Isolating non-adiabatically enhanced ground state quantum beats through two-dimensional electronic spectroscopy

Abstract

Resonant vibrational–electronic (vibronic) couplings in donor–acceptor systems may play a crucial role in driving non-adiabatic internal conversion reported in natural photosynthesis, organic photovoltaic polymers, and singlet exciton fission. Quantum beats arising from impulsive excitation are often employed as spectroscopic reporters of the specific vibrational modes driving this process. However, distinguishing these promoter modes from spectator modes, which do not participate in vibronic mixing and simply accompany ultrafast internal conversion, remains a challenge. This is so because vibrational quantum beats arising from uncoupled monomers can modulate pump–probe transients by themselves. In this paper, we show that vibronic mixing induces quantum beats whose amplitude is anisotropic with respect to the polarization of the light. We propose a readily implementable polarization-controlled two-dimensional electronic spectroscopy experiment to uniquely identify signatures of excited state vibronic resonance using ground state quantum beats by discriminating against vibrational motions (and corresponding quantum beats) that are simply spectators. Through analytical expressions and simulation of two-dimensional electronic spectra, we show that the resulting 2D spectra are expected to exhibit distinct spectral lineshapes with a strong temperature dependence that arises solely due to the excited state vibronic mixing. Our findings suggest an interesting experiment to decipher the presence of excited state vibronic resonances.