  Picosecond geminate recombination kinetics for electrons generated by
multiphoton ionization of liquid water become power dependent when the radiance
of the excitation light is greater than 0.3-0.5 TW/cm^2 (the terawatt regime).
To elucidate the mechanism of this power dependence, tri- 400 nm photon
ionization of water has been studied using pump-probe laser spectroscopy on the
pico- and femtosecond time scales. We suggest that the observed kinetic
transformations are caused by a rapid temperature jump in the sample. Such a
jump is inherent to multiphoton ionization in the terawatt regime, when the
absorption of the pump light along the optical path becomes very nonuniform.
The heating of water is substantial (tens of degrees C) because the 3-photon
quantum yield of the ionization is relatively low, ca. 0.42, and a large
fraction of the excitation energy is released into the solvent bulk as heat.
Evidence of the temperature jump is the observation of a red shift in the
absorption spectrum of (thermalized) electron and by characteristic
"flattening" of the thermalization dynamics in the near infra-red. The
temperature jump in the terawatt regime might be ubiquitous in multiphoton
ionization in molecular liquids. The implications of these observations for
femtosecond pulse radiolysis of water are discussed.
