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.