Formation of negative ions of water clusters by electron transfer from high-Rydberg atoms

Negative ions of H₂O and D₂O clusters were produced by collisional electron transfer from highly excited Rydberg atoms, where no evaporation of monomers after electron transfer is expected. The threshold sizes observed in the mass spectra of (H₂O)^-_n and (D₂O)^-_n, n ≥ 11 and 13 respectively, are interpreted as the critical sizes beyond which the vertical electron affinities of water clusters are positive. A comparison with previous reports shows that the negative-ion states of water clusters with near-threshold sizes are likely to be far from the solvated electron state.