Nonadiabatic effects induced by the coupling between vibrational modes via Raman fields

We study the effect of coupling between molecular vibrational modes attendant the excitation of Raman transitions using ultrafast chirped laser pulses. We model Raman-active vibrational modes by two-level systems (TLSs) with nondegenerate ground states and a predetermined initial relative phase. Chirp of the pump and Stokes pulses is the same in magnitude and opposite in sign for the whole pulse duration. To reveal the nonadiabatic effects induced by the coupling between the vibrational modes, we compare the model of two uncoupled TLSs with the one of two TLSs coupled by the external fields. The first model shows population inversion. Within the second model, the coupling induces nonadiabatic effects leading to a mixed population distribution. By introducing the time delay between the Stokes and pump pulses, nearly complete population transfer is realized in both coupled TLSs.