【作者】 郭静；

【导师】 刘学深； Shih-I Chu；

【作者基本信息】 吉林大学 ， 原子与分子物理， 2010， 博士

【摘要】 在物理和化学领域,对强场下多电子原子或分子系统的动力学研究一直是一个很重要的课题。外场中的原子分子系统是当前物理学中最为活跃的研究对象之一,而电子动力学的研究在阿秒科学中也占有重要地位。由于很多非线性的强场现象,比如多光子电离,多光子共振,高次谐波发射,等等,都超出了微扰的范畴。为了深入理解这些物理过程,需要引入一些新的理论计算方法。一般说来,理论计算方法分为三种,第一种,采用数值求解Schr?dinger方程的方法来研究强场中原子与分子的动力学,它的优点是理论严密,计算精确,但对计算机的要求很高,并且难以推广到高维多中心多电子系统。第二种是经典理论,它的优点是图像清晰,计算简便。不仅能够提供描述强激光场中分子过程的正确图像以便于我们了解激光与原子分子系统的相互作用,并也可以得到与量子计算一致的结果,只是精确性稍有欠缺。开展这方面的研究,将从另一方面深化人们对强场下多原子分子的动力学行为的了解。第三种是半经典理论,它可以综合经典理论与量子方法的优点,在哈密顿量中加入量子修正,只是在理论推导上较为复杂。本论文即采用了三种不同的计算方法,分别从经典理论,半经典理论以及纯量子理论出发,来研究强场下多电子原子及分子系统的动力学。本文主要分为三个部分:第一部分,介绍了经典理论方法及辛算法,并把它们推广到强场原子与分子动力学领域,具体研究了强场下氦原子和锂原子的电离动力学过程。1)我们将经典系综方法应用到强场下一维氦原子的电离动力学过程。先采用系综法选取无场时的初态,并用辛算法求解强场下He的哈密顿正则方程。在532 nm,780 nm和1024 nm时我们观察到表征非次序电离的“knee”结构,并论证出NSDI在低频时更易产生。当波长为532 nm, 780 nm和1024 nm时,随着激光强度的增大He从非次序双电离占主导地位变为次序电离占主导地位,而当波长为248 nm时不会产生非次序电离,这与相应的理论和实验结果是一致的。此外,我们的经典模拟也从单电子能量分布的角度给出了详细的NSDI和SDI图像。这些数值结果与实验和量子计算均相吻合。2)我们用经典系综方法研究了激光脉冲作用下的1维Li原子与强场作用的电离动力学。电子的运动可由哈密顿正则方程来描述。我们计算了单电离和双电离的比率随激光强度的变化情况,并用电子的能量分布加以解释。我们也研究了Li的三次电离及主要的三次电离通道。当波长为390 nm和780 nm时,Li的双电离过程从非次序向次序转变。在高频激光场下(39 nm),NSTI和NSDI均观察不到。而在低频激光场中(如390 nm, 780 nm),表征非次序双电离与非次序三电离的“knee”结构则非常明显。此外,我们还计算了不同参数下非次序双电离与非次序三电离的变化规律并用能量曲线加以解释。第二部分,我们在M. S. Child等人提出的耦合干涉态理论(CCS)基础上发展了这一方法并将其推广到强场多原子分子和多电子原子的动力学研究领域,去解决多维多体量子动力学问题。此方法可以减少维度,计算简洁直观。我们可以观察到He原子的低能态结构。我们也提出了电子位置和动量的含时演化,其结果也可证明“重散射”模型的正确性,即电离的电子在激光场作用下返回核附近,与未电离电子发生非弹性碰撞而产生双电离。此外,He的角分布也被研究,结果表明,在波长较短时,ATI谱线更为明显。随着波长的增大或者场强的增加,角分布离核更近,并且电子与轴线方向夹角的最大值更小。第三部分,简单介绍了Chu等人提出的一般伪谱方法和密度泛函理论,并将此应用于研究强场下原子与分子的高次谐波发射和多光子电离。我们首先用一般伪谱离散(GPS)来得到初态,这一方法的优点是可以用尽量少的点来得到高精度的电子态。接着采用密度泛函研究了Ne原子的高次谐波和多光子电离,其结果与实验符合得很好。更多还原

【Abstract】 The study of the strong-field multi-photon process of atomic and molecular system is a subject of current significance both theoretically and experimentally. Recent progress of laser technology has reached a burst of attosecond science where the electron dynamics plays an important role in physics. In particular, the recent advancement of intense laser field has led to a set of strong-field phenomena, such as multiphoton ionization, multiphoton resonance, high-order harmonic generation, etc, are all beyond the perturbative regime. To advance the strong-field physics, this dissertation aims at developing the new theoretical and computational methods for ab initio studies of atomic and molecular process in intense laser fields. The main achievements are summarized below:1. The ionization dynamics of helium atom and lithium atom under intense laser pulses was explored by using classical ensemble method. 1) Probabilities of double-ionization of helium in intense laser fields in different conditions are calculated by the symplectic method. The wavelength dependence of NSDI in He is investigated. The non-sequential 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 double to single ionization ratio of helium with different laser intensities are also investigated, and the results shows the double-ionization of helium shifts from non-sequential to sequential as the intensity increases at the wavelength of 532 nm, 780 nm and 1024 nm, which is in agreement with the corresponding experimental and quantum results qualitatively. The result demonstrated the non-sequential double ionization mechanism could be observed in terms of one-electron energy from a classical view. 2) The classical ensemble method is applied to study ionization processes of a 1D model lithium atom interacting with an intense laser pulse. The motion of electrons is described by the classical Hamiltonian canonical system of equations. The ratio of double-to-single ionization with the increasing laser intensity is calculated and explained in terms of the energy distribution of electron. The triple ionization of lithium is also investigated and the primary triple ionization path is found. Our results show a clear transition from non-sequential to sequential double ionization as the intensity increases, which is in agreement with quantum calculation.2. We extended the coupled coherent-states (CCS) approach to simulate the strong field ionization of atoms and molecules in long wavelength. This approach uses a basis of trajectories guided by Frozen Gaussian coherent states, sampled from a Monte-Carlo distribution, as the initial states of the quantum time-dependent Schrodinger equations. The CCS trajectories move over averaged potentials, which can remove the Columbic singularities exactly. 1) The behavior of helium in intense laser field is investigated by CCS method. The novel low-energy structure (LES) is predicted by our CCS calculation and the ’’rescattering’’ event is clearly identified in the higher energy regime. Besides, the non-sequential double ionization is also explored and the“rescattering”event can be identified as the major mechanism. In addition, we also studied the electron angular distribution of helium and found that the maximum angle between the electron and electric field directions becomes smaller with the increasing of laser intensity and wavelength. 2) The strong-field ionization, low energy structure region and the orientation dependence of CO2 and N2 in intense laser fields are investigated by the improved coupled coherent-states (CCS) approach. The effects of laser intensity on the orientation dependence of total ionization probability of N2 and CO2 are calculated and the results are in agreement with experimental research. The normalized electron yield and angular distribution with different parameters are also investigated.3. I have learned the time-dependent generalized pseudospectral (TDGPS) method and time dependent density functional theory (TDDFT) in Prof. Shih-I Chu’s group for atomic and molecular systems in intense laser fields, and applied them to Ar cases. The generalized pseudospectral technique is extended to perform an optimal spatial grid discretization, leading to significant improvement of the quality of the wavefunction over that obtained by the equal-spacing spatial discretization techniques. The accuracy of the method is demonstrated by several benchmark calculations including the excellent agreement of the HHG power spectra in length and acceleration form.更多还原