  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 femtoseconds, 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 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 stretch
and bending modes calculated for these specific interfaces.
