  Ultrafast photoinduced dynamics of electronic excitation in molecular dimers
is drastically affected by the dynamic reorganization of inter- and intra-
molecular nuclear configuration modeled by a quantized nuclear degree of
freedom [Cina et. al, J. Chem Phys. {118}, 46 (2003)]. The dynamics of the
electronic population and nuclear coherence is analyzed by solving the chain of
coupled differential equations for %mean coordinate, population inversion,
electron-vibrational correlation, etc. [Prezhdo, Pereverzev, J. Chem. Phys.
{113} 6557 (2000)]. Intriguing results are obtained in the approximation of a
small change of the nuclear equilibrium upon photoexcitation. In the limiting
case of resonance between the electronic energy gap and the frequency of the
nuclear mode these results are justified by comparison to the exactly solvable
Jaynes-Cummings model. It is found that the photoinduced processes in the model
dimer are arranged according to their time scales: (i) fast scale of nuclear
motion, (ii) intermediate scale of dynamical redistribution of electronic
population between excited states as well as growth and dynamics of
electron-nuclear correlation, (iii) slow scale of electronic population
approach to the quasi-equilibrium distribution, decay of electron-nuclear
correlation, and decrease of the amplitude of mean coordinate oscillation. The
latter processes are accompanied by a noticeable growth of the nuclear
coordinate dispersion associated with the overall nuclear wavepacket width. The
demonstrated quantum relaxation features of the photoinduced vibronic dynamics
in molecular dimers are obtained by a simple method, applicable to systems with
many degrees of freedom.
