  Resonant inelastic x-ray scattering (RIXS) provides remarkable opportunities
to interrogate ultrafast dynamics in liquids. Here we use RIXS to study the
fundamentally and practically important hydroxyl radical in liquid water,
OH(aq). Impulsive ionization of pure liquid water produced a short-lived
population of OH(aq), which was probed using femtosecond x-rays from an x-ray
free-electron laser. We find that RIXS reveals localized electronic transitions
that are masked in the ultraviolet absorption spectrum by strong
charge-transfer transitions -- thus providing a means to investigate the
evolving electronic structure and reactivity of the hydroxyl radical in aqueous
and heterogeneous environments. First-principles calculations provide
interpretation of the main spectral features.
