Imaging The Short-Lived Hydroxyl-Hydronium Pair In Ionized Liquid Water

The radiolysis of water is ubiquitous in nature and plays a critical role in numerous biochemical and technological applications. Although the elementary reaction pathways for ionized water have been studied, the short-lived intermediate complex and structural dynamic response after the proton transfer reaction remain poorly understood. Using a liquid-phase ultrafast electron diffraction technique to measure the intermolecular oxygen center dot center dot center dot oxygen and oxygen center dot center dot center dot hydrogen bonds, we captured the short-lived radical-cation complex OH(H3O+) that was formed within 140 femtoseconds through a direct oxygen center dot center dot center dot oxygen bond contraction and proton transfer, followed by the radical-cation pair dissociation and the subsequent structural relaxation of water within 250 femtoseconds. These measurements provide direct evidence of capturing this metastable radical-cation complex before separation, thereby improving our fundamental understanding of elementary reaction dynamics in ionized liquid water.