  We investigate non-equilibrium two-electron transfer in a model redox system
represented by a two-site extended Hubbard model and embedded in a dissipative
environment. The influence of the electron-electron interactions and the
coupling to a dissipative bosonic bath on the electron transfer is studied in
different temperature regimes. At high temperatures Marcus transfer rates are
evaluated and at low temperatures, we calculate equilibrium and non-equilibrium
population probabilities of the donor and acceptor with the non-perturbative
Numerical Renormalization Group approach. We obtain the non-equilibrium
dynamics of the system prepared in an initial state of two electrons at the
donor site and identify conditions under which the electron transfer involves
one concerted two-electron step or two sequential single-electron steps. The
rates of the sequential transfer depend non-monotonically on the difference
between the inter-site and on-site Coulomb interaction which become
renormalized in the presence of the bosonic bath. If this difference is much
larger than the hopping matrix element, the temperature as well as the
reorganization energy, simultaneous transfer of both electrons between donor
and acceptor can be observed.
