Liquid water is involved in almost all biological and chemical processes. Understanding radiation-induced chemical processes in condensed phase has importance for applications ranging from cancer therapies and space travel to environmental remediation of legacy nuclear waste. High-energy irradiation can ionize and electronically excite target atoms and molecules. These processes happening on the physico-chemical timescale, that is to say, within 10^-12 s, produce very reactive radical species, for example, the quasi-free electron and cationic hole (H₂O⁺) after the primary ionization. They will react amongst themselves and with the complicated environment, inducing damage that may be observed at longer time scale. Upon water radiolysis, the electron is quickly hydrated and has been a longstanding scientific problem and heavily studied, and the oxidizing OH radical formed from the cationic hole is now well understood experimentally as well as theoretically, while the ultrafast chemistry of the precursor H₂O⁺ has remained a very challenging topic.

The very fast processes following the photoionization of water has been revealed in our ab initio Ehrenfest dynamics simulations. They were performed to study both ultrafast coherence and non-equilibrium non-adiabatic energy transfer pathways upon photoionization of water. The cationic hole was created upon sudden ionization from the HOMO of the water nanodroplet. It quickly localized within 10 fs via hole trapping or hole transfer, leading to large OH oscillations. It was followed by proton transfer which results in an OH radical about 30~40 fs in our simulations. The resulting vibrationally-hot hydronium H₃O⁺ eventually shuttles one of its protons to another nearest neighbor water molecule 10~20 fs after its formation. During this stage, metastable Zundel complex ([H(H₂O)₂]⁺) is formed with a lifetime of 10~20 fs.

Scheme of valence hole dynamics upon water radiolysis from <i>ab initio</i> Ehrenfest simutions

Scheme of valence hole dynamics upon water radiolysis from ab initio Ehrenfest simutions

Andrew Wildman

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