Attosecond X-ray Spectroscopy of Ultrafast Dynamics in the Condensed Phase

我们都熟悉x射线可以“看到”物质内部的想法，毕竟这是许多成像技术的基础，从医用x射线到超人惊人的视力。我们计划使用新的x射线源，其独特的特性是它们的脉冲持续时间小于1飞秒（1 fs= 10^-15 s，即十亿分之一秒的百万分之一），使我们能够第一次对物质中发生的一些最快的事件拍摄快照，甚至制作电影。我们生产和使用这种短脉冲x射线的能力来自于我们过去六年的研究（由ERC高级资助和EPSRC项目资助）。我们证明了基于高谐波产生（HHG）的脉冲持续时间为~0.3 fs的源可以从150 - 600 eV产生，这是超快x射线源前所未有的成就。结合我们最近发明的超薄液体片射流，以及我们实验室中可以通过电子激发材料启动时钟的飞秒光脉冲，我们现在拥有了一种新的超快x射线光谱工具，可以应用于任何系统，无论是气体、液体还是固体状态。我们现在想用这种方法来研究物理和化学中一些最快的过程，从而把这项研究推向新的方向。在激光研究的同时，我们一直在与国际团队合作，开发使用x射线自由电子激光设备在几飞秒时间尺度上进行测量的方法。这包括与LCLS（加州SLAC）在双脉冲/双色模式下的工作，时间分辨率约为3fs。今年晚些时候，预计LCLS将产生第一个基于FEL的亚飞秒x射线脉冲，其亮度是我们的HHG源的10^6倍。我们计划利用这种新能力来开发x射线非线性光谱学的新概念，这将使我们能够在原子空间分辨率和比飞秒更快的时间尺度上精确地跟踪物质中的电子运动。我们将用这些非凡的新工具来研究什么？答案是物理学中的基本动力学事件，如激子形成和电荷迁移，以及化学中的关键过程，如电子转移和键断裂/形成。这些可以在初始电子激发后10秒内发生，迄今为止还无法直接测量。此外，利用我们的工具，我们可以在量子和经典行为之间的时域边界上跟踪微观系统的动力学。在凝聚态系统中，初始态的量子相干性在10飞秒内消失，这将使我们能够看到一个新的量子动力学体系。我们将特别关注包含离域电子的结构（π共轭系统），因为在这种情况下，电子是高度可移动的，因此可以显示出最快的动力学。此外，π共轭分子是聚合物和分子复合物的基本组成部分，在光化学和太阳能转换中具有重要意义。我们的测量不仅能捕捉到这些系统中的超快电子运动，还能让我们测量与异构化、开环和其他化学变化相关的结构动力学。我们的研究将扩展科学在时域测量能力的前沿，以比以前快100倍的时间尺度测量物理过程的可能性。这将导致我们对自然界和技术中的量子动力学过程的理解取得新的突破。

We are all familiar with the idea that X-rays can "see" inside matter, after all this is the basis of much imaging technology from medical X-rays to Superman's astounding vision. We plan to use new sources of X-rays with the unique property that they are in pulses of duration less than 1 femtosecond (1 fs= 10^-15 s that is a millionth of a billionth of a second) to allow us, for the first time, to take snapshots and even make movies of some of the fastest events that occur within matter. Our ability to produce and use such short pulses of X-rays has come out of our research over the past six years (funded by an ERC Advanced Grant and an EPSRC Programme Grant). We showed that high harmonic generation (HHG) based sources of ~0.3 fs pulse duration could be generated from 150 - 600 eV, an unprecedented achievement for ultrafast X-ray sources. Combined with the ultra-thin liquid sheet jets that we have recently invented and few-femtosecond optical pulses available in our laboratory that can start the clock by electronic excitation of the material, we have now in place the tools that enable a new kind of ultrafast X-ray spectroscopy that can be applied to any system whether it is in gas, liquid or solid state. We now want to advance this research in new directions by using this method to investigate some of the fastest processes in physics and chemistry. In parallel with our laser research we have been working with international teams to develop the methods of using X-ray free electron laser facilities to make measurements on the few-femtosecond timescale. This has included work with LCLS (SLAC, California) in a two-pulse/two-colour mode with ~ 3 fs temporal resolution. Later this year it is anticipated that LCLS will produce the first FEL based sub-femtosecond X-ray pulses of extreme brightness (> 10^6 times greater than our HHG source). We plan to use this new capability to develop a new concept in X-ray non-linear spectroscopy that will allow us to precisely follow electron motions in matter at atomic spatial resolution and with time-scales faster than a femtosecond.What will we investigate with these remarkable new tools? The answer is the fundamental dynamical events in physics, such as exciton formation and charge migration, and key processes in chemistry, such as electron transfer and bond-breaking/making. These can occur within 10 fs of initial electronic excitation and have hitherto not been accessible to direct measurement. Moreover with our tools we can track the dynamics of microscopic systems across the boundary in the temporal domain between quantum and classical behaviour. In condensed phase systems, where the quantum coherence of the initial state is lost in a few 10's of femtoseconds, this will allow us to see into a new quantum regime of dynamics. In particular we will focus on structures containing delocalised electrons (pi-conjugated systems) as in this case the electrons are highly mobile and so can display the very fastest dynamics. Additionally pi-conjugated molecules are the building blocks of polymers and molecular complexes of great interest in photochemistry and solar-energy conversion. Not only will our measurements capture the ultrafast electronic motion in these systems they will also allow us to measure the structural dynamics associated with isomerization, ring opening and other chemical changes.Our research will extend the frontier of science's measurement capability in the time domain with the likelihood of measuring physical processes at timescales 100 times faster than before. This will lead to new breakthroughs in our understanding of quantum dynamical processes in nature and technology.