Four-Dimensional Coherent Spectroscopy of Complex Molecular Systems in Solution

An understanding of microscopic interactions in solution is of fundamental importance in chemistry. However, the structure and dynamics of complex systems in the condensed phase, especially far from thermal equilibrium, are masked by broad and often featureless 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 is increased, the better the microscopic interactions are revealed, as spectral bands disperse in the large hyper-spectral volume. This capability, however, comes at a steep price, as the signal decreases exponentially at higher orders of optical response, and added experimental complexity increases noise. We discuss a 4D coherent spectroscopy known as gradient-assisted multidimensional electronic-Raman spectroscopy (GAMERS) 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 like GAMERS are needed to understand the microscopic interactions that connect structure to dynamics to function.