Dielectron Attachment and Hydrogen Evolution Reaction in Water Clusters*

Binding of excess electrons to nano-size water droplets, with a focus on the hitherto largely unexplored properties of doubly-charged clusters, were investigated experimentally using mass spectrometry and theoretically with large-scale first-principles quantum simulations. Doubly-charged clusters were measured in the range of 83 $\le $ n $\le $ 123, with (H$_{2}$O)$_{n}^{-2 }$ clusters found for 83 $\le $ n $<$ 105, and mass-shifted peaks corresponding to (H$_{2}$O)$_{n-2}$(OH$^{-})_{2}$ detected for n $\ge $ 105. Simulations revealed surface and internal dielectron, e$^{-}_{2}$, localization modes and elucidated the mechanism of the reaction (H$_{2}$O)$_{n}^{-2}\to $ (H$_{2}$O)$_{n-2 }$(OH$^{-})_{2}$ + H$_{2}$ (for n $\ge $ 105), which was found to occur via concerted approach of a pair of protons belonging to two water molecules located in the first shell of the dielectron internal hydration cavity, culminating in hydrogen formation 2H$^{+}$ + e$^{-}_{2 }\to $ H$_{2}$. Instability of the dielectron internal localization impedes the reaction for smaller ( n $<$ 105) clusters.