Primary photochemical processes in thymine in concentrated aqueous solution studied by femtosecond UV spectroscopy

Using a 282 nm fs light source, we have investigated the primary photochemical processes in liquid aqueous solution of thymine (Thy), one of the DNA bases using probe pulses in the range 282-588 nm. The studied processes include two-step Thy photoionization with the formation of an electron-cation pair (Thy⁺,^-) followed by partial geminate recombination, the formation of primary photoproducts, energy transfer from the excited Thy molecule to surrounding water molecules as well as the S₁ → S₀ relaxation. The two-photon absorption of the solvent water and the resulting generation of photoproducts was also taken into account. By comparison of the numerical simulations of the model derived with the experimental results, we have estimated the absorption cross section of the species mentioned above and determined the involved time constants; e.g., the S₁ lifetime τ₁ = (1.2 ± 0.2) ps. The theoretical model is supported by the measured intensity dependence.