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

光电子圆二色性（PECD）是一种基于电偶极子跃迁的CD效应，与普通的CD效应相比，它的跃迁更大。最近，我们在2+1共振增强多光子电离（REMPI）方案中，在400 nm激发波长下，用飞秒激光脉冲对随机定向的气相樟脑、枫香和去樟脑分子测量了高达10%的PECD。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具有相似性，但仍观察到贡献勒让德多项式的不同调制和振幅。我们的强度研究表明解离电离是PECD效应的起源，中间共振的电离是主导信号。到目前为止，对多光子情况下的PECD缺乏一致的理论描述，中间体的作用在理论上也不清楚。由于几个理论小组目前正在研究这一主题，预计会取得进展。本文提出将PECD测量提升到一个新的水平，利用锁相双色（400 nm / 800 nm）偏振定制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。实验旨在研究光子角动量对共振态和连续态的影响，以及在异常偏振场中研究PECD。利用双色偏振定制激光场对电离动力学进行系统的研究，目前还没有得到普遍的应用。这部分是由于缺乏适当的光学装置来产生这些定制的光场。因此，本提案的目标之一是实现和表征一个锁相双色双极化装置，该装置具有两个辐射场的独立可选偏振状态和强度，以及允许从光干涉制度（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针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，我们将首次将这种场应用于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开发的层析重建技术推断出来。在各种不同的物理主题中，我们期望从两种具体方法中获得有希望的结果：我们将研究使用激光制备的非各向同性分布可以在多大程度上增加PECD效应。我们将在不寻常的极化场中研究PECD，例如“三叶草”或“蝴蝶”形状，后者可能在一个极化场中开辟一条路线来产生PECD效应。

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.