Photoelectron angular distributions at the ionization of atoms by intense sub-one-cycle laser pulses

The phase sensitivity of the photoelectron angular distributions by intense sub-one-cycle linearly polarized laser pulses has been discussed within the analytic Landau-Dykhne approximation. In both cases of sine and cosine laser pulses most of the electrons are ejected along the polarization axis of the laser field. Nevertheless the electron yield and the electron kinetic energies are much larger for the cosine waveform pulse.     

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.     

Emergence of classical orbits in few-cycle above-threshold ionization of atomic hydrogen

The time-dependent Schr?dinger equation for atomic hydrogen in few-cycle laser pulses is solved numerically. Introducing a positive definite quantum distribution function in energy-position space, a straightforward comparison of the numerical ab initio results with classical orbit theory is facilitated. Integration over position space yields directly the photoelectron spectra so that the various pathways contributing to a certain energy in the photoelectron spectra can be established in an unprecedented direct and transparent way.     

Deflections of photoelectron classical trajectories in screened Coulomb potentials of H_2~+

The photoelectron momentum distribution of H+2in circularly polarized laser fields is studied based on classical trajectory calculations. We screen Coulomb potentials at different radii, and trace trajectories of an ensemble of electrons in such screened Coulomb potentials and circularly polarized laser fields. Simulations show that electron trajectories are bent by Coulomb fields, resulting in the laser-intensity-dependent drift of photoelectron momentum distributions in the laser polarization plane. This study intuitively explains how Coulomb potentials modify photoelectron momenta.     

Effect of polarization on the structure of electromagnetic field and spatiotemporal distribution of <Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>–</Superscript> pairs generated by colliding laser pulses

We have studied the production of electron–positron pairs due to polarization of vacuum in the presence of the strong electromagnetic field of two counterpropagating laser pulses. The structure of the electromagnetic field with the circular polarization has been determined using the 3D model of focused laser pulses, which was proposed by Narozhny and Fofanov. Analytic calculations have shown that the electric and magnetic fields are almost parallel to each other in the focal region when the laser pulses are completely transverse in the electric (E-wave) or magnetic (H-wave) field. On the other hand, the electric and magnetic fields are almost orthogonal when laser pulses consist of a mixture of E- and H-waves of the same amplitude. It has been found that although the latter configuration of colliding laser pulses has a much higher pair production threshold, it can generate much shorter electron–positron pulses as compared to the former configuration. The dependence of the production efficiency of pairs and their spatiotemporal distribution on the polarization of laser pulses has been analyzed using the structure of the electromagnetic field in the focal plane.     

Direct XUV probing of attosecond electron recollision

We demonstrate that the recolliding electron wave packet, fundamental to many strong field phenomena, can be directly imaged with sub-A spatial and attosecond temporal resolution using attosecond extreme ultraviolet (XUV) pulses. When the recolliding electron revisits the parent ion, it can absorb an XUV photon yielding high energy electron and thereby providing a measurement of the electron energy at the moment of recollision. The full temporal evolution of the recollision wave packet can be reconstructed by measuring the photoelectron spectra for different time delays between the driving laser and the attosecond XUV probe. The strength of the photoelectron signal can be used to characterize the spatial distribution of the electron density in the longitudinal direction. Elliptical polarization can be used to characterize the electron probability in transversal direction.     

PHOTOEXCITATION OF ATOM WITH ULTRASHORT LASER PULSES

The uncoupled basis set is used to describe the interaction between the ultrashort pulses and the Na atom. Several polarization configurations of laser pulses have been investigated, which produce different effects on the two-step photoexcitation process. The study shows that the final states of the transitions driven by ultrashort laser pulses have a strong dependence on the laser polarization configuration. A comparison of the two different basis sets shows that the uncoupled basis set may provide more information than the coupled basis set, such as angular distribution of the atom in the final states of the transitions.     

用变换方程测量窄带阿秒超紫外线XUV脉冲的强度时间结构

阿秒超紫外线(extreme ultra-violet, XUV)与飞秒超短激光脉冲共同激发惰性气体原子产生光电子，其终态能量与光电子产生时刻即激光相位有关. 介绍光电子的激光相位确定法，并利用光电子能谱本身(其比例谱)，计算出待测窄带XUV脉冲的强度时间结构. 研究表明，在与激光线性极化方向成0°或180°方向测量得到的光电子能谱动态范围大，容易解谱. XUV脉冲的时间宽度的测量范围为半个激光振荡周期，时间分辨率主要取决于测量系统的时间晃动和控制精度. 关键词： 光电子能谱 相位确定法 变换方程 脉冲强度时间结构     

How to measure the internuclear vector from photoelectron angular distributions

This paper uses a nonperturbative scattering theory to study photoelectron angular distributions of homonuclear diatomic molecules irradiated by circularly polarized laser fields. This study shows that the nonisotropic feature of photoelectron angular distributions is not due to the polarization of the laser field but the internuclear vector of the molecules. It suggests a method to measure the molecular orientation and the internuclear distance of molecules through the measurement of photoelectron angular distributions.     

用光电子能谱相位确定法同时测量阿秒超紫外线XUV脉冲的频率和强度时间分布

介绍阿秒超紫外线(XUV)激发惰性气体原子产生光电子并在强激光电场中运动的半经典近似 方法，以及同时、直接、全面地测量阿秒XUV脉冲频率和强度时间分布的光电子能谱相位确定法. 采用飞秒超短脉冲激光和XUV间的交叉关联技术，从不同激光强度下、与激光线性极化方向成0°或180°方向上测得的光电子能量积分谱，可以重建XUV的频率和强度时间分布. XUV脉冲时间宽度的测量范围从1／4到1／2激光振荡周期，时间分辨率取决于激光束和XUV脉冲之间的相对延时控制精度和光路抖动时间. 所述方法可用于在阿秒尺度上的超快速测量，以及有关电子在原子和分子中运动的超快速动力学过程研究. 关键词： 阿秒测量 光电子能谱 相位确定法 超紫外线 频率和强度时间分布     

氢原子在红外激光和极紫外脉冲组合场中的阈上电离

通过求解氢原子在强红外(IR)激光和极紫外（XUV）脉冲组合场中的三维含时薛定谔方程(TDSE),理论研究了XUV脉冲的加入对光电子能谱和二维光电子动量分布的影响.计算结果表明,与仅由红外场驱动的情况相比,组合场驱动下的光电子能谱和二维光电子动量分布中呈现出明显的干涉增强结构,干涉结构对XUV脉冲的强度、光子能量和时间延迟都有很强的依赖,该方案可实现高能阈上电离谱的选择性增强.     

Electron heating through a set of random levels in the conduction band of insulators induced by femtosecond laser pulses

We develop a theory describing the heating of electrons in crystalline insulators irradiated by high-intensity laser pulses. In agreement with photoelectron yield versus intensity measurements, we assume that electrons are excited into the conduction band from defect layers and traps. The electron dynamics due to direct inter-branch transitions within the conduction band is simulated by solving of time-dependant Schr?dinger equation. The set of levels for this equation is supposed to be random with a distribution function equal to the density of states in the conduction band. The influence of different parameters on the electron heating efficiency is studied. The theory is applied for diamond; the theoretical spectrum is in qualitative agreement with the experimental observations.     

Photoelectron angular distributions of molecules in bichromatic laser fields of circular polarization

Photoelectron angular distributions (PADs) from above-threshold ionization of O₂ and N₂ molecules irradiated by a bichromatic laser field of circular polarization are studied. The bichromatic laser field is specially modulated such that it can be used to mimic a sequence of one-cycle laser pulses. The PADs are greatly affected by the molecular alignment, the symmetry of the initial electronic distribution, and the carrier-envelope phase of the laser pulses. Generally, the PADs do not show any symmetry, and become symmetric about an axis only when the symmetric axis of laser field coincides with the symmetric axis of molecules. This study shows that the few-cycle laser pulses can be used to steer the photoelectrons and perform the selective ionization of molecules.     

不同偏振态下的飞秒激光脉冲与铝靶相互作用中超热电子的产生

研究了P偏振、S偏振和圆(C)偏振态下亚相对论强度飞秒激光脉冲与铝靶相互作用时产生的超热电子的能谱和靶背向角分布，并对S偏振产生的超热电子的特殊角分布通过引入表面磁场的概念进行了定性解释. 关键词： 超短超强激光 等离子体 超热电子 表面磁场     

Field-induced ionization and Coulomb explosion of nitrogen

Femtosecond-laser field-induced ionization and Coulomb explosion of diatomic nitrogen were systematically investigated using time-of-flight mass and photoelectron spectrometry. Both linearly and circularly polarized femtosecond laser pulses were used at intensities varying from 5×10¹³ to 2×10¹⁵ W/cm². Strong N₂ ⁺, N₂ ²⁺, N⁺, N²⁺ and N³⁺ ion signals were observed for horizontally polarized pulses. Moreover, signals from the atomic ions exhibited a double-peak structure. Suppression of ionization was observed for circularly polarized pulses, while for vertically polarized pulses, only N₂ ⁺ and N₂ ²⁺ ions were observed. The angular distributions of the ions were measured under zero-field conditions in the ionization zone. The atomic ions N⁺, N²⁺ and N³⁺ exhibited highly anisotropic distributions, with maxima along the laser polarization vector and zeroes normal to the laser polarization vector. In contrast to the atomic ions, N₂ ⁺ exhibited a strong isotropic angular distribution. These observations indicate that dynamic alignment is responsible for the observed anisotropic angular distribution of the atomic ions. The kinetic energy spectrum of the photoelectrons is featureless and broad, extending above the ponderomotive potential of the laser pulse. The angular distribution is markedly anisotropic, with a maximum along the laser polarization vector. These observations further support the notion that the field-ionization mechanism is dominant under our experimental conditions. Received: 29 January 2002 / Revised version: 15 March 2002 / Published online: 12 July 2002     

Dynamic properties of angle-dependent high-order harmonic generation from coherently rotating molecules

High-order harmonic generation from coherently rotating N2 and O2 molecules has been observed for different alignment angles in a pump and probe experiment using femtosecond laser pulses. The results obtained are in excellent agreement with those calculated using a recently developed theory, which represent the characteristic properties predicted for angle-dependent harmonic generation. It is shown that polarization geometry and alignment distribution play essential roles in potential applications to probe electronic structure and dynamics of molecular systems.     

Measurement of the carrier-envelope phase of few-cycle laser pulses by use of asymmetric photoionization

Using numerical solutions of the time-dependent Schr?dinger equation for a hydrogen and a helium atom in a linearly polarized, few-cycle laser field, we calculate the photoelectron left-right asymmetry measured by two opposing detectors placed along the laser polarization vector, with the laser focus in the center. We find a simple dependence of this asymmetry on carrier-envelope (CE) phase phi for laser intensities slightly below the tunneling regime, which may allow us to measure (or to calibrate) and to stabilize the CE phase. In particular, we suggest that the condition of zero asymmetry for few-cycle pulses may be useful for both these goals.     

Combination of high-intensity femtosecond laser pulses for generation of time-dependent polarization pulses and ionization of atomic gas

Time-dependent polarization pulses are generated by combining two perpendicularly polarized, high-intensity laser pulses. The time evolution of the polarization state of the combined laser pulse is measured by the POLLIWOG technique. We observed changes in the polarization state while varying the relative delay. In order to investigate the effect of pulse combination on the ionization of atoms, the electron signals and the ion signals are measured by irradiating combinations of two perpendicularly polarized pulses or two parallel polarized pulses. With the two parallel polarized pulses, high-order fringe-resolved autocorrelations are obtained by measuring the time-integrated ion signals as a function of the time delay. When two perpendicularly polarized pulses are combined, the fringe period of the time-integrated electron signal as a function of the time delay is different from that of the time-integrated ion signal. This is due to the fact that the electron signal depends on the direction of the field vibration and the number of generated electrons. We also measured the electron energy distributions at different relative delays and confirmed that these depend on the polarization state of generated pulses.     

Control for atom response in multicomponent laser fields

A theory of the nonlinear optical response of an atom interacting with a superposition of arbitrarily polarized fields is developed. The theory is based on the analytical solution of the boundary-value problem for an electron moving in a spherically symmetric intraatomic field and in the field of an external electromagnetic field. By means of the example of an argon atom interacting with a bichromatic field formed by the first and second harmonics of a Ti:sapphire laser, it is shown that, when an atom interacts with the field of two polarized pulses the polarization directions of which are not collinear, the response spectrum significantly depends on the laser radiation parameters—the duration and intensity of pulses, the time of delay between them, and the angle between the directions of polarization vectors. Generation of THz radiation is shown to be possible in the ionization-free regime due to intraatomic nonlinearity.     

超短脉冲激光与乙醇微滴相互作用中超热电子的双叶状角分布

测量了聚焦光强为1016W/cm2的超短脉冲激光与乙醇微滴相互作用中产生的能量大于50?keV的超热电子的角分布和电子能谱.观察到的超热电子角分布明显依赖于激光的偏振特性，在与激光偏振平面平行的平面上超热电子相对于激光入射方向呈对称的双叶状分布.超热电子的能谱显示超热电子的最大能量大于750?keV.以上超热电子的角分布可用一个基于共振吸收机制的模型加以解释. 关键词： 超短脉冲激光 超热电子 微滴 共振吸收