations of multiple orbitals, ranging from the highest occupied to the next two lower-lying molecular orbitals for various channels observed in our experiments. Not only the emission of a single electron, but also the sequential tunneling dynamics of two electrons from multiple orbitals are traced step by step. Our results confirm that the shape of the ionizing orbitals determine the strong laser field tunneling ionization in the CO molecule, whereas the linear Stark effect plays a minor role.     

Laser-induced recollision phenomena: interference resonances at channel closings

An analytical theory of the resonancelike phenomena in high-order above-threshold ionization is presented that explains details of the experimental spectra and theoretical simulations. It traces the observed features to the constructive interference of "quantum orbits" with long travel times at laser intensities where the N-photon ionization channels close. Characteristic differences show up between even and odd N. The effects are generic to all laser-induced recollision phenomena. For nonsequential double ionization, their signature in the momentum distribution of the final electrons is identified.     

Dynamics of strong-field double ionization of H_2 in co-rotating two-color circularly polarized laser fields

Double ionization of H_2 in a co-rotating two-color circularly polarized(TCCP) laser field is theoretically investigated. By changing the ratio of electric field peak amplitudes of the TCCP laser pulses, the double ionization probability as a function of the laser intensity shows a clear knee structure, which is suppressed significantly in the case of the atom. Due to the large spatial range of the electronic initial distribution, with the analysis of classical trajectories of ionized electrons, it is found that the ionization of the electron in the farther distance increases the probability of recollision. Furthermore, the yield of nonsequential double ionization created by the recollision can be enhanced by controlling the amplitude ratio of the TCCP laser field.     

Effect of electron excitation in nonsequential double ionization by intense laser fields

We study the double ionization process of atoms in intense laser fields. The momentum distributions of the correlated electrons are calculated. Contrary to the general expectation, we show an increasing proportion of the electrons ionized via excitation with the increasing laser intensity. These electrons generally have small energy thus they concentratedly distribute on the central region of the momentum diagram. Consequently, the central part of the momentum diagram becomes more notable in higher intensity laser fields. Further study suggests that this phenomenon is general in double ionization.     

Controlling attosecond double ionization dynamics via molecular alignment

We investigate the dynamics of double ionization in aligned nitrogen molecules. An ultrashort, weak laser pulse creates an aligned ensemble of molecules that is ionized with a subsequent, strong probe pulse. We find that the two electrons involved in nonsequential double ionization more likely exit the molecule in the same direction if it is parallel to the probe laser polarization, indicating that they are ejected within a few hundred attoseconds of each other. Double ionization is less probable and takes longer for perpendicular molecules.     

Ellipticity dependence of high-order above-threshold ionization from aligned diatomic molecules

We investigate high-order above-threshold ionization of the Ar₂ molecule by an elliptically polarized laser field. In order to describe this process we use the modified molecular strong-field approximation. We have found that the differential ionization rate is a very sensitive function of the ellipticity of the laser field and of the molecular orientation. For parallel orientation the rate has the maximum value for linear polarization, independently of the laser intensity. On the other hand, for perpendicular orientation the rate is maximal for a particular value of the ellipticity which is different from zero. Such a behavior can be related to the molecular symmetry. We consider the differential ionization rate of high-energy electrons as a function of the ellipticity for different internuclear distances. We also present, in the electron energy-emission angle plane, our new results for the focal-averaged spectra of electrons ionized by an elliptically polarized laser field. We have found that, in this case, the two-source two-rescattering-centers interference minima are blurred.     

高温气体中的激光击穿效应

 通过对电子能量分布函数所满足的Boltzmann方程的求解,得到了高温氧原子气体中激光产生电离击穿效应时的束流强度阈值。这一阈值远小于在低温氧分子气体中产生电离击穿的阈值,最多可相差两个数量级。导致这一现象出现的主要原因是原子和分子的能级分布和激发自由度有很大的差别。     

Mechanism of delayed double ionization in a strong laser field

When intense laser pulses release electrons nonsequentially, the time delay between the last recollision and the subsequent ionization may last longer than what is expected from a direct impact scenario [recollision excitation with subsequent ionization (RESI)]. We show that the resulting delayed ionization stems from the inner electron being promoted to a sticky region. We identify the mechanism that traps and releases the electron from this region. As a signature of this mechanism, we predict oscillations in the ratio of RESI to double ionization yields versus laser intensity.     

分子对飞秒激光场响应的含时密度泛函理论研究

采用含时密度泛函理论方法研究线性分子碳化锂(Li2C2)对飞秒激光场响应的电子-离子动力学行为.在典型的近共振和非共振的激光频率作用下,分别对比分析了分子的共振和非共振电离过程.研究发现:分子在共振频率激光场的作用下发生更强的电离过程,并倾向于发生库伦爆炸,键长的振荡断裂与电离相互促进影响,而分子在较弱的激光场作用下发生单光子电离过程;随着双脉冲时间间隔的增加,离化电子数在一定范围内呈振荡上升趋势,随后趋于常数.     

高频强激光场中H-电离特性理论研究

高频强激光场中高价负离子（比如H-,H2-,He-,He2-等等）的稳定存在虽然在理论上早已得到得到证明,但在实验上至今没有被直接验证。为了探索可行的实验验证方案,通过以高频强场中的负一价氢离子H-为例,采用相空间平均法,详细研究了其在外加激光场中的电离率及光电子角分布情况。研究发现 的电离率比氢原子高,并且不存在氢原子在较强激光场中的电离抑制现象。另外随着激光场强度的增大和激光频率的减小,电子角分布与H-在高频场直接卸载后电子角分布的差别也增大。这些结果为将来在实验上验证高频强场中高价负离子的存在性提供了很好的理论依据。     

Binary and recoil collisions in strong field double ionization of helium

We have investigated the correlated momentum distribution of both electrons from nonsequential double ionization of helium in a 800 nm, 4.5 x 10(14 W/cm2 laser field. Using very high resolution coincidence techniques, we find a so-far unobserved fingerlike structure in the correlated electron momentum distribution. The structure can be interpreted as a signature of the microscopic dynamics in the recollision process. We identify features corresponding to the binary and recoil lobe in field-free (e,2e) collisions. This interpretation is supported by analyzing ab initio solutions of a fully correlated three-dimensional helium model.     

Few-photon multiple ionization of ne and ar by strong free-electron-laser pulses

Few-photon multiple ionization of Ne and Ar atoms by strong vacuum ultraviolet laser pulses from the free-electron laser at Hamburg was investigated differentially with the Heidelberg reaction microscope. The light-intensity dependence of Ne2+ production reveals the dominance of nonsequential two-photon double ionization at intensities of I<6x10(12) W/cm2 and significant contributions of three-photon ionization as I increases. Ne2+ recoil-ion-momentum distributions suggest that two electrons absorbing "instantaneously" two photons are ejected most likely into opposite hemispheres with similar energies.     

分子对飞秒激光场响应的含时密度泛函理论研究

采用含时密度泛函理论方法研究线性分子碳化锂（Li2C2）对飞秒激光场响应的电子-离子动力学行为。在典型的近共振和非共振的激光频率作用下,分别对比分析了分子的共振和非共振电离过程。研究发现：分子在共振频率激光场的作用下发生更强的电离过程,并倾向于发生库伦爆炸,键长的振荡断裂与电离相互促进影响,而分子在较弱的激光场作用下发生单光子电离过程;随着双脉冲时间间隔的增加,离化电子数在一定范围内呈振荡上升趋势,随后趋于常数。     

Carrier-envelope-phase effect in nonsequential double ionization of Ar atoms by few-cycle laser pulses

A classical ensemble method is used to investigate nonsequential double ionization(NSDI) of Ar atoms irradiated by linearly polarized few-cycle laser pulses. The correlated-electron momentum distribution(CMD) exhibits a strong dependence on the carrier-envelope phase(CEP). When the pulse duration is four cycles, the CMD shows a cross-like structure, which is consistent with experimental results. The CEP dependence is more notable when the laser pulse duration is decreased to two cycles and a special L-shaped structure appears in CMD. Recollision time of returning electrons greatly depends on CEP, which plays a significant role in accounting for the appearance of this structure.     

Bifurcation of ion momentum distributions in sequential double ionization by elliptically polarized laser pulses

Using a fully classical model, we have studied sequential double ionization (SDI) of argon driven by elliptically polarized laser pulses at intensities well in the over-barrier ionization region. The results show that the width of the ion momentum along the major elliptical axis increases with the ellipticity while that in the laser propagation direction decreases with the ellipticity. The ion momentum in the minor elliptical axis bifurcates from one peak structure to three-peak, then to four-peak, finally to six-peak structure. Analysis shows that this ellipticity dependence of the ion momentum distribution is a result of the subcycle nature of the ionization dynamics in SDI. By changing the ellipticity and wavelength of the driving pulses, this subcycle ionization dynamics is more observable. This provides a simple and efficient way to study the subcycle ionization dynamics in strong field processes.     

The Study of “Coulomb Asymmetry” in an Elliptically Polarized Laser Field

Due to the effect of Coulomb potential, the angular distribution of electron ionized in an elliptically polarized field presents an asymmetric structure, which is called "Coulomb asymmetry". In this paper, we study how to modulate the asymmetric degree of the electron angular distribution by using a semi-classical simplified tunneling model. It is found that the asymmetric structure is easily affected by three parameters:the ionization potential, the laser ellipticity, and the laser wavelength. However, the laser intensity has little effect on the asymmetric structure. To explain these phenomena we have derived an analytical formula, which clearly demonstrates the relationship between the asymmetric degree and these parameters. Moreover, we find that in elliptically polarized laser field only those electrons that are released in a certain narrow window of initial field phase are greatly effected by the Coulomb potential and play the key role in the formation of asymmetric structure. This study provides some reference values in the development of attoclock technique, which can be used to probe the tunneling process.