Spectroscopic study of the ion-radical H-bond in H4O2+.

The primary event in the ionization of water involves rapid proton transfer, leading to charge localization on H(3)O(+) and the creation of a hydroxyl radical. We trap the nascent [H(3)O(+).(*)OH] exit channel intermediate in the bimolecular reaction by Ar-mediated ionization of the neutral water dimer and characterize the nature of this ion-radical complex using vibrational predissociation spectroscopy of the Ar-tagged species. The resulting bands involving the displacement of the bridging proton are broad and appear as a strong triplet centered around 2000 cm(-1). The observed band pattern is analyzed with theoretical calculations to identify the origin of the anhamonic effects evident in the spectrum. In the course of this work, expressions were derived for treating the coupling terms within a sinc-DVR. Although this level of treatment did not reveal the assignment of the triplet structure, its characteristic approximately 100 cm(-1) spacing suggests activity involving the frustrated rotation of the hydroxyl radical upon excitation of the bridging-proton vibration parallel to the heavy atom axis. The behavior of this system is considered in the context of that reported previously for the related H(5)O(2)(+), H(3)O(2)(-), and F(-).H(2)O complexes.