Primary photochemical processes in water

The primary photochemical processes which occur in neat water after 282 nm high-intensity excitation via a two-photon mechanism were studied. Using probe pulses in UV, visible, and near-IR spectral ranges, the absorption process itself and the time evolution of the generated electrons and other photoproducts in the 0.1-80 ps scale were investigated. Different to previous femtosecond laser studies of water a quantitative analysis of the absorption process and yields of primary photoproducts was carried out. The two-photon absorption coefficient of liquid water for femtosecond pulses at 282 nm was measured to be β = (1.9 ± 0.5) × 10^-12 m/W, while the quantum yield of the hydrated electron at the same wavelength was determined to be Φ (e_aq^-) = 0.11 ± 0.03. The results of the electron solvation and geminate recombination dynamics at 282 nm (excitation energy E_2hv = 8.8 eV) are in accordance with the findings of other groups for different excitation wavelengths. The numerical simulations of our data suggest that the energy threshold for H₂O⁺ ion formation is above 8.8 eV.