  The two-dimensional spectroscopy has recently revealed oscillatory behavior
of excitation dynamics in molecular systems. However, in the majority of cases
it is strongly debated if excitonic or vibrational wavepackets, or evidences of
quantum transport have been observed. In this letter, the method for
distinguishing between vibrational and excitonic wavepacket motion is
presented, based on the phase and amplitude relationships of oscillations of
distinct peaks, which has been revealed using fundamental analysis of
two-dimensional spectrum of two representative systems.
