Femtosecond Electron-Transfer Dynamics across the D₂O/Cs⁺/Cu(111) Interface: The Impact of Hydrogen Bonding

Hydrogen bonding is essential in electron-transfer processes at water-electrode interfaces. We study the impact of the H-bonding of water as a solvent molecule on real-time electron-transfer dynamics across a Cs⁺-Cu(111) ion-metal interface using femtosecond time-resolved two-photon photoelectron spectroscopy. We distinguish in the formed water-alkali aggregates two regimes below and above two water molecules per ion. Upon crossing the boundary of these regimes, the lifetime of the excess electron localized transiently at the Cs⁺ ion increases from 40 to 60 fs, which indicates a reduced alkali-metal interaction. Furthermore, the energy transferred to a dynamic structural rearrangement due to hydration is reduced from 0.3 to 0.2 eV concomitantly. These effects are a consequence of H-bonding in the water-water interaction and the beginning formation of a nanoscale water network. This finding is supported by real-space imaging of the solvatomers and vibrational frequency shifts of the OH stretching and bending modes calculated for these specific interfaces.