Theoretical ab initio study of the water trimer anion: Ground and excited state

Abstract

The ground and excited state of the water trimer anion, (H2O)3−, are studied via high-level ab initio calculations. A systematic search for the stable (H2O)3− configuration is carried out, based on the configuration formed by the stable (H2O)2− plus one additional water molecule. Four isomers, including three surface structures and one interior structure, are located on the (H2O)3− potential energy surface. While the linear chainlike structure is determined to be the major species detected in supersonic expansion experiments, consistent with previous theoretical studies, the other three isomers may also exist in molecular beams as evidenced by the matches of their vertical detachment energies with high-energy shoulders observed in photoelectron spectra. A significant geometric distortion of water molecules directly interacting with the excess electron indicates that the Franck–Condon effects are the major cause for the experimental observation of the excitation of water vibrational modes upon the photodetachment of (H2O)3−. This is in contrast to the vibronic effects for the case of (H2O)2−. In addition, the excited states of (H2O)3− are also investigated. The results show the existence of electronically bound excited states in surface structures of (H2O)3−. These states possess considerable p character, and therefore, can be regarded as a precursor of three p-like excited states of bulk hydrated electrons.