  Liquid water at ambient temperature displays ultrafast molecular motions and
concomitant fluctuations of very strong electric fields originating from the
dipolar H2O molecules. We show that such random intermolecular fields induce
tunnel ionization of water molecules, which becomes irreversible if an external
terahertz (THz) pulse imposes an additional directed electric field on the
liquid. Time-resolved nonlinear THz spectroscopy maps charge separation,
transport and localization of the released electrons on a few-picosecond time
scale. The highly polarizable localized electrons modify the THz absorption
spectrum and refractive index of water, a manifestation of a highly nonlinear
response. Our results demonstrate how the interplay of local electric field
fluctuations and external electric fields allows for steering charge dynamics
and dielectric properties in aqueous systems.
