Attosecond Electronic Dynamics of the Valence States in Matter Measured with XFELs

2019年首次演示了X射线自由电子激光(XFEL)产生的阿秒持续时间的单色和双色脉冲，开启了超快科学的新纪元。增强型自放大自发辐射(ESASE)，又称X射线激光阿秒脉冲(XLEAP)，用于在多GeV电子束进入波荡器之前对其进行控制，使其照射在单个大电流峰值上，产生持续数百阿秒的变换受限X射线脉冲。这一里程碑是在LCLS XFEL(斯坦福直线加速器中心)实现的，我们的团队在这项工作中发挥了重要作用，这些早期发展(发表在《自然·光子学2020》上)。我们很快使用这些脉冲进行了开创性的新科学研究，例如，首次观测到由于核心激发电子态的波包(叠加)的形成而导致的俄歇-迈特纳发射中的几飞秒量子拍子(发表在《科学2022》上)。XLEAP脉冲的显著特征不仅是它们持续时间很短，约为300阿秒，即十亿分之一秒的千分之三，以及X射线波长(从200 eV到1500 eV的光子能量)，而且这些脉冲具有数十微焦耳的能量(包含约100万个x射线光子)，使它们的强度比任何替代阿秒技术都高10亿倍。这种高亮度使超快X射线测量中的新概念成为可能。X射线泵浦探测测量电子价态动力学和电子拉曼激发需要高强度和阿秒脉冲宽度，我们将在我们提议的工作中使用这些测量来瞄准新的科学。我们将通过研究：(A)阿秒时间尺度的物质中的电子动力学来捕捉光激发的基本事件以及它如何驱动化学，(B)新型的电子介导的X射线非线性相互作用，有可能揭示物质中电子成键的全部动力学，以及(C)充分解释这些实验中的见解的理论能力的发展，重点放在物质的价/键态的超快电子动力学上。这项研究将在超快测量能力和科学认识上取得阶段性的变化。这项研究将为探索控制物质中最快转变的动力学事件开辟新的途径，并将：1)利用特定的位置和状态探测，以前所未有的分辨率(10^-16 S)测量所有相(即气体、等离子体、固体、液体)的物质2/提供对导致电荷迁移现象的短暂电子量子叠加态的访问，并追踪电子相干如何通过与核自由度的耦合而受到抑制(这是了解x射线辐射损伤和电荷定向化学反应的关键)3/允许电子激发的分子与其周围环境之间的相互作用得到解决，并跟踪如何在凝聚相环境中出现光化学和光物理过程(这是物质转换过程中能量和电荷流动的关键)4/提供一系列新的x射线非线性相互作用，能够揭示物质内部电子耦合的基本原理(这是控制量子动力学和测量的关键)5/加强英国的地位作为超快x射线科学的世界领先者，并为国家配备更多熟练的科学家，以开发未来的x射线自由电子激光机会

A new era in ultrafast science began with the first demonstration in 2019 of attosecond duration single and two-colour pulses from an x-ray free-electron laser (XFEL). Enhanced Self Amplified Spontaneous Emission (eSASE), aka x-ray Laser Attosecond Pulses (XLEAP), was used to control the multi-GeV electron bunch before it enters the undulators so that it lased on a single high current peak to generate a transform limited x-ray pulse of a few hundred attoseconds duration. This landmark was achieved at the LCLS XFEL (Stanford Linear Accelerator Centre) with our team playing a strong role in this work from these early developments (published in Nature Photonics 2020). We soon used these pulses to carry out ground-breaking new scientific research, e.g. by making the first observation of a few-femtosecond quantum-beat in Auger-Meitner emission due to the formation of wave-packets (superpositions) of core excited electronic states (published in Science 2022). The remarkable feature of XLEAP pulses is not only their very short duration of ~300 attoseconds, i.e. three hundredth millionth of a hundred millionth of a second, and x-ray wavelength (from 200 eV to 1500 eV photon energy), but that these pulses have tens of microjoule energy (containing about a million million x-ray photons) making them a billion times more intense than any alternative attosecond technology. This high brightness makes possible new concepts in ultrafast x-ray measurement. The high intensity and attosecond pulse duration are required for x-ray pump-probe measurements of electronic valence state dynamics and electronic Raman excitation that we will use to target new science in our proposed work. We will focus on the ultrafast electronic dynamics in the valence/bonding states of matter through investigating: (a) attosecond timescale electronic dynamics in matter to capture the fundamental events of photoexcitation and how it can drive chemistry, (b) new types of electronic mediated x-ray non-linear interactions with the potential to uncover the full dynamics of electronic bonding in matter, and (c) the development of the theoretical capabilities to fully interpret the insights from these experiments. Together this research will make a step-change in ultrafast measurement capability and scientific understanding. This research will open-up new ways to probe the dynamical events that control the fastest transformations in matter and will:1/ Enable ultrafast measurement at unprecedented resolution (10^-16 s) in matter of all phases (i.e. gas, plasma, solid, liquid) using site and state specific probes2/ Provide access to fleeting electronic quantum superposition states that lead to the phenomena of charge migration, and trace how electronic coherence is damped through coupling to the nuclear degrees of freedom (this is key to understanding x-ray radiation damage and charge directed chemical reactivity)3/ Allow the role of interaction between an electronically excited molecule and its surroundings to be resolved and to track how photochemical and photophysical processes emerge in a condensed phase environment (this is key to the flow of energy and charge during matter transformation)4/ Offer a new array of x-ray non-linear interactions capable of revealing the fundamentals of electronic coupling within matter (this is key to controlled quantum dynamics and measurement)5/ Enhance the UK position as a world leader in ultrafast x-ray science and equip the nation with more skilled scientists to exploit future x-ray FEL opportunities