Photoelectron spectra of water and simple aqueous solutions at extreme conditions

Determining the electronic structure of aqueous solutions at extreme conditions is an important step towards understanding chemical bonding and reactions in water under pressure (P) and at high temperature (T). We present calculations of the photoelectron spectra of water and a simple solution of NaCl under pressure at conditions relevant to the Earth's interior (11 GPa and 1000 K). We combine first-principles and deep-potential molecular dynamics with electronic structure calculations with dielectric-dependent hybrid functionals. These functionals are defined with a fraction of exact exchange determined from the dielectric constant of the liquid computed in extreme conditions. We find a broadening of the spectra relative to ambient conditions, particularly prominent in the merging of the two main peaks below the onset of the spectra. Furthermore we find an overall red shift at high pressure and temperature, which is however not constant over the whole energy range and varies between 1.1 and 2.4 eV. Our results also show that the anion energy levels are closer to the valence band maximum of the liquid than at ambient conditions, indicating that as P and T are increased, the defect levels of Cl^- and OH^- in water may eventually lie below the valence band maximum of water. Finally, we characterize the ionization potential of hydrated species deriving from rapid water dissociation, e.g. hydrated hydroxide and hydronium, and we elucidate the electronic states associated with proton transfer events at high pressure. Our results represent a first, important step in predicting the electronic properties of solutions in super-critical conditions.