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The near threshold vacuum-UV photoionization of water clusters has been performed by using a resonance line emission of argon at 11.83 eV. The well-known intensity anomaly at the cluster ion (H2O)21H+ is observed even in this threshold photoionization, for the first time, with very small excess energy. Structures for the water cluster ions (H2O)21H+ and (H2O)28H+ which exhibit enhanced structural stability (magic number), are presented based on Monte Carlo simulations as well as on the analogy of our previous study on the stability of the (H2O)20NH+4 ion. The Monte Carlo calculations are carried out at the temperatures of 200, 150, 100, and 50 K for the ionized water clusters (H2O)nH+ around n=21 and also around n=28, which includes the ionic hydrogen-bond interactions between an H3O+ ion and neutral H2O molecules. The clusters (H2O)21H+ and (H2O)28H+ have greater binding energies per molecule than their neighbors although the enhancement of the latter is somewhat temperature dependent. The calculations suggest that the stable structures for (H2O)21H+ and (H2O)28H+ are represented by the ion–clathrate (ion-centered cage) configurations with either an H2O+ or (H2O ⋅ H3O)+ ion trapped inside the cage, respectively. The simulations for (H2O)20 ⋅ H3O+ also suggest that the cluster is especially stable due to the strong Coulombic interaction between the encaged H3O+ ion and the surrounding 20 water molecules which form a deformed pentagonal dodecahedral cage. The stability of the (H2O)26(H2O ⋅ H3O)+ ion can also be explained consistently within the framework of the present ion–clathrate model.
Nagashima et al. (1986) studied this question.