What do Ultrafast Photoelectron Spectroscopy and Temperature-Dependent Transient Absorption Experiments Tell Us About the Structure of the Hydrated Electron?

Despite intense investigation, it is still unclear whether the structure of an excess electron in liquid water is best thought of as something similar to a halide ion, where most of the electron resides in a solvent cavity, as a non-cavity object, with many water molecules packed within the electron's wavefunction, or something in between. To address this question, we performed a series of mixed quantum/classical simulations with the goal of connecting the structure of the simulated hydrated electron with ultrafast spectroscopy experiments. We find that traditional cavity models are unable to predict the temperature dependence of the hydrated electron's excited-state lifetime, whereas a non-cavity model provides excellent agreement with temperature-dependent pump-probe experiments. Cavity models also fail to reproduce features of both static and ultrafast time-resolved photoelectron spectroscopy (TRPES) experiments, including observations such as the facts that hydrated electrons are not found near the air/water interface, that solvation dynamics lower the excited-state energy prior to internal conversion, and that solvation of the ground-state is significantly slower than the excited-state lifetime, features that are all well captured with a non-cavity model.