Abstract
An understanding of microscopic interactions in liquids is of fundamental importance in chemistry. However, the structure and dynamics of complex systems in the condensed-phase, especially far from thermal equilibrium, is masked by broad, and often features, absorption and emission spectra. Nonlinear optical spectroscopy has proven to be a powerful and general approach to disentangling congested spectra by spreading information across multiple dimensions, revealing features oftentimes hidden in lower-order projections. As the dimensionality of the measurement increases, the better the microscopic interactions are revealed as spectral bands disperse in the large hyper-spectral volume. This, however, comes at a steep price as the signal decreases exponentially with increasing noise and experimental complexity. Here, we discuss a four-dimensional coherent spectroscopy that reveals coupling between electronic and vibrational transitions in complex, condensed-phase systems ranging from organic molecules to semiconductor nanocrystals. We reveal that high-resolution spectra may be extracted from these systems even in the presence of severe spectral broadening, both homogeneous and inhomogeneous in origin. The theoretical and experimental underpinnings of this method are discussed. Increasingly higher-order and higher-dimensionality spectroscopies are needed to understand the microscopic interactions that connect structure to dynamics to function.