Excited-State Relaxation of Hydrated Thymine and Thymidine Measured by Liquid-Jet Photoelectron Spectroscopy: Experiment and Simulation

Time-resolved photoelectron spectroscopy is performed on thymine and thymidine in aqueous solution to study the excited-state relaxation dynamics of these molecules. We find two contributions with sub-ps lifetimes in line with recent excited-state QM/MM molecular dynamics simulations (J. Chem. Phys. 2013, 139, 214304). The temporal evolution of ionization energies for the excited pi pi* state along the QM/MM molecular dynamics trajectories were calculated and are compatible with experimental results, where the two contributions correspond to the relaxation paths in the pi pi* state involving different conical intersections with the ground state. Theoretical calculations also show that ionization from the n pi* state is possible at the given photon energies, but we have not found any experimental indication for signal from the n pi* state. In contrast to currently accepted relaxation mechanisms, we suggest that the n pi* state is not involved in the relaxation process of thymine in aqueous solution.