Xian-ting Liang*
The evolutions of 2D, third-order, non-linear photon echo rephasing spectra are investigated theoretically by using an exact numerical path integral method. The coherence and relaxation times for an excitonic model are short, but those for a vibronic model are greatly extended. The couplings between electronic and vibrational modes play important roles in keeping long-lived coherence in light-harvesting complexes.