Dynamics of water clusters upon UV‐excitation leading to ionization: Nonempirical study

Nonempirical direct molecular dynamics of neutral and cationic water clusters involving four to six molecules is studied to shed light on the character of the ionization of water clusters. Potential energy and forces are calculated in the second‐order of the Møller‐Plesset perturbation theory with a 6–31++G(1d, 1p) basis set. Upon vertical ionization of a water cluster, H3O+ and OH fragments appear. The process needs no less than 40 fs, and in some cases, up to 100 or 150 fs. The resulting configuration of the cluster (neglecting its generally larger volume) is close to that obtained in the geometry optimization run. Vibrational excitation of a neutral cluster with a total energy about half its adiabatic ionization potential may promote the appearance of structures similar to that of the stable cation at certain moments in time. In particular, a configuration in which only one water molecule separates H3O+ and OH− fragments forms much faster than upon vertical electron detachment from a stable neutral cluster, in nearly 18 fs. Such structure reorganization is noticeably impeded by the rotational–vibrational excitation of the whole cluster, especially when hydrogen bonds are substantially distorted. The effect of distorting motions becomes very pronounced after 20 fs.