Light-induced temperature jump causes power-dependent ultrafast kinetics of electrons generated in multiphoton ionization of liquid water

摘要

Picosecond geminate recombination kinetics for electrons generated bymultiphoton ionization of liquid water become power dependent when the radianceof 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 photonionization of water has been studied using pump-probe laser spectroscopy on thepico- and femtosecond time scales. We suggest that the observed kinetictransformations are caused by a rapid temperature jump in the sample. Such ajump is inherent to multiphoton ionization in the terawatt regime, when theabsorption of the pump light along the optical path becomes very nonuniform.The heating of water is substantial (tens of degrees C) because the 3-photonquantum yield of the ionization is relatively low, ca. 0.42, and a largefraction 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 theabsorption spectrum of (thermalized) electron and by characteristic"flattening" of the thermalization dynamics in the near infra-red. Thetemperature jump in the terawatt regime might be ubiquitous in multiphotonionization in molecular liquids. The implications of these observations forfemtosecond pulse radiolysis of water are discussed.