Isotopic effects in two-body fragmentation process of water dications induced by electron-impact ionization

The two-body fragmentation dynamics of water isotopologues dications (H₂O²⁺, HOD²⁺, and D₂O²⁺) induced by 200 eV electron impact is investigated. Two fragment ions and an emitted electron are detected in coincidence, and their momentum vectors are determined by employing a reaction microscope. The complete kinematical information of four two-body fragmentation channels of H⁺ + OH⁺, H⁺ + OD⁺, D⁺ + OH⁺, and D⁺ + OD⁺ is obtained. By analyzing the projectile energy-loss spectrum, the initial electronic state of the two-body dissociation channel is determined. Upon examining the kinetic energy release (KER) distributions of the four fragmentation channels, a clear difference is found between the two-body fragmentation channel H⁺ + OD⁺ and the other three channels. The isotopic effect in the two-body fragmentation is demonstrated by the analysis of the relative yields of the two-body fragmentation channels originating from different isotopologues, which shows preferential cleavage of the O-H bond over the O-D bond. These results provide deeper insight into the microscopic dynamic mechanisms in water radiolysis.