强激光场中极性分子奇偶谐波辐射的时间分辨动力学研究

The harmonic emission from polar molecules exposed to strong laser field is one hot topic in strong-field physics. Due to symmetry breaking, polar molecules emit both odd and even harmonics. Present studies mainly focus on the power spectra of odd-even harmonic emission from polar molecules. However, in order to understand the complex origin of the odd-even harmonic emission, the study of their time-resolved dynamics is desired. In this project, the time-frequency properties of the odd-even harmonic emission from polar molecules with diverse inter-nuclear distances in strong laser fields will be investigated theoretically. The competition mechanism in the time domain between different ionization channels (the ground state channel and the first excited state channel) and recollision channels (odd harmonic channels and even harmonic channels) will be deeply analyzed. From the time-frequency phenomena, the inner relation between orientation dependence of ionization and harmonic emission, the interaction between two-center interference and permanent dipole of the molecule are also studied. Moreover, we will study the odd-even harmonic emission from polar molecules in two-color laser field, with aiming at the relation between the laser-field induced odd-even harmonic emission and that arising from the asymmetric potential of the polar molecule. These studies will benefit understanding of ionization and harmonic emission mechanism and provide reference to the related experimental work.

强激光场中极性分子谐波辐射是目前强场物理的一个研究热点。由于对称破缺，极性分子的谐波辐射谱中一般包含奇偶谐波。目前对极性分子奇偶谐波辐射的研究主要集中在谐波功率谱，要深入了解奇偶谐波的复杂起源，需要研究其时间分辨动力学，而此项工作尚未见报道。本项目欲从理论上详细研究单色激光场辐射下，具有不同核间距的极性分子奇偶谐波辐射的时频特性，深入分析不同电离通道（基态通道和激发态通道）和谐波辐射通道（奇次谐波通道和偶次谐波通道）在时域上的竞争机制。通过对时域现象的研究，深入剖析极性分子电离与谐波辐射取向依赖的内在关系，两中心干涉效应和固有偶极矩效应之间的相互作用。此外，本项目还将研究双色场中极性分子的奇偶谐波辐射，探究激光场导致的奇偶谐波辐射与不对称的库伦势导致的奇偶谐波辐射之间的关系。这些研究将有助于人们理解极性分子的电离及谐波辐射机制，并为相关的实验研究提供有益的参考。

由于对称破缺，极性分子辐射的谐波既包含奇次谐波，又包含偶次谐波。本人及其合作者较早的注意到极性分子奇偶谐波辐射可能具有不同的时频特性，并在此后的工作中创新性地对奇次谐波和偶次谐波辐射分别进行研究，深入分析了奇偶谐波辐射与分子结构的关系，提出了奇偶谐波辐射的模型：实现了奇偶谐波辐射在时域上的分离，指出了干涉效应会影响实验上对极性分子取向度的正确评估。从时间分辨动力学的角度分析了极性分子固有偶极子与强激光场相互作用对分子电离过程的影响，进而对奇偶谐波辐射不同释放通道谐波产量的影响。指出极性分子谐波辐射时频现象是两中心干涉与固有偶极子效应的相互作用的结果。为了进一步探索极性分子谐波辐射中的动力学行为，研究了双色强激光场中极性分子电离时间与返回时间。数值结果表明，在分子轴与基频场偏振方向平行与反平行时，低阶谐波的电离时间呈现数十阿秒的差异。通过这种差异，我们尝试获得不对称系统中取向依赖的有效电离能。另外，我们也比较研究了极性与非极性分子在双色场中的高次谐波辐射特性。通过数值求解薛定谔方程对双色场的相对相位延迟进行扫描，分析了电离取向效应和两中心干涉效应对谐波辐射强度随相对相位的变化规律，建立了以半经典模型和量子轨道理论为基础的半解析理论模型。