Dynamics of photoexcited 5-bromouracil and 5-bromo-2'-deoxyuridine studied by extreme ultraviolet time-resolved photoelectron spectroscopy in liquid flat jets.

The UV-induced photo-relaxation dynamics of 5-bromouracil (BrU) and 5-bromo-2'-deoxyuridine (BrUrd) in aqueous solution were investigated using femtosecond time-resolved photoelectron spectroscopy with an extreme ultraviolet (XUV) probe in a flat liquid jet. Upon excitation to the ππ* state by 4.66 eV UV photons, both molecules exhibited rapid relaxation into lower-lying electronic states followed by decay to the S ground state. By employing a 21.7 eV XUV probe pulse, we were able to differentiate the relaxation of the excited state population from the initially excited ππ* state to an intermediate electronic state with 100 fs. Computational results identify this intermediate as the πσ* excited state, accessed by a ππ*/πσ* conical intersection, and the signal from this intermediate state disappears within ∼200 fs. In contrast to thymine, formation of neither the nπ* state nor a long-lived triplet state was observed. Although the πσ* state is largely repulsive, prior studies have reported a low quantum yield for dissociation, and we observe weak signals that are consistent with production of hot S ground state (for BrUrd) on a time scale of 1.5-2 ps. It thus appears that solvent caging effects limit the dissociation yield in solution.