Theoretical study on ultrafast chemical processes in terms of pump-probe photoelectron spectroscopy

我们发展了飞秒泵浦探测波包电离的能量和角度分辨光电子能谱理论，并将其应用于取向Na_2的^1Σ^+_μ双极小态。该公式非常适合于包含分子光电离的基本动力学及其对分子几何的依赖。选取了三种典型的泵浦激光能量，定性地研究了波包在双极小势曲线上的时空传播及其相关的光电子能谱，得到了一些结果。给出了线偏振泵浦和探测脉冲不同取向下的光电子角分布。由此得到的光电子能谱说明了在非绝热区域从泵浦-探测光电子信号中解开波包动力学的重要性，在非绝热区域中，电子结构随着几何形状的快速演化，适当地描述了潜在的光电离幅度及其对几何的依赖。将本理论推广到处理NaI分子这样的非绝热跃迁系统，由于分子内的电子转移，其基势曲线和第一激发势曲线相交。首次证明了用泵浦探测光电子能谱可以直接和实验地观察到核波包的分叉。这在化学、波场工程、量子混沌、量子基础理论等方面都具有重要意义。

We have developed a theory of energy- and angle-resolved photoelectron spectra for femtosecond pump-probe ionization of wavepackets and results of its application to the ^1Σ^+_μ double-minimum state of aligned Na_2. The formulation is well suited for inclusion of the underlying dynamics of molecular photoionization and its dependence on molecular geometry. Results have been obtained for three typical pump laser energies selected so as to investigate qualitatively different patterns of the spatio-temporal propagation of wavepackets on the double-minimum potential curve and of their associated photoelectron spectra. Photoelectron angular distributions have been given for different orientations of linearly polarized pump and probe pulses. The resulting photoelectron spectra illustrate the importance of a proper description of the underlying photoionization amplitudes and their dependence on geometry for unravelling wavepacket dynamics from pump-probe photoelectron signals in non-adiabatic regions where the electronic structure evolves rapidly with geometry. The present theory was extended so as to treat a system of nonadiabatic transition such as that in NaI molecule, whose ground and first excited potential curves intersect with each other due to intramolecular electron transfer. It has been shown for the first time that the bifurcation of nuclear wavepackets can be directly and experimentally observed with the pump-probe photoelectron spectroscopy. This is of great importance both in chemistry, wave-pacekt engineering, quantum chaos, the fundamental quantum theory, and so on.