  Understanding heterogeneous charge transfer is crucial if we are to build the
best electrolyzers, fuel cells and photoelectrochemical water splitting devices
that chemistry allows. Because the elementary processes involved have
timescales ranging from femto- to milliseconds, direct simulation is not
generally possible. Model Hamiltonian approaches thus have a crucial role in
gaining mechanistic insight. Current generations of such theories describe a
reactant(s) or product(s) that interacts with electrolyte via a single
effective interaction. Such approaches thus obscure the extent to which
particular solvent fluctuations influence charge transfer. Here we demonstrate
experimentally that for a prototypical system, a ferrocene terminated alkane
thiol self-assembled monolayer (SAM) on gold in contact with aqueous
electrolyte, charge transfer from the Au to the ferrocene can be induced by
vibrational excitation of the ferrocene aromatic CH. Intriguingly the energy of
the aromatic CH vibration, 0.38 eV, is a large fraction of the effective
solvent interaction strength inferred for the ferrocene/ferrocenium system in
prior electrochemical studies: 0.85 eV. Our results thus demonstrate the
coupling of charge transfer to a specific solvent motion and more generally
imply that solvent may affect reduction/oxidation rates in electrocatalysis by
coupling to a few distinct solvent motions. Identifying these motions is
crucial in rationalizing trends in reactivity with change in electrolyte and
thus in pursuing electrolyte engineering from first principles.
