飞秒激光作用下分子离子激发态研究

Femtosecod laser has become an important tool to measure and manipulate the ultrafast dynamic behaviors of molecules due to its ultrashort pulse duration and ultrahigh peak intensity. However, the laser-molecule interaction is very complicated in intense laser fields. The molecular reaction pathway is determined by the correlation dynamics of electron-electron, electron-nuclear, and nuclear-nuclear in intense laser fields. In the case of ionization of molecules driven by intense laser fields, it is still concerntrated on the dynamics of the ground electronic state of molecular cations generated in intense 800-nm laser field even though more and more experiments show that the generated molecular cations can be in the ground electronic state as well as the excited electronic states. Because it is very challenging for both experimentalist and theorist to carry out the study of excited molecular ions generated in intense femtosecond laser fields, it is still lack of accurate experimental data as well as quantitative investigation of theoretical models until now. In this proposal, the excited state dynamics of molecular ions will be extensively studied in intense laser fields with various laser wavelengths. Through identifying the inner quantu states, and measuring the three-dimensional momentum and quautm yields of reation porducts, we explore the evolution of molecular quantum states in intense laser fields. In combination with the laser-driven molecular dynamic simulation, the formation mechanism of excited states will be revealed for molecular ions generated in intense laser fields.

飞秒激光具有超短脉冲宽度和超高峰值功率，已经成为测量和操控分子动力学行为的重要工具。飞秒强激光与分子相互作用过程非常复杂，涉及到强激光场中的电子与电子、电子与核、以及核与核之间的关联效应。对于飞秒强激光作用下的分子电离研究，目前主要集中在波长800纳米激光作用下分子离子基电子态。尽管实验表明飞秒强激光场下分子电离产生的离子也可以处在电子激发态，但是分子离子激发态研究既缺乏精确的实验数据，也缺少理论模型的定量描述。本申请拟开展不同波长的飞秒激光作用下分子电离，通过对飞秒激光作用后分子离子量子态的精确指认以及平动能量和离子产率的精确测量，获得飞秒强激光场下分子量子态演化精确数据。结合强激光场下分子动力学理论模拟，揭示飞秒强激光和分子相互作用过程中分子离子激发态的形成机理。

飞秒激光具有超短脉冲宽度和超高峰值功率，它与氮气分子作用，在激光的传输方向上观察到多条相干辐射谱线，这些辐射谱线对应氮气分子离子的第二电子激发态到电子基态的不同振转能级之间的跃迁。这种以空气主要成分氮气分子作为增益介质、不需要谐振腔、可在远程产生的相干辐射被称为空气激光。作为一种新兴光源，空气激光在大气远程探测领域有巨大的应用潜力。为了揭示氮气分子离子相干辐射的物理本质以及产生条件，项目组系统地研究了飞秒激光和氮气分子的相互作用过程，探讨了氮气分子离子激发态产生机理及弛豫动力学。实验发现当种子光通过波长800nm的飞秒激光电离的氮气分子时，依赖于种子光的波长，种子光呈现放大或者吸收现象。结合理论计算，项目组确定了氮气分子离子的量子态布居，证实第二电子激发态的基振动态与电子基态的基振动态之间的布居实现了反转。进一步地利用双泵浦光或者双种子光对氮气分子离子相干辐射进行调控，实验观察到亚飞秒分辨的周期性调制信号。理论上建立半经典光学布洛赫方程，模拟了飞秒激光与氮气分子离子的相互作用动力学过程，再现了实验观察到的氮气分子离子相干辐射的时间特性、光学增益特性、以及双种子光或者双泵浦光作用下的干涉条纹。这些研究揭示了氮气分子离子激发态的形成机理，深化了氮气分子离子激光的物理本质，为优化氮气分子离子激光提供了重要参考。