Time-resolved extreme ultraviolet photoelectron spectroscopy of 4-bromophenotate anions in a liquid jet

Abstract

The dynamics of photo-excited 4-bromophenolate anions in aqueous solution are investigated using femtosecond extreme ultraviolet (XUV) time-resolved photoelectron spectroscopy (TRPES) in a liquid flat jet. Excitation to the first singlet S₁ (1¹ππ^*) excited state at 290 nm yields lifetimes of ~160 fs and 1.1 ps that are attributed to the timescale for the transition from the initially excited S1 state to the optically-dark ¹πσ^* state and the subsequent decay of the ¹πσ^* state, respectively. Excitation at 240 nm to the second singlet excited state (S₂, 2¹ππ^*) reveals an additional ultrafast component assigned to decay from the S₂ to the S₁ state with a lifetime of 60 fs. Subsequent decay components show lifetimes of 130 fs and 1.4 ps, corresponding to the transition from S₁ to the ¹πσ^* state and its subsequent decay. In contrast to phenolate anions, which result in hydrated electron formation within ~20 ps upon S₁ excitation, 4-bromophenolate undergoes rapid decay through the ¹πσ^* state, which is repulsive along the C-Br bond length and correlates to C₆H₄O + Brˉ. Quantum chemical calculations suggest that the S₁-¹πσ^* state crossing possesses a small potential energy barrier that promotes a facile non-adiabatic transition. By comparison with phenolate photo-oxidation dynamics, this study highlights the competing dynamics between photo-oxidation and non-adiabatic relaxation within the framework of anions in aqueous solution.