AQuA DIP: Advanced Quantum Approaches to Double Ionisation Processes

为了“观察”原子中的电子，我们需要能够在与相互作用本身相当的时间尺度上描述它们的运动。这类似于拍摄照片-物体移动得越快，拍摄它所需的曝光时间就越短。在原子和光学物理学中，我们有效地使用超短“相机”闪光（激光脉冲）来拍摄运动中的电子。虽然阿秒物理学（1阿秒=十亿分之一秒的十亿分之一）的这一领域已经建立得很好，但其基本机制的理论描述和计算模型还相对落后。在这个项目中，我们特别感兴趣的是双电离过程：两个电子从原子靶中逃逸的机制。这种兴趣有两方面的技术动机。首先，X射线自由电子激光系统（简称XFEL）允许传递大量高能辐射，能够用一个或两个光子多次电离目标。其次，能够在可见光到中红外区提供短而强的激光脉冲的桌面光源在所谓的“强场”区驱动动力学，其中激光的电场足够强以扭曲将电子结合到目标的势阱，使它们逃逸。诸如“非顺序双电离"（NSDI）或”共振激发和随后的电离“（RESI）等过程是令人着迷的，因为它们不仅取决于激光场的特性，还取决于许多电子之间发生的详细而复杂的相互作用。由于这种复杂性，缺乏能够为这些过程提供理论见解的方法。计算或分析方法往往专注于一个特定的制度（XFEL或强场），即使这样，许多人选择专注于激光驱动的动力学，忽略了图片的电子相互作用部分。出于这个原因，在这个项目中，我们将开发三种互补的方法来解决原子中的双电离动力学。一种是具有时间依赖性的R矩阵，或“RMT”方法，它将使用高性能计算技术将极其复杂的计算分解为较小的部分，由数千个处理器并行解决。第二种是库仑量子强场近似，或“CQSFA”，这是一种基于电子轨迹的分析技术。这增强了CQSFA的解释能力，因为它提供了一个清晰的动力学物理图像，这有时会被RMT所采用的完整的量子波函数图像所掩盖。我们将开发的第三种方法是RMT和CQSFA的混合：两者都是最好的：对核心动力学（激光场中原子靶的行为）进行复杂而准确的处理，并对连续动力学（电离电子的行为）进行清晰的解释。在开发了这三种互补的工具之后，我们将把它们应用于XFEL和强场激光脉冲中的双电离问题。特别是，我们将解决这些过程中包含了强烈的影响，从激光场和量子力学行为的目标，因为这是制度基本上不服务于当前的方法。因此，我们将进行NSDI和RESI的研究，以及核心电离后的俄歇衰变和单光子双电离驱动的XFEL光。了解这些机制将有助于我们了解所有光介导的电子过程，包括重要的生物作用，如光合作用和眼睛的运作。attoscience的最终目标是控制这些过程，并实现对社会的潜在利益。工具，如在这个项目中开发的工具，在这方面可能会证明是无与伦比的价值。

In order to 'view' electrons in the atom, we need to be able to describe their motion on a time-scale comparable to the interactions themselves. This is akin to taking a photograph- the faster an object is moving, the shorter the exposure time required to capture it. In atomic and optical physics we effectively use ultrashort 'camera' flashes (laser pulses) to image electrons in motion. While this field of attosecond physics (1 attosecond = 1 billionth of a billionth of a second) is well established, the theoretical description and computational models of the underlying mechanisms are relatively underdeveloped.We are particularly interested, in this project, in double-ionisation processes: mechanisms whereby two electrons escape from an atomic target. There is a two-fold technological motivation for this interest. Firstly, X-ray Free-Electron Laser systems (or XFELs for short) allow the delivery of massively energetic radiation capable of multiply ionising a target with just one or two photons. Secondly, table-top sources capable of delivering short, intense laser pulses in the visible to mid-infrared regime drive dynamics in what is known as the 'strong-field' regime, where the electric field of the laser is strong enough to distort the potential well binding the electrons to the target, allowing them to escape. Processes such as 'non-sequential double ionisation' (NSDI) or 'resonant excitation and subsequent ionisation' (RESI) are fascinating because they depend not only on the characteristics of the laser field, but also on the detailed and complex interactions taking place between the many electrons.Because of this complexity, there is a dearth of methods capable of providing theoretical insight to these processes. Computational or analytical methods tend to specialise in one particular regime (either XFEL or Strong-field) and even then, many choose to focus on the laser-driven dynamics, ignoring the electronic-interaction part of the picture. For this reason, in this project we will develop three complementary methods to address double-ionisation dynamics in atoms. One is the R-matrix with time-dependence, or 'RMT' approach, which will use techniques of high-performance computing to break the immensely complex calculation into smaller parts, solved in parallel by many thousands of processors. The second is the Coulomb Quantum Strong Field Approximation, or 'CQSFA', which is an analytical technique based on electron trajectories. This lends enhanced interpretational power to the CQSFA, because it provides a clear physical picture of the dynamics, which can sometimes be obscured in the full, quantum-wavefunction picture employed by RMT. The third approach we will develop is a hybrid of RMT and CQSFA: giving the best of both: a sophisticated and accurate treatment of the core dynamics (the behaviour of the atomic target in the laser field) and a clear interpretation of the continuum dynamics (the behaviour of the ionised electrons). Having developed these three, complementary tools we will apply them to problems of double ionisation in XFEL and strong-field laser pulses. In particular we will address those processes which contain a strong influence from both the laser field and the quantum mechanical behaviour of the target, as this is the regime largely unserviced by current methodology. Hence we will perform studies of NSDI and RESI, as well as core-ionization followed by Auger decay and single photon double ionization driven by XFEL light. Understanding these mechanisms will inform our understanding of all light-mediated electronic processes, including important biological actions such as photosynthesis and the operation of the eye. The ultimate goal of attoscience is to control these processes and realise the potential benefits for society. Tools, such as those developed in this project, may prove to be incomparably valuable in this pursuit.