Phase locked bichromatic polarization tailored femtosecond laser fields to study and control electron dynamics in chiral molecules.

光电子圆二向色性(PECD)是一种基于电偶极跃迁的CD效应，与普通的CD效应相比，它的电偶极跃迁更大。最近，我们在2-1共振增强多光子电离(REMPI)方案中测量到了高达10%的PECD。该方案采用飞秒激光脉冲，在400 nm激发波长下，在气相中对随机取向的樟脑、芬香酮和降樟脑分子进行了激发。我们观察到高阶勒让德多项式的贡献高达吸收光子数的两倍。尽管在化学结构和吸收光谱上相似，但贡献勒让德多项式的调制和幅度都不同。我们的强度研究表明，离解电离是PECD效应的来源，而中间共振的电离是信号的主导。到目前为止，对于多光子情况下的PECD缺乏一致的理论描述，中间体的作用在理论上也不清楚。随着几个理论小组目前正在就这一主题开展工作，预计将取得进展。在这里，我们建议将PECD测量提高到一个新的水平，并使用锁相双色(400 nm/800 nm)偏振定制的飞秒激光场来研究和控制手性分子中的电子动力学，以上述双环酮为原型。实验的目的是研究光子角动量对共振和连续态的影响，并研究反常偏振场中的PECD。到目前为止，还没有人利用双色偏振定制的激光场对电离动力学进行系统的研究。这在一定程度上是由于缺乏适当的光学设置来创建这些定制的光场。因此，该方案的目标之一是实现和表征具有两个辐射场的独立可选择偏振态和强度的锁相双色双偏振设置，以及允许从光学干涉制度(两个辐射场的时间重叠)到纯量子干涉制度(时间分离脉冲)和具有衰减脉冲的泵浦-探测制度的相位稳定调谐。除了对非手性钾原子的设置进行测试外，我们还将首次应用这种场来电离随机取向的手性分子。三维动量分布可以从我们实验室最近发展起来的层析重建技术中推断出来。在一系列不同的物理主题中，我们期待从两个具体的方法中获得有希望的结果：我们将调查使用激光制备的各向同性分布可以在多大程度上增加PECD效应。我们将研究像“三叶草”或“蝴蝶”形状这样的不寻常偏振场中的PECD，后者可能会开辟一条在一个偏振场中产生PECD效应的途径。

Photoelectron Circular Dichroism (PECD) is a CD effect based on an electric dipole transition being large in comparison to ordinary CD effects. Recently we have measured a PECD up to the ten percent regime in a 2+1 resonance enhanced multi photon ionization (REMPI) scheme with femtosecond laser pulses on randomly oriented molecules of Camphor, Fenchone and Norcamphor in the gas phase at an excitation wavelength of 400 nm. We observed contributions from higher order Legendre polynomials up to two times the number of photons absorbed. Different modulations and amplitudes of the contributing Legendre polynomials are observed despite the similarity in chemical structure and absorption spectrum. Our intensity studies revealed dissociative ionization as the origin of the PECD effect and ionization of the intermediate resonance is dominating the signal. So far there is a lack of a consistent theoretical description of the PECD in the multiphoton case and the role of the intermediate is theoretically unclear. Progress is expected as several theoretical groups are currently working on this topic. Here we propose to put PECD measurements to a new level and use phase locked bichromatic (400 nm / 800 nm) polarization tailored femtosecond laser fields to study and control electron dynamics in chiral molecules, where the above mentioned bicyclic ketones serve as prototypes. The experiments are directed to study the influence of the photons angular momentum on resonances and continuum states and to study PECD in unusual polarization fields. A systematic study using bichromatic polarization tailored laser fields on the ionization dynamics in general has not been performed so far. This is partly due to the lack of a proper optical set-up to create these tailored light fields. One of the objectives of this proposal is therefore the implementation and characterization of a phase locked bichromatic bipolarization set-up with independent selectable polarization states and intensities for the two radiation fields as well as allowing for phase stable tuning from the optical interference regime (temporal overlap of the two radiation fields) to the pure quantum interference regime (temporally separated pulses) and a pump-probe regime with attenuated pulses. Besides a test of the set-up on achiral potassium atoms, we will for the first time apply such fields to ionize randomly oriented chiral molecules. The three dimensional momentum distribution can be inferred from tomographic reconstruction techniques developed recently in our laboratories. Amongst a whole variety of different physical topics, we expect promising results from two specific approaches: We will investigate to what extent the PECD effect can be increased using laser prepared nonisotropic distributions. We will study PECD in unusual polarization fields like for example 'cloverleaf' or 'butterfly' shapes, where the latter might open a route to create a PECD effect within one polarization field.