AQuA DIP: Advanced Quantum Approaches to Double Ionisation Processes

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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.