Photoionization studies of (H2O)2 and (HF)2 producing H3O+ and H2F+ yield 7.18±0.08 eV (165.8±1.8 kcal/mole) and 4.09±0.06 eV (94.3±1.4 kcal/mole) as the proton affinities of H2O and HF, respectively. The measured ionization potential of (H2O)2, 11.21±0.09 eV, along with the known ionization potential of H2O, 12.615±0.001 eV, allow the deduction of a lower bound for the dissociation energy of (H2O)2:1.58±0.13 eV (36±3 kcal/mole). The experiments have demonstrated that photoionization of dimers is one of the most useful general methods for the determination of proton affinities.
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