  A comprehensive description of molecular electron transfer reactions is
essential for our understanding of fundamental phenomena in bio-energetics and
molecular electronics. Experimental studies of molecular systems in
condensed-phase environments, however, face difficulties to independently
control the parameters that govern the transfer mechanism with high precision.
We show that trapped-ion experiments instead allow to reproduce and
continuously connect vastly different regimes of molecular charge transfer
through precise tuning of, e.g., phonon temperature, electron-phonon
interactions, and electronic couplings. Such a setting allows not only to
reproduce widely-used transport models, such as Marcus theory. It also provides
access to transfer regimes that are unattainable for molecular experiments,
while controlling and measuring the relevant observables on the level of
individual quanta. Our numerical simulations predict an unconventional quantum
transfer regime, featuring a transition from quantum adiabatic- to
resonance-assisted transfer as a function of the donor-acceptor energy gap,
that can be reached by increasing the electronic coupling at low temperatures.
Trapped ion-based quantum simulations thus promise to enhance our microscopic
understanding of molecular electron transfer processes, and may help to reveal
efficient design principles for synthetic devices.
