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 1 pi pi* state by 4.66 eV UV photons, both molecules exhibited rapid relaxation into lower-lying electronic states followed by decay to the S0 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 1 pi pi* state to an intermediate electronic state with 100 fs. Computational results identify this intermediate as the 1 pi sigma* excited state, accessed by a 1 pi pi*/1 pi sigma* conical intersection, and the signal from this intermediate state disappears within similar to 200 fs. In contrast to thymine, formation of neither the 1n pi* state nor a long-lived triplet state was observed. Although the 1 pi sigma* 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 S0 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. The UV-induced photo-relaxation dynamics of 5-bromouracil and 5-bromo-2 '-deoxyuridine in aqueous solution were investigated using femtosecond time-resolved photoelectron spectroscopy with an extreme ultraviolet probe in a flat liquid jet.