Four-Dimensional Coherent Spectroscopy of Complex Molecular Systems in Solution

Abstract

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.

Equation-of-Motion Methods for the Calculation of Femtosecond Time-Resolved 4-Wave-Mixing and N-Wave-Mixing Signals

“…With the one-sided CW pulse EOM-PMA, one can perform 21 propagations of the auxiliary DMs in time t and compute P k (ω τ , τ′, T , T ′, t ) for fixed ω τ or I 3 D (ω τ , τ′, ω t , T , T ′) for fixed ω τ and τ′. Fifth-order two-quantum 2D, three-quantum 2D, and 8WM 4D signals can also be treated in a similar manner.…”

Uncovering the Functional Role of Coherent Phonons during the Photoinduced Phase Transition in a Molecular Crystal

“…Abundant research has identified the important roles of vibrational coherences in important natural processes, including in the photoisomerization of retinal that is responsible for vision, − exciton transport in light-harvesting complexes, − excited state proton transport in the green fluorescent protein, and a variety of charge transport materials. − However, many researchers have provided pertinent counterarguments to the functional role of these vibrational coherences, particularly with respect to the photosynthetic light-harvesting complex. − Worse yet, identifying the influence of molecular vibrations on photochemistry can be quite complicated, especially for complex systems in which energy can be partitioned over many possible pathways. Discerning the direct role of vibrations and phonons in photochemical reactivity requires further investigation.…”