Photo-relaxation Dynamics of Phenolate Anions by Extreme Ultraviolet Time-resolved Photoelectron Spectroscopy in Liquid Jets

Abstract

The dynamics of photo-excited phenolate anions in aqueous solution are investigated using femtosecond extreme ultraviolet (XUV) time-resolved photoelectron spectroscopy (TRPES) in liquid jets. Excitation to the first singlet excited state (S₁, 1¹ππ^*) at 288.5 nm yields a lifetime of 18.3 ps attributed to parallel decay pathways: hydrated electron production and internal conversion to the S₀ ground state. Excitation at 240 nm to the second singlet excited state (S₂, 2¹ππ^* ) shows that this state undergoes internal conversion within 160 fs to internally hot S₁ (1¹ππ^*) anions. These anions subsequently relax with a 2.4 ps lifetime through both hydrated electron formation and vibrational relaxation to internally cold S₁ anions, which then undergo similar dynamics as seen for direct S₁ excitation. In contrast to the charge-transfer-to-solvent (CTTS) dynamics of atomic iodide, no distinct spectral shifts are observed in phenolate, indicating the absence of the transient electron species seen in iodide CTTS excitation. These differences are attributed to the nature of the excited electronic states for the two systems. The hydrated electron yield for S₁ excitation appears to be lower in the TRPES experiments than in previous transient absorption studies. This effect may reflect differing sensitivities of the two methods to excited electronic states relative to hydrated electrons as well as the enhanced concentration of phenolate anions at the liquid interface.