The ionization of aqueous indole at low excitation energies

The relaxation of indole after excitation at 267 nm (4.6 eV) in water and methanol solutions has been explored by fs time-resolved experiments and the observations have been rationalized using quantum dynamical calculations on the excited states of solvated water and methanol clusters. The research explores the autoionization channel, which in parallel to the neutral relaxation route leads, only in aqueous environments, to the formation of the fully solvated cations and electrons. The pump-probe and pump-repump-probe measurements identify a charge-transfer character state, formed simultaneously or shortly after the ππ* state, that is the precursor of the charged species. This state relaxes with a lifetime analogous to the measured fluorescence lifetime, ∼4 ns, indicating that both locations are related. Dynamical calculations show how, in the solvated four water cluster, the formation of a charge transfer state with the electron delocalized in the explicit water molecules occurs shortly (∼20 fs) after the excitation, by coupling to the prepared ππ* state. On the other hand, in good agreement with the experimental findings, the formation of the charge transfer state is absent from the dynamics computed for the methanol clusters.