  A general theory of electronic excitations in aggregates of molecules coupled
to intramolecular vibrations and the harmonic environment is developed for
simulation of the third-order nonlinear spectroscopy signals. The model is
applied in studies of the time-resolved two-dimensional coherent spectra of
four characteristic model systems: weakly / strongly vibronically coupled
molecular dimers coupled to high / low frequency intramolecular vibrations. The
results allow us to classify the typical spectroscopic features as well as to
define the limiting cases, when the long-lived quantum coherences are present
due to vibrational lifetime borrowing, when the complete exciton-vibronic
mixing occurs and when separation of excitonic and vibrational coherences is
proper.
