Hydrated Electron Motifs in Water Cluster Anions: Structures and Isomer-Specific Femtosecond Real-Time Relaxation Dynamics

Abstract We reexamined hydrated electron motifs in water cluster anions using photoelectron and electronic absorption spectroscopies under identical ion source conditions. Previously uncharacterized structural motifs were identified and reclassified, refining the framework of hydrated electron isomers. Time-resolved photoelectron spectroscopy revealed motif-dependent relaxation dynamics, with certain surface-bound species undergoing more efficient excited-state autodetachment than others. Extending the temporal probing window uncovered long-lived vibrationally excited ground-state anions (τ∼340 ps), far exceeding prior expectations, while increasing cluster size enhanced internal conversion efficiency. These results closely align with theoretical predictions, reinforcing the essential role of computational modeling in understanding hydrated electrons. More broadly, they demonstrate that excited-state dynamics are highly sensitive to local structure, giving rise to distinct relaxation pathways under different hydration environments. The structure–dynamics relationships established here provide a foundation for extending such studies to other hydrated species, advancing fundamental insight into electron solvation and relaxation in aqueous systems.