Enhanced fragmentation of water dications upon high Rydberg states

The ionization and fragmentation of water are fundamental processes across numerous scientific and technological fields, yet the relaxation pathways of water dications-particularly the rare D⁺ + O⁺ + D channel-remain poorly understood. Here, we report enhanced fragmentation pathways within this channel induced by electron-impact. Using multi-particle coincidence momentum spectroscopy supported by electron-capture-mediated molecular dissociation calculations, we identify a Rydberg state-controlled fragmentation mechanism and resolve its ultrafast relaxation dynamics. Furthermore, we demonstrate that this mechanism also occurs in ammonia and strong-field ionization experiments. In the laser-induced processes, electron-recollision with the cation leads to additional ionization plus excitation into dicationic Rydberg states on a sub-cycle (~2 fs) timescale, effectively freezing nuclear motion. Our findings reveal a general molecular fragmentation pathway governed by high Rydberg states, providing a molecular clock to probe electron-nuclear coupling and offering new insights into water radiolysis.