Probing molecular chirality and chiral dynamics using High Harmonic generation

我们项目的主要目标是开发对分子手性敏感的新方法，包括其结构和动力学。我们的目标包括在其自然的亚飞秒时间尺度上解析和控制分子中的多电子手性动力学。到目前为止，这种手性响应的超快时间尺度仍然隐藏在实验中。该方法基于随机取向分子(手性HHG，cHHG)气体的高次谐波产生。它的关键组成部分是应用定制的强脉冲来诱导、增强和操控手性响应。手性是生物的基本属性，对其功能至关重要。分子的左、右对映体具有相同的化学和物理性质，除非它们与另一个手性物体相互作用，例如手性光。旋转各向同性介质中手性识别的传统方法是线性光谱技术，它测量左右对映体对手性(例如圆偏振)光的光学响应的差异。在这些技术中，手势效应产生于光诱导的电偶极跃迁和磁偶极跃迁之间的相互作用。光-物质相互作用中的磁效应一般很弱。这会导致较弱的手部反应，通常比光吸收小四到六个数量级。手性响应的弱性对时间分辨测量提出了挑战。为了增强手性响应的持续努力，最近在非常具有挑战性的气相研究中取得了显着的进展。最灵敏的技术之一是光电子圆二色谱(PECD)，它预示着手性光学识别中的“偶极革命”：不使用手性光进行手性识别。在这一重大突破之后，几种新的方法完全在电偶极子近似下工作，包括对映灵敏微波探测、光激发圆二色谱(PXCD)和光激发诱导光电子圆二色谱(PXECD)。PXCD和PXECD的概念是在这个提议的第一阶段提出的，涉及手性电子或振动动力学的超快激发和探测。我们的理论统一而简单地描述了所有基于偶极近似的技术，并阐明了在没有手性脉冲的情况下实现手性识别的机制。有了这些知识，我们为关键的新步骤做好了准备：我们提议给手性氢化物带来“偶极革命”。使用新的cHHGd方法，我们的目标是实现手性二色性的高值-所有基于偶极的技术的标志，同时还能够以比现有最先进技术高出两个数量级的时间分辨率来同时成像潜在的手性动力学。

The main objective of our project is to develop new methods sensitive to molecular chirality, both its structure and dynamics. Our goals include resolving and controlling multielectron chiral dynamics in molecules at its natural, sub-femtosecond time-scale. So far, this ultrafast time-scale in chiral response has remained hidden in experiments. The proposed method is based on high harmonic generation from a gas of randomly oriented molecules (chiral HHG, cHHG). Its key component is the application of tailored intense pulses to induce, enhance and manipulate the chiral response.Chirality is a basic property of living matter, crucial for its function. Left and right enantiomers of molecules have identical chemical and physical properties unless they interact with another chiral object, such as chiral light. Traditional methods of chiroptical discrimination in rotationally isotropic media are linear spectroscopic techniques, which measure the difference of the optical response of left and right enantiomers to chiral (e.g. circularly polarized) light. In these techniques, the chiroptical effects arise from the interplay between the light-induced electric- and magnetic-dipole transitions. The magnetic effects in light-matter interaction are generally very weak. This results in a weak chiroptical response, often four to six orders of magnitude smaller than e.g. light absorption. The weakness of the chiral response poses challenges for time-resolved measurements.Constant efforts towards the enhancement of the chiral response have led to a remarkable recent progress in very challenging gas-phase studies. One of the most sensitive techniques is photoelectron circular dichroism spectroscopy, PECD, which heralded the "dipole revolution" in chiro-optical discrimination: chiral discrimination without using chiral light. Several new methods followed this remarkable breakthrough, working purely in electric dipole approximation, including enantio-sensitive microwave detection, photoexcitation circular dichroism (PXCD) and photoexcitation induced photoelectron circular dichroism (PXECD) . The concepts of PXCD and PXECD were developed in phase I of this proposal and involve ultrafast excitation and probing of chiral electronic or vibrational dynamics. Our theory provides a unified and simple description of all dipole-approximation based techniques and clarifies the mechanisms enabling chiral discrimination without chiral pulses. With this knowledge we are ready for the key new step: We propose to bring the "dipole revolution" to chiral HHG. With the new cHHGd method we aim to achieve high values of chiral dichroism – the hallmark of all dipole-based techniques, while also being able to simultanelously image the underlying chiral dynamics with the time resolution that is two orders of magnitude better than the available state-of-the art techniques.