Evidence for resonance-enhanced multiphoton ionization of liquid water using 2 eV laser light: Variation of hydrated electron absorbance with femtosecond pulse intensity

The multiphoton ionization of liquid water that occurs in femtosecond laser photolysis is investigated with an experimental setup in which 2 eV photons are sent into the sample. Hydrated electrons are identified as the generated absorbing species and their absorbance A is measured as a function of the laser irradiance E. The power law A∝${\mathit{E}}^{1.8}$ which is found differs markedly from that expected from energetic considerations. This is explained in terms of a resonant four-photon absorption involving the à ${}_{}{}^1{}_{}{}^{}$ ${\mathit{B}}_1$ electronic state of water which, in the liquid phase, is partly autoionizing.