Coherent population transfer in molecules coupled with a dissipative environment by an intense ultrashort chirped pulse

We have studied the intense chirped pulse excitation of a molecule coupled with a dissipative environment taking into account electronic coherence effects. We considered a two-state electronic system with relaxation treated as a diffusion on electronic potential energy surfaces. This relaxation model enables us to trace continuously the transition from a coherent population transfer to incoherent one. An inhomogeneously broadened system with frozen nuclear motion is invoked to model a purely coherent transfer. We show that the type of population transfer (coherent or incoherent) strongly depends on the pulse chirp, its sign, and the detunings of the exciting pulse carrier frequency with respect to the frequency of the Franck-Condon transition. For positive chirped pulses and moderate detunings, relaxation does not hinder a coherent population transfer. Moreover, under these conditions the relaxation favors more efficient population transfer with respect to the "coherent" system with frozen nuclear motion. (C) 2002 American Institute of Physics.