Site- and State-Resolved Observation of Ultrafast Asymmetric Stretching Dynamics of Water Dication

We report a combined experimental and theoretical study of ultrafast structural dynamics of water dication (D 2 O 2 + ) dissociating into D + + D + + O channel induced by electron-impact. Three-dimensional momenta of two fragment ions are measured in coincidence with the scattered electron, which obtain the projectile energy loss and the kinetic energy release versus the angle between asymptotic proton momenta of the dissociation channel. This measurement enables the determination of the initial dicationic states for detailed observation of the dynamics. The state-resolved measurements show clearly asymmetric and symmetric stretching dynamics accompanied by unbending deformations prior to dissociation, which are supported by our full quantum time-dependent wave-packet evolution calculations. Our further studies of HDO 2 + dynamics reveal an intriguing mechanism of the dissociation pathway wherein the stretch of OD can exceed that of OH. This study verifies previous predictions with a multireference configuration approach and contributes to a more complete understanding of the fundamental processes in water radiolysis.