强激光场中原子单电离的半经典方法

Semiclassical methods for strong field ionization of atoms

随着激光器技术的发展,实验室中可获得的激光场强度不断增大,可以观测到各种高度非线性的实验现象,比如原子分子中的电子可以从强激光场中吸收比克服电离能需要的光子数更多的光子而发生电离,被称为阈上电离.这些非线性现象向量子力学的微扰理论提出了挑战,而精确求解含时薛定谔方程的方法只局限在少数的简单原子或分子,并且计算复杂.在这种情形下,基于电子轨迹的半经典理论由于其物理过程清晰、计算简单等优点应运而生,在强场光电离领域已经广泛应用.本文从原子分子电离的经典描述出发,介绍了半经典理论的发展.半经典理论结合了量子力学隧穿效应、电子经典的轨道描述以及量子的相位信息,能够很好地解释在强场隧穿区的各种实验现象.文中重点介绍两种已经发展得较为成熟的半经典方法,即量子轨道蒙特卡罗方法 (quantum trajectory Monte Carlo)和库仑修正下的强场近似方法 (Coulomb-corrected strong field approximation).这两种方法的优点在于同时考虑了电子轨迹的库仑势作用以及电子轨迹之间的干涉效应,能够通过追踪电子的运动轨迹对电离电子动量谱中各种结构的形成给出直观清晰的解释.

As the advances of laser technology,more and more nonlinear phenomena are observed in the atoms and molecules driven by strong laser pulses.Systematic investigations on these findings,such as above threshold ionization and highorder harmonic generation,will lead us to understanding the mechanisms in the microscopic world.The most exact way to simulate the experimental measurements is to solve the time-dependent Schr？dinger equation（TDSE） numerically,in which the system is described by the wave function and thus one cannot have an intuitive insight into the underling process.Therefore,several semiclassical methods have been developed to understand the strong field ionization.In the classical point of view,the electrons tunnel out when the strong laser field suppresses the Coulomb potential.Then the electrons are driven by the laser electric field according to the Newtonian equations.Semiclassical methods take into account the tunnelling of the electron,the classical orbit of the electron,and the action as the phase of trajectory,which have successfully explained main structures in the ionization spectrum.Two of the most popular semiclassical methods are the quantum trajectory Monte Carlo method and the Coulomb-corrected strong field approximation method.In the present review,we will introduce these basic methods and show how they have been developed step by step,covering the most relevant and important works in the strong field physics.Finally we give two example of applications to show how these methods work.With the advantage of the classical picture,we can identify different kind of structures in the2 D photoelectron momentum distributions and tell how the structures are formed.Nonadiabatic effects can be studied by comparing the results of the two methods,together with accurate simulation from the numerical solution of TDSE.The current semiclassical methods can be further developed into advanced ones,which can be used in more complex molecular systems or multi-electron systems,and be widely used in the study of dynamics of molecule and atoms in strong laser fields.