IR-VUV Photoionization Spectra of Hydrated BaOH Reveal Base Dissociation in Growing Water Clusters

Investigating the dissociation of base in a microhydration environment is important for revealing various fundamental physical and chemical processes. In this study, neutral open-shell BaOH(H₂O)_n (n = 1-5) clusters were characterized by size-specific infrared-vacuum ultraviolet photoionization spectroscopy combined with quantum chemical calculations and ab initio molecular dynamics simulations. The results show that the Ba-OH bond is undissociated for n = 1 and 2, and the transition from contact ion pair (CIP) to solvent-shared ion pair (SIP) spontaneously starts at n = 3. The present findings clarify the previous debate that BaOH shows no sign of dissociation even at n = 5. This work reveals the microscopic mechanisms of base dissolution processes and lays a solid foundation for broader application across systems.