The Exploration on Molecular Alignment Employing Two Ultrafast Laser Pulses

强场超快激光技术的进步拓展了人类对原子分子光物理领域的认识。随着人类驾驭更为复杂的实验系统和激光装置能力的提高，研究焦点也从原子向更为复杂的分子转移。对于分子在强场下动力学的研究，如电子跃迁，电离机制，高次谐波产生等，对我们更深入地了解化学反应和生物过程的本质，确立分子结构和量子信息学的研究，具有深远的意义。 不同于原子，分子具有各向异性的结构特征。因此很多分子动力学的研究要求对分子的空间排列有进行控制的能力。在超短激光脉冲和线型分子系综相互作用过程中，分子会按照激光场的偏振方向取向。通过改变激光脉冲之间的延时，利用两束超短激光脉冲可以增强或者减弱分子取向。本论文主要研究了利用双脉冲激光场控制分子取向的物理机制，包含以下两个方面： 1. 找到并证明了双脉冲增强分子取向的最佳延时是一个转动全周期。不同于之前文献中的经典力学解释，我们建立双脉冲分子取向增强的量子物理图像。我们指出双脉冲拉曼激发的跃迁振幅受到两束激光脉冲间隔的调制。最大的激发概率发生在两束脉冲间隔为一个转动全周期处，导致最优的取向增强。 2. 给出了之前发现的关于双脉冲分子取向增强和减弱普遍规律的“导数法则”的物理解释。这个现象是双脉冲拉曼激发过程中，多量子路径干涉在分子热力学系综中的体现。我们进一步确立了量子路径干涉条件和第一束脉冲分子取向的导数之间的联系。这样可以简单地通过第一束脉冲后的分子取向图线预言第二束脉冲加入后对分子取向的影响。这样的发现使得利用双脉冲激光场对分子取向进行主动控制成为可能。The constant progress in intense ultrafast laser technology significantly extends human’s knowledge on atomic, molecular and optical physics. With the increased ability to harness more complicated experimental systems and laser facilities, researchers’ focus is shifted from atoms to more sophisticated molecules. The research of molecular dynamics in high field regime, such as electron transition, ionization scheme, high order harmonic generation, etc, is of significance to help us gain more insights into the chemical reaction, biological process, molecular structure and quantum information. Different from atoms, molecule has an anisotropic structure. Therefore many researches on molecular dynamics desire the capability to control the molecular alignment in the space. During the interaction between ultrafast laser pulse and linear molecular ensemble, molecules are aligned along the direction of laser polarization. By changing the delay between pulses, two ultrafast laser pulses can enhance or suppress molecular alignment. This thesis is focused on understanding the physical mechanics of two-pulse alignment, including two aspects: 1. The optimal two-pulse delay to enhance the molecular alignment is proved to be a full rotation period. Different from the previous classical interpretation, we build the quantum picture to explain the alignment enhancement. The Raman excitation probability is modulated by the delay between two pulses. The maximum excitation occurs when the pulse delay is a full rotation period, leading to optimal alignment enhancement. 2. An interpretation on the physical mechanisms of the “rule of slope”, a general rule about the alignment enhancement and suppression, is proposed. It is due the manifestation of multiple quantum path interference on the molecular ensemble during two-pulse Raman excitation. We further build the connection between the interference condition and the slope of the alignment curve due to the first pulse. It is thus possible to predict the timing of the second pulse to cause the enhancement or suppression of molecular alignment, simply according the alignment signal by the first pulse. This phenomenon allows one to actively control the molecular alignment using two ultra-short laser pulses