Charge Separation Dynamics of Aromatic Molecules at Aqueous Interfaces Revealed by Ultrafast Photoelectron Spectroscopy

Hydrophobic molecules can preferentially accumulate at the air-water interface, leading to interfacial concentrations far exceeding those in bulk solution and causing reactions to occur at higher rates. However, it remains unclear whether the reaction rate constants are intrinsically altered by the interfacial solvation environment. Here, we employ extreme ultraviolet time-resolved photoelectron spectroscopy (EUV-TRPES) to investigate the photoinduced charge-separation dynamics of the surface-active organic molecules indole, phenol, and phenolate. Ultraviolet photoexcitation of interfacial indole was found to produce both valence excited states and electron-cation contact pairs, both of which decayed faster than in the bulk solution. In contrast, interfacial phenol produced only valence excited states, with no detectable hydrated electrons or their precursors. Interfacial phenolate exhibited faster photodetachment than in the bulk solution. Both indole and phenol exhibit concentration-dependent dynamics due to interfacial molecular aggregation. These results contribute to a more comprehensive understanding of interfacial dynamics when considered alongside prior nonlinear optical spectroscopy studies. We further demonstrate EUV-TRPES with 18 fs time resolution, enabling direct observation of the 5 fs internal conversion of interfacial indole from the ¹L_a to ¹L_b state, followed by clear vibrational quantum beats. Thus, EUV-TRPES provides a powerful means for probing electronic and vibrational dynamics at interfaces.