Where have all the electrons gone?

The process whereby an electron is dissolved in water is one of the longest standing problems in physical chemistry despite its central role in many areas, including cancer therapy. Even thought it is conceptually simple, no complete description exists, partly because of the ultra-fast nature of the process. This computational project will develop a multi-scale model of the solvation process, benefitting from close collaboration and data exchange with experimentalists using Taranis, a state-of-the-art high-power laser facility at QUB, and Diamond, the world's brightest mid-energy synchrotron facility in Harwell, Oxfordshire.

At present, the resolution of the experiments is not yet capable of probing the femtosecond to picosecond regime, in which the excited electrons are expected to thermalize and diffuse. The understanding of this regime is crucial to describe and control the processes that occur at later times, e.g. the evolution of the optical properties and the possibility of thermal spikes or Coulomb explosions.

Through a close collaboration between experimentalists and theoreticians a first phenomenological model has been developed to explain the time-evolution of the absorbance (how much light is absorbed by the sample) observed in the experiments [1]. This model, however, involves certain parameters of yet unclear origin, like the duration of the electronic excitations (recently shown in Taranis to be much longer than the proton pulse). The understanding of such processes requires supplementing experiments and phenomenological modelling with first-principles numerical simulations of the irradiation process. This should be done by solving the coupled dynamics of the excited electrons and the moving incident ion, as done for example in [2], where the coupled proton-electron dynamics was treated within a first-principles molecular dynamics framework. Specifically, the electron dynamics was treated within time-dependent density functional theory (TDDFT), while the ion and the atomic nuclei in the target were treated classically. However, in TDDFT the electron dynamics misses effects responsible for thermalisation and, on a longer time-scale, for returning the electronic system to equilibrium. Those effects are naturally included within the non-equilibrium Green's function (NEGF) framework [3] and also in a method under development that implements, efficiently, a correlated electron-phonon dynamics [4].

The goal of this project is to integrate the various sizes and time scales involved in the process of ion irradiation into a multi-scale framework that begins with the highly accurate NEGF approach, through (TD)DFT and classical force fields, and ends up with phenomenological models that are informed by the atomistic simulations. A particularly attractive challenge here is to develop methodologies to connect the various approaches.

[1] B. Dromey et al. Nature Communications 7, 10642 (2016)

[2] A. A. Correa, J. Kohanoff, E. Artacho, D. Sanchez-Portal and A. Caro, Phys. Rev. Lett. 108, 213201 (2012)

[3] C. Attaccalite, M. Gruening, A. Marini, Phys. Rev. B 84, 245110 (2011)

[4] V. Rizzi, T. N. Todorov, J. Kohanoff, and A. A. Correa, Phys. Rev. B 93, 024306 (2016)