Controlling photophysics and photochemistry via quantum superpositions of electronic states: towards attochemistry

当分子吸收足够能量的光时，产生激发电子态。电子的分布--化学键--然后发生变化，导致原子核相应地移动。这样的电子变化相对于随后的核运动来说似乎是瞬时的。我们现在知道这些电子的变化不是瞬时的，但它们通常比核运动快得多，这限制了我们迄今为止控制它们的影响的范围。我们打算探索激光操纵电子态运动如何能够提供对光化学的前所未有的控制：化学反应是由电子激发态中的键合变化引发的。我们能否通过控制一个分子的电子态的相干量子叠加的初始演化来指导分子中光化学过程的结果？我们提出的研究将探索一种新的方法来控制分子系统中的动力学在一个重要的跨学科的交界处。它有望帮助我们理解和掌握超快化学过程，并将我们操纵物质量子态的能力扩展到阿秒时间域。最近阿秒分子物理学一直在探索“电荷迁移”的概念：分子或其他扩展量子系统中电子突然激发后的电子动力学。为了理解这种现象，我们必须认识到电子和原子核的量子性质。由于电子和核量子态的耦合，超快退相干被发现是快速和普遍的，发生在几十飞秒的时间尺度上。因此，在退相干消除我们的控制范围之前，光激发量子态动力学的控制只能通过在更快的时间尺度上施加光场来实现。这项研究的一个中心目标是通过圆锥形交叉点附近的超快光场控制量子演化：电子态之间的交叉可以导致多种化学途径的分子几何结构。控制这里将使我们控制不同的化学结果。光脉冲的激发-控制-探测序列将被应用于选定的分子。几个飞秒的紫外激发脉冲将启动电子态叠加，随后是几个周期的红外脉冲后，短和精确控制的时间延迟。当系统流经临界圆锥交叉点时，该脉冲序列将操纵状态之间的相干性。通过以这种方式在几十飞秒的时间间隔内改变叠加-在比退相干更快的时间尺度上-我们将改变量子演化路径和最终结果。这一路径将通过X射线光谱学与亚飞秒X射线脉冲进行实时测量，这是一种对分子结构和电子状态具有高灵敏度的技术。计算机模拟使用最先进的代码和大量的计算能力来解决耦合的电子-核运动将被用来预测和解释实验。我们将研究小的，孤立的，分子系统的动力学和控制在这个建议。尽管如此，我们通过这项研究所学到的东西很可能适用于许多其他具有超快化学动力学的光吸收系统的量子尺度操纵。因此，这项工作与广泛的纳米系统有关：纳米颗粒，催化复合物，生物分子，有机光电子学，二维材料和其他先进材料。除了提供对分子基本行为的新见解外，我们正在研究的超快量子科学可能会导致未来的量子设备，其中量子系统内的电荷，能量和信息的流动可以由超快光场控制。

When molecules absorb light of sufficient energy, an excited electronic state is generated. The distribution of electrons - the chemical bonding - then changes, causing the nuclei to move in response. Electronic changes such as these appear to be instantaneous relative to the nuclear motion that follows. We now know that these electronic changes aren't instantaneous, but they are usually much faster than the nuclear motions, which has limited our scope for controlling their effects until now.We propose to explore how laser manipulation of electronic state motion can offer unprecedented control over photochemistry: chemical reactions that are initiated by changes in bonding in electronic excited states. Can we direct the outcome of a photochemical process in a molecule by controlling the initial evolution of a coherent quantum superposition of its electronic states? Our proposed research will explore a new approach to controlling dynamics in molecular systems at an important interdisciplinary junction. It promises to benefit our understanding of - and mastery over - ultrafast chemical processes, and to extend our ability to manipulate quantum states of matter into the attosecond time domain.Recently attosecond molecular physics has been exploring the concept of "charge migration": electronic dynamics following sudden excitation of an electron in a molecule or other extended quantum system. To understand such phenomena we must recognise the quantum nature of both electrons and nuclei. Ultrafast decoherence due to coupling of the evolving electronic and nuclear quantum states is found to be rapid and general, taking place on a timescale of a few tens of femtoseconds. The control of photoexcited quantum state dynamics can therefore only be achieved with light fields applied on a faster timescale, before decoherence removes our scope for control. A central target of this research is the control of quantum evolution by ultrafast light fields in the vicinity of conical intersections: molecular geometries where crossings between electronic states can lead to multiple chemical pathways. Control here will give us control over different chemical outcomes. Excitation-control-probe sequences of light pulses will be applied to selected molecules. A few-femtosecond UV excitation pulse will initiate the electronic state superposition, followed by a few-cycle infrared pulse after a short and precisely controlled time delay. This pulse sequence will manipulate the coherence between states as the system flows through the critical conical intersection. By varying the superposition within a time interval of a few tens of femtoseconds in this way - on a time-scale faster than decoherence - we will change the quantum evolution path and final outcomes. This path will be measured in real-time via X-ray spectroscopy with sub-femtosecond X-ray pulses, a technique with a high sensitivity to molecular structure and electronic states. Computer simulations using state-of-the-art codes and substantial computing power to solve the coupled electronic-nuclear motions will be used both to predict and to explain the experiments.We will study the dynamics and control of small, isolated, molecular systems in this proposal. Nevertheless, it is likely that what we will learn through this research will be applicable to the quantum scale manipulation of many other light-absorbing systems with ultrafast chemical dynamics. This work is therefore pertinent to a wide range of nanoscale systems: nanoparticles, catalytic complexes, biomolecules, organic optoelectronics, two-dimensional materials and other advanced materials. As well as providing new insight into the fundamental behaviour of molecules, the ultrafast quantum science we are researching may lead to future quantum devices where the flow of charge, energy and information within a quantum system can be controlled by ultrafast light fields.