![]() ![]() In this work, we present an approach that provides direct correlation between impulsively driven low-frequency modes such as phonons, vibrations and (multi-)excitons with quantum coherence selectivity through control of resonance. The shortcomings of current four-wave mixing (4WM) techniques, such as 2DFT spectroscopy 23, 24, motivate the development of higher-order spectroscopies that may enable unambiguous assignment of signals to specific coherent pathways. Although much progress has been made in better understanding and modelling these protein complexes 13, 14, 18, 19, 20, 21, 22, until now there has been no direct (that is, model independent) method for assigning these signals to specific coherent electronic or vibrational pathways, therefore leaving their physical origins unresolved. For instance, the physical origins of some coherent phenomena observed 15, 16, 17 in photosynthetic pigment–protein complexes using two-dimensional Fourier-transform (2DFT) spectroscopy remain elusive, partly due to the ambiguity of spectral signatures arising from related signal pathways. Despite tremendous effort, revealing these interactions experimentally has proved extremely challenging. In natural systems, interactions between electronic and vibrational degrees of freedom may have important implications for rapid and efficient energy transfer 9, 10, 11, 12, 13, 14. For instance, electron–phonon interactions underlie the carrier properties of a wide range of promising light harvesting materials including hybrid perovskites 1, 2, organic crystals that undergo singlet fission 3, 4 and quantum-confined nanostructures 5, 6, 7, 8. The interaction between different degrees of freedom in coupled molecular systems-electrons, nuclei and phonons-dictate many of their most important physical properties. This method is suited for studying elusive quantum effects in which electronic transitions strongly couple to phonons and vibrations, such as energy transfer in photosynthetic pigment–protein complexes. The vibronic structure of the system is then revealed within an otherwise broad and featureless 2D electronic spectrum. A critical feature of this method is electronic and vibrational frequency resolution, enabling isolation and assignment of individual quantum coherence pathways. ![]() By combining near-impulsive resonant and non-resonant excitation, the desired fifth-order signal of a complex organic molecule in solution is measured free of unwanted lower-order contamination. Specifically, we measure a fully coherent four-dimensional spectrum which simultaneously encodes vibrational–vibrational, electronic–vibrational and electronic–electronic interactions. Here, we demonstrate a method that probes correlations between states within the vibrational and electronic manifold with quantum coherence selectivity. Electronic and vibrational correlations report on the dynamics and structure of molecular species, yet revealing these correlations experimentally has proved extremely challenging. ![]()
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