We study the effects of coupling between molecular vibrational modes attendant the excitation of Raman transitions. Raman active vibrational modes are described by two two-level systems (TLSs) with nondegenerate ground states. The population inversion is investigated under the condition of zero relative phase between initially populated states. We use two ultrafast laser pulses for the pump and the Stokes fields that have chirps the same in the magnitude and opposite in the sign. Within this model, the coupling induces nonadiabatic effects leading to a diverse population distribution. By introducing the time delay between the Stokes and the pump pulse, we realized a complete population transfer in both coupled TLSs. We demonstrated the population inversion in a more generic case of the averaged initial relative phase. © 2012 Wiley Periodicals, Inc.