Electron-impact ionization of the water molecule at large momentum transfer above the double-ionization threshold

The single and double ionization of the water molecule at large momentum transfer has been studied using a combination of (e,2e) and (e,3-1e) spectroscopy, with the binding energy spectrum being measured from 0 to 100 eV. The experiment has been performed in the symmetric noncoplanar geometry at an incident electron energy of 2055 eV. In this way we have achieved a large momentum transfer of 9 a.u. In particular, we present an observation of a relatively intense band at around 58 eV. Symmetry-adapted cluster configuration interaction (SAC-CI) general-R calculations for single ionization indicate that the observed band is at least partly generated by a cluster of satellites with small intensities, which predominantly belong to states possessing 2A ₁ symmetry originating from the mixing of the (2a ₁)⊃-1 state with two electron processes. Nevertheless, it has been found that the entire spectrum above the lowest double-ionization threshold cannot be understood, even qualitatively, with the SAC-CI calculations. This result suggests that the (e,3-1e) double-ionization processes have a significant contribution to the observations.