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 (H 2O)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 (H 2O)20NH4+ ion. The Monte Carlo calculations are carried out at the temperatures of 200, 150, 100, and 50 K for the ionized water clusters (H2O)n H+ 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 (H 2O)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 (H 2O · H3O)+ ion trapped inside the cage, respectively. The simulations for (H2O)20 · H 3O+ 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.
|Number of pages||6|
|Journal||The Journal of Chemical Physics|
|Publication status||Published - Dec 1 1985|
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry