Experimental evidence for metastable states of D₃O and its monohydrate by neutralized ion beam spectroscopy

The oxonium radical (H₃O) has been generated in its ground state by neutralizing a fast beam of ions in the near resonant electron transfer reaction H₃O⁺ + K(g)→H₃O* + K ⁺. Analysis of neutral beam scattering profiles and collisionally reionized mass spectra indicate that the fully deuterated species (D ₃O) can be formed in a distribution of dissociative and metastable states (τ > 0.6 μs). Thermalization of the precursor D ₃O⁺, prior to electron transfer, is required for production of metastable D₃O. Neither H₃O nor D ₂HO is observed in metastable states. These isotope effects support earlier theoretical predictions of a shallow local minimum on the oxonium potential surface. The ionization potential of D₃O is calculated to be 4.3 ± 0.1 eV. Some spectroscopic implications for this radical are discussed. The oxonium monohydrates (H₃O·H₂O) are also observed to exist in metastable states for several H/D isotopic variants. The ionization potential of D₃O·D₂O is estimated to be ≥3.4 eV.