Theoretical estimation of the ionization potential of water in condensed phase. II. Superficial water layers

Quantum chemical calculations of neutral and charged (H2O)(n) water clusters modeling fragments of a real hydrogen-bond network of water and dynamic simulations of the clusters either upon the electron removal from a stable neutral cluster or upon the excitation of various cluster vibrations enabled us to distinguish separate stages of the structure reorganization. Thermal motion of molecules in a liquid modeled by the low-frequency rotational vibrations and swinging of molecules prevents the formation of cations with the optimum mutual arrangement of the OH radical and H3O+ ion. Judging from the typical periods of reactive vibrations and those motions, which impede the desired structure reorganization, the most probable should be the formation of OH...H2O... H3O+ fragments in liquid water. The energy necessary for the ionization of superficial water layers (at the irradiation) can be estimated from the intermediate ionization potentials of water clusters. Extrapolating the dependence of these potentials on the number of water molecules constituting the cluster provided the first ever theoretical estimate of the threshold ionization energy of water: 9.5 eV.