High-intensity laser interactions with matter produce extreme environments with very high temperatures and densities such that the electrons within the atoms of the material no longer remain bound to the atomic nuclei and the material becomes a plasma. These interactions can create conditions for studying astrophysical phenomena, including supernova shocks and solar flares, as well as supporting very high electric fields that can be used to accelerate charged particles over distances 100s to 1000s times shorter than the limits of radio-frequency accelerator technology. These compact accelerators have been shown to generate ion beams with highly desirable properties for key applications in materials testing, radiobiology, and inertial fusion energy. So far, full exploration and exploitation of these interactions has been hampered by the difficulty in reproducing their complex behaviour in numerical and computational models and by the limited data available which is caused by the low repetition rate of the high-energy pulsed laser (typically <<0.002 Hz - a shot every 10 mins) used to create the plasma and drive particle acceleration. This is particularly the case in the study of fragile ultra-thin opaque targets where the absorption of energy from the laser causes the target to heat and expand leading to the target becoming transparent as the density falls. When this occurs the laser can propagate through the target and the transfer of laser energy to the plasma is no-longer localised at the target surface. This interaction is of significant interest as it is here that the highest energy laser-accelerated protons have been recorded. A new generation of multi-Hz high-energy laser-technology is facilitating orders of magnitude increase in data acquisition rate. In order to exploit these new lasers, it is also necessary to test target technology that can provide fresh ultra-thin foils with high positional stability at multi-Hz repetition rate. In addition, despite the enormous increase in data acquisition-rate the dependence of the interaction dynamics on a large number of variables (e.g. laser energy, laser spatial and temporal energy distribution, target density profile) means that `grid-scanning' each parameter is not an efficient method to map their interdependence. By incorporating machine learning tools the high data rate enabled by the lasers and target can be used to intelligently sample the parameter space to model the interaction and quantify the stability of these novel accelerators. The proposed collaboration will address this challenge by coupling a liquid sheet target, developed at the US SLAC National Accelerator Laboratory (SLAC), with a new computer-guided approach to laser-plasma experiments, pioneered by researchers at Queen's University Belfast (QUB). The development of this novel experimental platform will enable deeper understanding of the key energy transfer pathways between laser and plasma and their dependence on experimental variables. The research will directly impact on plasma modelling, advanced accelerator research, plasma astrophysics, inertial confinement fusion, materials testing and FLASH radiobiology. The research outputs will feed into EPSRC 2022-2025 strategic priorities on the physical and mathematical sciences powerhouse, frontiers in engineering and artificial intelligence up-skilling through the research themes: AI and Data Science for Engineering, Health and Government by exploiting AI for experimental science; Energy through inertial confinement fusion; Plasma and lasers by developing crucial technology to facilitate deeper understanding and broader exploitation of novel radiation sources; and research infrastructure by enhancing the capabilities of high-intensity laser facilities.

All material systems are made up of positively charged nuclei, and negatively charged electrons. The way in which the arrangement of electrons and nuclei respond to external stimuli, such as photons from light, determines their physical and chemical properties. The electrons in materials do not behave as independent particles, and their position and momenta are highly correlated due in part to their mutual Coulombic repulsion. An approach to probing and understanding the correlated behaviour of electrons is to remove one of the electrons and then observe how the remaining electrons adjust in response to the sudden removal of an electron. That is precisely the approach we will follow here; we will employ a short X-ray pulse from a free-electron laser to remove an electron from the Glycine molecule leaving a "hole" in the molecule, and then, with a second time-delayed short X-ray pulse we will probe the evolution of that hole as the electrons adjust. The second "probe" pulse will excite an inner core electron to the energy where the hole was created. As the other electrons readjust, the accessibility of that hole to the core electron will vary. As such, the probability of re-populating the hole with the core electron will evolve in time, providing us with a way to view the electron motion in the molecule. This experiment requires two synchronised laser pulses with duration of 5 fs and with a photon energy of 280 eV. The only place in the world that light pulses with these characteristics are available is at the Linear Coherent Light Source (LCLS) facility at Stanfrod, US.

Light triggers many important chemical reactions. These include photosynthesis (converting sunlight to chemical energy), human vision (detecting photons via light-induced changes in molecules), and new technologies such as photodynamic therapy for cancer, photocatalysis, fluorescent tags for healthcare diagnostics, and photovoltaics. Light-triggered processes in molecules are difficult to study experimentally and involve a complex interplay of concerted changes in molecular structure and rapid rearrangements of the electrons in the molecule. Conical intersections play a decisive role for the outcome of photochemical reactions, analogous to that of a transition state in standard ground-state chemistry. These are regions on photochemical pathways where molecules can transition efficiently between electronic states. Being able to map the path of molecules through conical intersections would open avenues to controlling photochemical reactivity via modification of excited state dynamics. To achieve this we must simultaneously observe the electronic characteristics of the molecule and the corresponding changes in molecular structure. The challenge is compounded by the short timescales involved, on the order of femtoseconds. Notably, there are as many femtoseconds in a second as there are seconds in 30 million years. In contrast, standard techniques for structural determination require long observation times. New facilities known as X-ray Free-Electron Lasers (XFELs) deliver extremely short pulses of intense high-energy x-ray photons, making completely new types of measurements possible. In recent work, we have demonstrated that we can track the changes in molecular structure in excited molecules and, in separate experiments, detect the nearly instantaneous re-arrangement of electrons when molecules absorb light. Exploiting these advances, the proposed project will develop measurements that track the motion of electrons alongside the motion of the nuclei, allowing conical intersections to be identified, and the structure of molecules at conical intersections to be determined. The resulting experimental technique will yield a powerful tool for fundamental research and provide images of electrons and nuclei that can be used to customise photoactive molecules, ultimately contributing to new technologies in catalysis, new cancer treatments, and energy harvesting from sunlight.

Light triggers many important chemical reactions. These include photosynthesis (converting sunlight to chemical energy), human vision (detecting photons via light-induced changes in molecules), and new technologies such as photodynamic therapy for cancer, photocatalysis, fluorescent tags for healthcare diagnostics, and photovoltaics. Light-triggered processes in molecules are difficult to study experimentally and involve a complex interplay of concerted changes in molecular structure and rapid rearrangements of the electrons in the molecule. Conical intersections play a decisive role for the outcome of photochemical reactions, analogous to that of a transition state in standard ground-state chemistry. These are regions on photochemical pathways where molecules can transition efficiently between electronic states. Being able to map the path of molecules through conical intersections would open avenues to controlling photochemical reactivity via modification of excited state dynamics. To achieve this we must simultaneously observe the electronic characteristics of the molecule and the corresponding changes in molecular structure. The challenge is compounded by the short timescales involved, on the order of femtoseconds. Notably, there are as many femtoseconds in a second as there are seconds in 30 million years. In contrast, standard techniques for structural determination require long observation times. New facilities known as X-ray Free-Electron Lasers (XFELs) deliver extremely short pulses of intense high-energy x-ray photons, making completely new types of measurements possible. In recent work, we have demonstrated that we can track the changes in molecular structure in excited molecules and, in separate experiments, detect the nearly instantaneous re-arrangement of electrons when molecules absorb light. Exploiting these advances, the proposed project will develop measurements that track the motion of electrons alongside the motion of the nuclei, allowing conical intersections to be identified, and the structure of molecules at conical intersections to be determined. The resulting experimental technique will yield a powerful tool for fundamental research and provide images of electrons and nuclei that can be used to customise photoactive molecules, ultimately contributing to new technologies in catalysis, new cancer treatments, and energy harvesting from sunlight.

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 probes 2/ 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