The hydrated electron, e–(aq), which forms when an excess electron is introduced into water, has often served as a model system to understand the influence of condensed-phase environments on electronic structure and dynamics, in addition to its importance as a reactive species in radiation chemistry. Despite over 60 years of study, however, the solvation structure of e–(aq) is still controversial. The structure of e–(aq) was long assumed to be an electron localized within a solvent cavity, expelling the water molecules and behaving roughly as a particle in a quasi-spherical box. Recently, however, this picture was challenged by theoretical studies which suggested that e–(aq)’s wavefunction encompasses several waters in a structure with only a small or even no central cavity. Unfortunately, experiments have so far been unable to fully resolve this controversy as the competing models predict many of the same observables, including electron radius and optical absorption spectrum. Additional experimental probes of the structure of e–(aq) are therefore urgently needed. X-ray Absorption Spectroscopy (XAS), being sensitive to local geometric and electronic structure, might help resolve the controversy of e–(aq)’s structure; however, such an experiment has yet to be performed. Thus, in this study we use Time-Dependent Density Functional Theory to simulate a UV-pump XAS-probe spectrum of e–(aq). We consider three different structural models of e–(aq): (1) a cavity model using the TB potential,(2) a non-cavity model using the LGS potential, and (3) an intermediate structural model using the TB-OptPol potential. It is found that in all models there are features of the XAS associated with transitions to the hydrated electron’s energy levels, but the energetic location and shape of these features are model dependent. Thus, we suggest that XAS can serve as a useful experimental probe of the hydrated electron’s local solvation structure.

Simulating the ultrafast X-ray absorption spectroscopy of the hydrated electron

256th National Meeting (2018)

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