Controlling photochemistry via quantum superpositions of electronic states: towards attochemistry

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In this project we will explore - using computational simulations - how laser manipulation of electronic motion in molecules might offer unprecedented control over photochemistry. The central question we seek to answer is: can we direct the outcome of a photochemical process by controlling the initial evolution of a coherent quantum superposition of electronic states in the vicinity of a crossing between these states? Recently attosecond molecular physics has been investigating the concept of "charge migration" i.e. electronic dynamics initiated by sudden excitation of an electron in a molecule or other extended quantum system. It is now clear that a full interpretation of such phenomena must recognise the quantum nature of both electronic and nuclear parts of the system. Decoherence due to the interplay of the evolving coupled nuclear and electronic quantum states is found to be exceptionally rapid and general, taking place on a timescale of a few tens of femtoseconds in all systems studied to date. Any control of photoexcited quantum state dynamics can therefore only be achieved by using light fields which are applied within this decoherence timescale. A target of particular significance is the control of quantum evolution by ultrafast light fields in the vicinity of conical intersections: crossings of potential energy surfaces where non-adiabatic electronic-nuclear couplings lead to crucial transitions between potential chemical pathways. Control here is ultimately control of chemical outcomes. Computer simulations using state-of-the-art code to solve the coupled nuclear-electronic motion will be used to predict and explain the experiments carried out as part of EP/T006943/1. As well as providing new insight into the fundamental behaviour of molecules, the ultrafast quantum science developed here 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.