  The time-convolutionless master equation for the electronic populations is
derived for a generic electron-phonon Hamiltonian. The equation can be used in
the regimes where the golden rule approach is not applicable. The equation is
applied to study the electronic relaxation in several models with the finite
number normal modes. For such mesoscopic systems the relaxation behavior
differs substantially from the simple exponential relaxation. In particular,
the equation shows the appearance of the recurrence phenomena on a time-scale
determined by the slowest mode of the system. The formal results are quite
general and can be used for a wide range of physical systems. Numerical results
are presented for a two level system coupled to Ohmic and super-Ohmic baths, as
well as for a model of charge-transfer dynamics between semiconducting organic
polymers.
