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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 ) (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 2hν ) 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 H 2 O + ion formation is above 8.8 eV. IntroductionAmong all the solvents, water occupies an extraordinary position. Being the natural environment of such important biological molecules as nucleic acids and peptides, water is present in almost any living organism. Therefore, a detailed understanding of charge-transfer reactions initiated in water by light absorption and of light absorption processes in water itself is necessary for a proper description of photochemical experiments performed in water solution. It is well-known that at low intensities of light (below 10 11 W/m 2 ) the water is transparent for UV light with λ > 190 nm; i.e., the linear and nonlinear absorption is negligible. However, with intense UV light at λ > 190 nm, it is possible to excite a water molecule via a two-photon absorption (TPA) mechanism 1 and initiate its chemical decomposition. It was shown in 1980 that under highintensity picosecond UV excitation with λ ) 266 nm the water molecule undergoes two-photon absorption 2,3 with subsequent dissociation and ionization 4In the first experiments 2-4 the total two-photon energy value was E total ) 9.3 eV, and the ionization threshold for liquid water was determined to be E ion ) 6.5 ( 0.5 eV, 4 consistent with earlier estimates of water ionization potential of 6.5 ( 0.1 5 and 6.0-6.5 eV. 6 An updated value E ion ) 6.36-6.41 eV follows from recent experiments. 7,8 The dissociation energy for neat water is reported to be E dis ) 6.41-6.71 eV. 9 The development of high-intensity femtosecond laser systems made it possible to investigate the dynamics of primary photochemical reactions in water. Using the common pump and probe techniques with two-photon excitation in the UV and probing in the visible and near-IR ranges, the electron trapping and solvation as well as geminate recombination were studied by many experimental groups with time resolution up to 20 fs. In many of these investigations a colliding pulse mode-locked dye laser with second harmonic generation (SH...

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