System-Dependent Signatures of Electronic and Vibrational Coherences in Electronic Two-Dimensional Spectra

J Phys Chem Lett. 2012 Jun 7;3(11):1497-502. doi: 10.1021/jz300362k. Epub 2012 May 16.

Abstract

In this work, we examine vibrational coherence in a molecular monomer, where time evolution of a nuclear wavepacket gives rise to oscillating diagonal- and off-diagonal peaks in two-dimensional electronic spectra. We find that the peaks oscillate out-of-phase, resulting in a cancellation in the corresponding pump-probe spectra. Our results confirm the unique disposition of two-dimensional electronic spectroscopy (2D-ES) for the study of coherences. The oscillation pattern is in excellent agreement with the diagrammatic analysis of the third-order nonlinear response. We show how 2D-ES can be used to distinguish between ground- and excited-state wavepackets. On the basis of our results, we discuss coherences in coupled molecular aggregates involving both electronic and nuclear degrees of freedom. We conclude that a general distinguishing criterion based on the experimental data alone cannot be devised.

Keywords: DFT-calculations; four-wave mixing; two-dimensional electronic spectroscopy; ultrafast dynamics; vibrational wavepackets.