Ionizing radiation can trigger ultrafast proton transfer, a central mechanism in many chemical and biological functions, that in turn can enhance or suppress electron relaxation processes and consequently cause abrupt changes in the reaction pathway. This study combines theory and experiment to probe ultrafast relaxation and dissociation in water dimers following valence photoionization. By tracking electron and nuclear motion simultaneously, we reveal competing fragmentation pathways that produce low-energy electrons (LEEs), which are key agents in radiation-induced chemistry, including DNA damage. While LEEs are known to arise via inner-valence ionization and intermolecular Coulombic decay, we identify a faster, previously unobserved relaxation channel, which we call proton-transfer-mediated autoionization (PTM-AI). Occurring within 10 femtoseconds, PTM-AI alters fragmentation outcomes, yielding either D3O+ + OD+ or D2O+ + D2O+, depending on the interplay of proton migration and hydrogen back-transfer. Our findings underscore the intricate coupling between electronic and nuclear dynamics in hydrogen-bonded systems and establish PTM-AI as a significant new pathway for LEE generation.
Iskandar et al. (Tue,) studied this question.
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