Imaging chemical dynamics through laser-induced electron diffraction in the molecular-frame

激光诱导电子衍射（LIED）是一种可以对分子结构和结构动力学进行原子分辨率成像的方法。再散射的电子，从强场电离的分子发出的强烈的中红外脉冲，已知包含的信息的准确（时间依赖）的结构的简单分子，可以提取记录化学动力学的“分子电影”。然而，将这种技术应用于复杂分子和分子动力学需要对分子样品进行强有力的控制，需要根据尺寸，结构异构体和量子状态进行空间分离，并进行强对齐和定向。在这里，我们着手推进方法，以强有力地控制复杂分子的分子样品，并使用它们来记录分子的精确结构和它们的动态使用LIED。我们将使用超音速膨胀来创建冷分子束，并根据量子状态分散这些束，以创建单个状态，物种或簇大小的纯样品。随后，分子将强烈的三维排列和定向使用中等强度，定制的激光和直流电场。这些样品，所有分子在实验室框架中看起来都是一样的，将受到强烈的中红外飞秒脉冲的照射。很强的电场会使分子加速，并使产生的电子加速。最终，电子将重新散射在留下的分子离子上。我们将测量这些电子在分子框架中的动量分布，并提取电子衍射图案，将其反转以产生分子的精确结构。加入超短紫外脉冲启动化学动力学将使我们能够进行泵浦-探测实验，并记录光引发动力学的快照。此外，我们将发展严格的理论模型，将实验数据转化为原子分辨率的分子结构和动力学电影。我们将对复杂的多原子非对称顶部分子和分子簇进行这些研究，例如，原型肽生色团吲哚及其水簇，研究结构重排反应和所谓的半碰撞，以创建这些复杂的化学动力学过程的清晰图像，具有高时空分辨率。所研究的体系范围从OCS分子的解离动力学到吲哚-水团簇中的溶剂-溶质相互作用。我们的研究结果将为化学和化学反应的分子基础提供新的见解。此外，这些方法的成功实施将为受控分子和强场物理在（结构）生物学和（生物）化学中的应用开辟道路。

Laser-induced electron diffraction (LIED) is an approach that allows for the atomic resolution imaging of structures and structural dynamics of molecules. Re-scattered electrons, emitted from strong-field ionization of molecules by intense mid-infrared pulses, are known to contain information on the exact (time-dependent) structures of simple molecules that can be extracted to record a "molecular movie" of chemical dynamics. However, the application of this technique to complex molecules and molecular dynamics requires strong control over the molecular sample, which needs to be spatially separated according to size, structural isomer, and quantum-state and to be strongly aligned and oriented.Here, we set out to advance methods to strongly control molecular samples of complex molecules and to use them to record precise structures of molecules and their dynamics using LIED. We will create beams of cold molecules using supersonic expansions and disperse these beams according to quantum-state, in order to create pure samples of individual states, species, or cluster sizes. Subsequently, the molecules will be strongly three-dimensionally aligned and oriented using moderately strong, tailored laser and dc electric fields. These samples, with all molecules looking identical in the laboratory frame, will be irradiated by an intense, mid-infrared, femtosecond pulse. The very strong electric field will ionize the molecules and accelerate the produced electrons. Eventually, the electron will re-scatter at the molecular ion, which was left behind. We will measure the momentum distribution of these electrons, in the molecular frame, and extract the electron diffraction pattern that will be inverted to yield a precise structure of the molecule. Adding an ultrashort UV pulse to start chemical dynamics will allow us to perform pump-probe experiments and to record snapshot of photo-initiated dynamics. In addition, we will develop rigorous theoretical models to invert the experimental data into atomic resolution molecular structures and dynamics movies.We will implement these investigations for complex polyatomic asymmetric-top molecules and molecular clusters, e.g., the prototypical peptide-chromophore indole and its water cluster, to investigate structural rearrangement reactions and so-called half collisions, in order to create clear pictures of these complex chemical-dynamics processes with high spatio-temporal resolution. The investigated systems range from the dissociation dynamics of the OCS molecule to the solvent-solute interaction in indole-water clusters. Our results will provide new insight into the molecular basis of chemistry and chemical reactions. Furthermore, the successful implementation of these methods will open avenues for applications of controlled molecules and strong-field physics in (structural) biology and (bio)chemistry.