Angular distributions for double ionization of Li- by an ultrashort, intense laser pulse.

We predict photoelectron angular distributions for double ionization of Li- by both weak and intense ultrashort, linearly polarized laser pulses by direct numerical integration of the three-dimensional, time-dependent Schr?dinger equation. Li- is treated as a two-active electron system. Near threshold, for low intensity we recover general features of angular distributions for one-photon double ionization. For the intense field (multiphoton) case, the photoelectron angular distribution changes significantly, particularly in directions parallel and perpendicular to the laser polarization axis.     

Multiphoton double ionization of barium

The multiphoton double ionization of Ba from ~280 to 700 nm was investigated using laser pulses 5 ns long of peak intensity ~10¹⁰ W/cm². The spectrum consists of a number of strong resonances, which can be assigned to Ba⁺ transitions. Most of the assignments have been verified by pump-probe techniques. Thus, the Ba⁺⁺ observed is due to sequential ionization. The multiphoton ionization probability is highest for λ~500 nm, which matches a series of strong Ba and Ba⁺ transitions leading to double ionization     

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.     

Angular distribution of ejected electrons at the laser-cluster interactions

The histograms of deflection angles of electrons ejected from Xe clusters irradiated by femtosecond super-intense laser pulses are presented. The dependence of the angular distribution on the peak laser intensity, the pulse duration, and the cluster position is considered. A clear relationship between the final electron energy and the deflection angle is shown. The deflection angles are calculated by solving the relativistic equation of motion taking into account the Lorentz force and the Coulomb field of the ionized cluster. The ions in the cluster undergo sequential multiple ionization up to charge multiplicity Z = 26. The measurements of the electron angular distributions allow us to reproduce the imaging dynamics of outer ionization of the cluster at the leading edge of the relativistic femtosecond laser pulse.     

非序列双电离向饱和区过渡的电子最大关联度

本文采用三维半经典再散射模型系统地研究了 He 原子在 光强从域下到近序列双电离区强场作用下的非序列双电离问题, 统计了各种场强下最终电离的两个电子正负关联的几率之比, 发现发生最大正关联的场强处于向非序列 双电离饱和区过渡的区域, 并在关联纵向动量谱中展示了形成最大关联度的过程. 最后通过轨道"回溯"分析, 进一步研究了其背后的物理机理, 发现在碰撞后其中一个电子再散射和隧穿电子高速弱碰撞分别 是低场强和高场强下负关联比例增加的原因.     

Probing electron-electron correlation with attosecond pulses

We study two-photon double ionization of helium in its ground state at sufficiently low laser intensities so that three and more photon absorptions are negligible. In the regime where sequential ionization dominates, the two-photon double ionization one-electron energy spectrum exhibits a well defined double peak structure directly related to the electron-electron correlation in the ground state. We demonstrate that when helium is exposed to subfemtosecond or attosecond pulses, both peaks move and their displacement is a signature of the time needed by the He⁺ orbital to relax after the ejection of the first electron. This result rests on the numerical solution of the corresponding non-relativistic time-dependent Schrödinger equation.Received: 17 January 2003, Published online: 18 March 2003PACS: 32.80.Rm Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) - 32.80.Dz Autoionization     

Intense-field double ionization of helium: identifying the mechanism

We present quantum mechanical calculations of the electron and ion momentum distributions following double ionization of a one-dimensional helium atom by ultrashort laser pulses (780 nm) at various intensities. The two-electron momentum distributions exhibit a clear transition from nonsequential to sequential double ionization. We provide strong evidence that rescattering is responsible for nonsequential ionization by calculating the momentum spectrum of the He2+ recoil ions-which we find in excellent agreement with recent experiments-and by analyzing the electronic center-of-mass motion via Wigner transforms.     

Transition from strong-field sequential to nonsequential double ionization at near-infrared wavelengths and low intensities

Within a quantum-mechanical model, we investigate strong-field double ionization of a model helium atom by near-infrared, linearly polarized laser pulses at intensities far below the recollision threshold. The quantum simulations show a clear mechanism change from sequential to nonsequential double ionization (NSDI) as the laser intensity increases. For NSDI, the two-electron correlated momentum distribution exhibits a strong final-state Coulomb repulsion effect for high-energy photoelectrons, but absent for low-energy photoelectrons. This repulsion effect is ascribed to field double ionization from doubly-excited states populated by recollision of the first ionized electron when it returns to the parent ion. Such recollision-induced excited states are absent at ultraviolet wavelengths due to the very low returning kinetic energies, resulting to the absence of final-state repulsion effect in NSDI.     

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.     

飞秒时间分辨实验中相关函数和时间零点的确定

结合飞秒激光在研究分子激发态弛豫动力学中的应用,介绍了几种飞秒时间分辨实验中确定泵浦激光脉冲与探测激光脉冲的相关函数和时间零点的方法.对于波长在可见波段的泵浦和探测激光脉冲,我们可以利用非线性光学的技术手段,探测泵浦光与探测光的和频光的强度随二者间的时间延迟的变化来确定相关函数和时间零点;对于波长在紫外甚至更短的波段的泵浦和探测激光脉冲,由于单脉冲能量比较低,目前还很难利用技术手段来测定泵浦激光与探测激光的相关函数及时间零点,可以利用某些原子气体(如Xe)或某些具有短寿命态的分子作平行实验进行间接测量.     

Subcycle electron emission in sequential double ionization by elliptical laser pulses

Using a classical ensemble method, we have investigated sequential double ionization (SDI) of Ar atoms driven by elliptical laser pulses. The results show that the ion momentum distribution of the Ar atoms depends strongly on the pulse duration. As the pulse duration increases, the ion momentum distribution changes from two bands to four bands and then to six bands and finally to an eight-band structure. Back analysis of double ionization trajectories shows that the variation of the band structure originates from pulse duration dependent multiple ionization bursts of the second electron. Our calculations indicate that the subcycle electron emission in the SDI could be more easily accessed by using elliptical laser pulses with a longer wavelength. Moreover, we show that there is good correspondence between the scaled radial momentum and the ionization time.     

Evidence for rescattering in intense, femtosecond laser interactions with a negative ion

It is now well established that energetic electron emission, nonsequential ionization, and high harmonic generation, produced during the interaction of intense, femtosecond laser pulses with atoms (and atomic positive ions), can be explained by invoking rescattering of the active electron in the laser field, the so-called rescattering mechanism. In contrast for negative ions, the role of rescattering has not been established experimentally. By irradiating F- ions with ultrashort laser pulses, F+ ion yields as a function of intensity for both linearly and circularly polarized light have been measured. We find that, at intensities well below saturation for F+ production by sequential ionization, there is a small but significant enhancement in the yield for the case of linearly polarized light, providing the first clear experimental evidence for the existence of the rescattering mechanism in negative ions.     

Few cycle dynamics of multiphoton double ionization

In intense field ionization, an electron removed from the atomic core oscillates in the combined fields of the laser and the parent ion. This oscillation forces repeated revivals of its spatial correlation with the bound electrons. The total probability of double ionization depends on the number of returns and therefore on the number of optical periods in the laser pulse. We observed the yield of Ne(2+) relative to Ne(+) with 12 fs pulses to be clearly less compared to 50 fs pulses in qualitative agreement with our theoretical model.     

Effects of molecular structure on ion disintegration patterns in ionization of O2 and N2 by short laser pulses

We demonstrate that the structure of the outermost orbitals of oxygen and nitrogen can be observed in the angular distribution of coincident ion pairs generated by the double ionization of these molecules by 8 fs laser pulses. We do this by establishing that these ions emerge from well defined excited electronic states of O2+2 and N2+2 respectively and that they are produced dominantly through a process which involves electron rescattering. The angular distributions of the ions from the two targets are very different, reflecting the different structures of the outermost orbitals of the two molecules.     

Generation of intense sub-20-fs vacuum ultraviolet pulses compressed by material dispersion

We present our latest results on the generation of ultrashort vacuum UV (VUV) pulses by nonresonant four-wave mixing of chirped broadband pulses generated by filamentation of the fundamental of a Ti:sapphire laser with relatively narrowband pulses at the third harmonic. Positive chirp at the broadband idler yields negatively chirped VUV pulses necessary to compensate for material dispersion of a MgF(2) window in the VUV beam path. Pulse energies exceeding 400 nJ are available for time-resolved experiments. Pulse duration is measured by pump-probe ionization of Xe gas, providing the cross correlation between the fifth harmonic and the fundamental.     

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.     

Correlated two-electron momentum spectra for strong-field nonsequential double ionization of He at 800 nm

We report on a kinematically complete experiment on nonsequential double ionization of He by 25 fs 800 nm laser pulses at 1.5 PW/cm;{2}. The suppression of the recollision-induced excitation at this high intensity allows us to address in a clean way direct (e,2e) ionization by the recolliding electron. In contrast with earlier experimental results, but in agreement with various theoretical predictions, the two-electron momentum distributions along the laser polarization axis exhibit a pronounced V-shaped structure, which can be explained by the role of Coulomb repulsion and typical (e,2e) kinematics.     

Ab initio calculation of the double ionization of helium in a few-cycle laser pulse beyond the one-dimensional approximation

We present ab initio computations of the ionization of two-electron atoms by short pulses of intense linearly polarized Ti:sapphire laser radiation beyond the one-dimensional approximation. In the model the electron correlation is included in its full dimensionality, while the center-of-mass motion is restricted along the polarization axis. Our results exhibit a rich double ionization quantum dynamics in the direction transversal to the field polarization, which is neglected in the previous models based on the one-dimensional approximation.     

Photon and positron generation by ultrahigh intensity laser interaction with electron beams

This study investigates the generation of high energy photons and positrons using focused ultrahigh intensity femtosecond laser pulses on a relativistic electron beam with a set of two-dimensional particle-in-cell simulations. We consider circularly and linearly polarized, single and spatially separated double laser pulses. We model both 500 MeV and 1 GeV electron beams. Higher positron production is obtained using circularly polarized laser pulses. Using double pulses, the focusing effect of the ponderomotive force confines the electrons to a small volume, generating additional energetic photons and positrons. The positron spectral distributions are effectively modified by these variations. When the electron beam energy is doubled, the number of positrons increased, while the cutoff energy remained nearly constant.     

Double ionization of helium with classical ensemble simulations

The classical ensemble method is used to study the dynamic process of the 1D helium interacting with intense laser pulses. Probabilities of double-ionization of helium in intense laser fields are calculated by the symplectic method. The wavelength dependence of double ionization in He is investigated. The non-sequential double ionization (NSDI) is observed in classical simulations at the laser wavelength of 532 nm, 780 nm and 1024 nm, respectively, while the sequential double ionization (SDI) is the dominant process at the laser wavelength of 248 nm. The pulse duration dependence on NSDI is also studied and the result is in agreement with the corresponding experimental results qualitatively.