The ultrafast dynamics of the cationic hole formed in bulk liquid water following ionization is 
investigated by ab initio molecular dynamics simulations and an experimentally accessible
signature is suggested that might be tracked by femtosecond pump-probe spectroscopy. 
This is one of the fastest fundamental processes occurring in radiation-induced chemistry in aqueous 
systems and biological tissue. However, unlike the excess electron formed in the same process, the nature 
and time evolution of the cationic hole has been hitherto little studied. Simulations show that 
an initially partially delocalized cationic hole localizes within ~30 fs after which proton transfer 
to a neighboring water molecule proceeds practically immediately, leading to formation of the OH 
radical and the hydronium cation in a reaction which can be formally written as H2O+ + H2O --> OH + H3O+. 
The exact amount of initial spin delocalization is, however, somewhat method dependent, being realistically 
described by approximate density functional theory methods corrected for the self-interaction error. 
Localization, and then the evolving separation of spin and charge, changes the electronic structure of 
the radical center. This is manifested in the spectrum of electronic excitations which is calculated for 
the ensemble of ab initio molecular dynamics trajectories using a QM/MM formalism applying the 
EOM-IP-CCSD method to the radical core. A clear spectroscopic signature is predicted by the theoretical 
model: as the hole transforms into a hydroxyl radical, a transient electronic absorption in the 
visible shifts to the blue, growing toward the near ultraviolet. Experimental evidence for this 
primary radiation-induced process is sought using femtosecond photoionization of liquid water excited 
with two photons at 11 eV. Transient absorption measurements carried out with ~40 fs time resolution 
and broadband spectral probing across the near-UV and visible are presented and direct comparisons 
with the theoretical simulations are made. Within the sensitivity and time resolution of the current 
measurement, a matching spectral signature is not detected. This result is used to place an upper 
limit on the absorption strength and/or lifetime of the localized H2O+(aq) species.