Four-dimensional coherent spectroscopy

The ground- and excited-state electronic and vibrational structure of condensed-phase systems dictates many of their functions. This structure, however, is oftentimes difficult to observe experimentally as the system increases in size and complexity due to short lifetimes and rapid decoherence. One powerful, and, general, approach to elucidating these interactions is to perform increasingly higher-order and higher-dimensionality optical spectroscopies, which have found success in a wide variety of chemical and biological systems and materials. While this approach indeed leads to increased spectral resolution by spreading the spectral information across multiple, coherently coupled dimensions, it also increases the number of signal pathways, and the signal strength falls exponentially. Experiment complexity, long acquisition times, and challenging spectroscopic interpretation has, thus far, made this approach impractical beyond three dimensions. Here, we discuss advances in four-dimensional spectroscopy that addresses, and overcomes, these challenges. We demonstrate that resonance may be used to control which pathways contribute to the signal, thereby greatly simplifying the physical interpretation in comparison to lower-order and fully-resonant experiments. Further, we show that contrary to expectation, and given the dramatically lower signal strength, orders-of-magnitude higher dynamic range is achievable than with lower-order measurements. The analogy of ideas presented here for sampling and reconstruction to existing approaches in NMR are discussed. Finally, we outline a strategy to utilize these methods to interrogate complex molecular systems in solution, show experimental results on organic molecules, and discuss prospects for studying more complex systems such as semiconductor nanocrystals and photosynthetic proteins.