分子的取向和激光强度对非时序双电离的影响

利用半经典再散射模型,分别研究分子的取向平行和垂直于电场方向时反冲离子的动量分布、两电子的动量相关和隧穿电子的电离能分布.计算结果表明,分子的取向和激光强度对非时序双电离过程有重要的影响.     

Nonsequential double ionization of the aligned hydrogen molecule in strong field

This paper studies the nonsequential double ionization(NSDI) process of diatomic molecules aligned parallel and perpendicular to an intense linearly polarized laser field by using a three-dimensional semiclassical model.With this model,it achieves insight into the ion momentum distribution under the combined influence of a two-centre Coulomb potential and an intense laser field,and this result shows the significant influence of molecular alignment on the ratio between double and single ionization rate.Careful investigations show that the NSDI process for different alignment molecules has a close relation to the laser intensity and the different bounding electron distribution has a significant influence on the final ion momentum distribution.     

Fully differential rates for femtosecond multiphoton double ionization of neon

We have investigated the full three-dimensional momentum correlation between the electrons emitted from strong field double ionization of neon when the recollision energy of the first electron is on the order of the ionization potential. The momentum correlation in the direction perpendicular to the laser field depends on the time difference of the two electrons leaving the ion. Our results are consistent with double ionization proceeding through transient double excited states that field ionize.     

Semiclassical dynamics of electron wave packet states with phase vortices

We consider semiclassical higher-order wave packet solutions of the Schr?dinger equation with phase vortices. The vortex line is aligned with the propagation direction, and the wave packet carries a well-defined orbital angular momentum (OAM) variant Planck's over 2pil (l is the vortex strength) along its main linear momentum. The probability current coils around the momentum in such OAM states of electrons. In an electric field, these states evolve like massless particles with spin l. The magnetic-monopole Berry curvature appears in momentum space, which results in a spin-orbit-type interaction and a Berry/Magnus transverse force acting on the wave packet. This brings about the OAM Hall effect. In a magnetic field, there is a Zeeman interaction, which, can lead to more complicated dynamics.     

Vacuum pair-production in a classical electric field and an electromagnetic wave

Using semiclassical WKB-methods, we calculate the rate of electron–positron pair-production from the vacuum in the presence of two external fields, a strong (space- or time-dependent) classical field and a monochromatic electromagnetic wave. We discuss the possible medium effects on the rate in the presence of thermal electrons, bosons, and neutral plasma of electrons and protons at a given temperature and chemical potential. Using our rate formula, we calculate the rate enhancement due to a laser beam, and discuss the possibility that a significant enhancement may appear in a plasma of electrons and protons with self-focusing properties.     

Electron-electron momentum exchange in strong field double ionization

We have investigated the momentum balance between the two electrons from strong field double ionization of argon at 780 nm and 1.9 x 10(14) W/cm(2). Experimental data show that perpendicular to the laser polarization direction the electrons emerge preferentially in opposite directions. Results of model calculations are found to agree well with the data and reveal a dominant role of the Coulomb correlation between the two outgoing electrons in this kinematical geometry. Differences between the experimental observations and the theoretical results for the ion momentum distribution indicate the importance of the further effects during the three-body breakup.     

Interference oscillations in the angular distribution of laser-ionized electrons near ionization threshold

We analyze the two-dimensional momentum distribution of electrons ionized by few-cycle laser pulses in the transition regime from multiphoton absorption to tunneling by solving the time-dependent Schr?dinger equation and by a classical-trajectory Monte-Carlo simulation with tunneling (CTMC-T). We find a complex two-dimensional interference pattern that resembles above threshold ionization (ATI) rings at higher energies and displays Ramsauer-Townsend-type diffraction oscillations in the angular distribution near threshold. CTMC-T calculations provide a semiclassical explanation for the dominance of selected partial waves. While the present calculation pertains to hydrogen, we find surprising qualitative agreement with recent experimental data for rare gases [A. Rudenko, J. Phys. B 37, L407 (2004)].     

Electron localization of linear symmetric molecular ion H_3~(2+)

Electron localization in the dissociation of the symmetric linear molecular ion H_3~(2+) is investigated. The numerical simulation shows that the electron localization distribution is dependent on the central frequency and peak electric field amplitude of the external ultrashort ultraviolet laser pulse. When the electrons of the ground state are excited onto the 2pσ~2Σ_u~+ by a one-photon process, most electrons of the dissociation states are localized at the protons on both sides symmetrically. Almost no electron is stabilized at the middle proton due to the odd symmetry of the wave function. With the increase of the frequency of the external ultraviolet laser pulse, the electron localization ratio of the middle proton increases, for more electrons of the ground state are excited onto the higher 3pσ~2Σ_u~+ ustate. 50.9% electrons of all the dissociation events can be captured by the middle Coulomb potential well through optimizing the central frequency and peak electric field amplitude of the ultraviolet laser pulse. Besides, a direct current(DC) electric field can be utilized to control the electron motions of the dissociation states after the excitation of an ultraviolet laser pulse, and 68.8% electrons of the dissociation states can be controlled into the middle proton.     

Laser radiation-induced acceleration of forbidden captures of orbital electrons by nuclei

The effect of linearly polarized laser radiation on the rate of the capture of atomic electrons by nuclei has been investigated. The allowed capture of s-state bound electrons from (inner) K,L, and M shells can only be weakened by the external electric field due to the shift of the maximum of the wavefunction of the bound electron with respect to the nucleus; at the same time, the wavefunctions of electrons in states with a nonzero orbital angular momentum on the nucleus can increase. The probability of various forbidden unique electron captures involving these states increases correspondingly. The problem has been examined in the simple Slater approximation. The calculations indicate that laser fields with the amplitude larger than the atomic value can significantly accelerate the first forbidden capture of p electrons, whereas laser fields with the amplitude smaller than the atomic value can significantly accelerate the second forbidden capture of d electrons.     

Relativistic MeV photoelectrons from the single atom response of argon to a 10 19 W/cm2 laser field

We present photoelectron measurements from argon ionization at 10(19) W/cm(2). Photoelectrons with energies above 400 keV, including a 1.2 MeV cutoff, are in quantitative agreement with a semiclassical, relativistic 3D ionization model that includes a nonparaxial laser field. L-shell photoelectrons have energies and momentum dominated by the field, including the acceleration out of the focus. Yields and angular distributions at 60 keV come from valence shell ionization by strong fields where rescattering and atomic processes determine photoelectron final states.     

Direct measurement of the spatial displacement of Bloch-oscillating electrons in semiconductor superlattices

We present a novel experimental technique which allows to precisely measure the spatial displacement of Bloch-oscillating electrons in semiconductor superlattices as a function of time: The dipole field caused by the motion of the electrons is traced by small shifts of the Wannier–Stark ladder states. The electron wave packet displacement can then be derived from these shifts with the excitation density as the only free parameter. Using this method, we show that the optically generated electron wave packets perform harmonic oscillations, as predicted by Zener for the semiclassical motion of electrons in 1934. The absolute amplitudes of the wave packets depend inversely on the static field and are close to the values expected from semiclassical theory.     

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.     

Exploration of strong-field double ionization of C_3H_6 with the structures of propene and cyclopropane in intense laser fields

By using classical ensemble method, we investigate the double ionization of C_3H_6 molecule with different structures(propene and cyclopropane) in intense laser fields. The numerical results show that the non-sequential double ionization occurs in propene molecule rather than cyclopropane molecule in 1200 nm laser field. To further explain this interesting phenomenon, the momentum distribution of double ionized electrons is presented and the result presents the "finger-like" structure at about 30 TW/cm~2 of propene molecule, and this structure is more obvious than that in cyclopropane molecule.The above phenomena are also demonstrated by analysing the energy distributions of double-ionized electrons versus time. Moreover, we also investigated the angular distribution at the end of pulse, which is different between propene and cyclopropane.     

以两种方法研究强激光场中的非时序双电离现象

以半经典再散射模型和量子S矩阵模型出发来研究强激光场中原子和分子的非时序双电离现象.分别利用这两个模型计算了电离率随激光强度的变化、反冲离子的动量分布、两出射电子的能量分布.数值结果表明半径典再散射模型和量子S矩阵模型在研究强场中原子和分子的非时序双电离现象时有很好的一致性.     

Semiclassical diagonalization of quantum Hamiltonian and equations of motion with Berry phase corrections

It has been recently found that the equations of motion of several semiclassical systems must take into account terms arising from Berry phases contributions. Those terms are responsible for the spin Hall effect in semiconductor as well as the Magnus effect of light propagating in inhomogeneous media. Intensive ongoing research on this subject seems to indicate that a broad class of quantum systems may be affected by Berry phase terms. It is therefore important to find a general procedure allowing for the determination of semiclassical Hamiltonian with Berry Phase corrections. This article presents a general diagonalization method at order ħ for a large class of quantum Hamiltonians directly inducing Berry phase corrections. As a consequence, Berry phase terms on both coordinates and momentum operators naturally arise during the diagonalization procedure. This leads to new equations of motion for a wide class of semiclassical system. As physical applications we consider here a Dirac particle in an electromagnetic or static gravitational field, and the propagation of a Bloch electrons in an external electromagnetic field.     

强激光场中H2^＋离解行为的研究

在多电子态模型下，利用二阶劈裂传播子算法计算了Ｈ２＾＋在强激光场中的离散行为。结果发现在高频场时分子的多光子离散碎片是近对称的，而对于低频场，在所考虑的激光强度下Ｈ２＾＋是被隧道离散的，因而其离散碎片Ｈ＾＋分布是反对称的，另外，文中还探讨了三脉冲形状对分子离解几率、离解碎片动力学行为的影响。     

Initial wavefunction dependence on atom interferometry phases

In this paper we present a mathematical procedure to analytically calculate the output signal of a pulsed atom interferometer in an inertial field. Using the well-known ABCDξ method we take into account the full wave dynamics of the atoms with a first order treatment of the wavefront distortion by the laser pulses. Using a numerical example we study the effect of both the length of the beam splitting laser pulses and of the width of the initial spatial distribution of the atoms. First, we find that in a general inertial field the interferometer only has a limited window in terms of the initial width (centered around 100 μm in the example calculation) in which interference fringes are visible at all. This effect is caused by the inevitable statistical spread in atomic parameters, such as initial position and momentum, and the dependence of the interferometer phase on these. In the optimum case, the useful range of the initial width is formed by the range in which both the spatial distribution and the diffraction limited momentum spread are small enough to avoid large phase differences over the atomic wavefunction. As a second result we find that the interferometer phase depends strongly on the length of the laser pulses and, to a smaller extent, on the initial width of the atomic cloud. This spatial dependency is relatively small (～10^?5 rad) and justifies semiclassical approximations, as used in other calculations, for most experiments. New high-accuracy experiments, however, will come in the range where this effect is no longer negligible.     

Nonsequential double ionization of Xe by mid-infrared laser pulses

With the three-dimensional semiclassical ensemble model, we studied the correlated electron dynamics in strong-field nonsequential double ionization of Xe by mid-infrared laser pulses over a wide range of laser intensities. Numerical results show that the correlated electron momentum distribution exhibits the strong back-to-back pattern at the lower laser intensity. It evolves into the side-by-side behavior as the laser intensity increases and presents the experimentally observed crosslike shape at high laser intensity. Different from the case of near-infrared region where recollision mainly occurs at the first returning, at the mid-infrared region recollision dominantly occurs at the later returnings. The windows of initial transverse velocity for the various multiple-returning recollision trajectories are different and are unambiguously determined by this semiclassical ensemble model.     

The effect of electron initial longitudinal velocity on the non-sequential double ionization process in an elliptically polarized laser field

The effect of initial longitudinal velocity of the tunnelled electron on the non-sequential double ionization(NSDI) process in an elliptically polarized laser field is studied by a semiclassical model.We find that the non-zero initial longitudinal velocity has a suppressing effect on single-return collision(SRC) events in the double ionization process,more specifically,it results in an obvious reduction in the center part of the correlation momentum distributions in the direction of the major polarization axis(z axis) and makes the distribution of single-return collision in the minor polarization axis(x axis) become narrower.