Broadband Spectral Probing Revealing Ultrafast Photochemical Branching after Ultraviolet Excitation of the Aqueous Phenolate Anion

Abstract

Electron photodetachment from the aromatic anion phenolate excited into the π−π* singlet excited state (S1) in aqueous solution is studied with ultrafast transient absorption spectroscopy with a time resolution of better than 50 fs. Broad-band transient absorption spectra from 300 to 690 nm are recorded. The transient bands are assigned to the solvated electron, the phenoxyl radical, and the phenolate S1 excited state, and confirmation of these assignments is achieved using both KNO3 as electron quencher and time-resolved fluorescence to measure singlet excited state dynamics. The phenolate fluorescence lifetime is found to be short (∼20 ps) in water, but the fast decay is only in part due to the electron ejection channel from S1. Using global target analysis, two electron ejection channels are identified, and we propose that both vibrationally hot S1 state and the relaxed S1 state are direct precursors for the solvated electron. Therefore, electron ejection is found just to compete with picosecond time scale vibrational relaxation and electronic radiationless decay channels. This contrasts markedly with <100 fs electron detachment processes for inorganic anions