| Low-frequency motions (<1000 cm−1) in liquids are directly connected to the complex many-body potential energy surfaces upon which the majority of chemical reactions take place. A detailed description of pure liquids enhances our understanding of solution phase processes such as reaction and solvation dynamics. However, the congested and featureless low-frequency spectra of liquids pose serious impediments to extracting important information related to vibrational anharmonicities and dephasing mechanisms. Since one-dimensional (single time or frequency) observables are only able to access essentially harmonic dynamics and are entirely insensitive to inhomogeneous and homogeneous contributions to the line shape, a higher dimensional observable is required. Fifth-order Raman spectroscopy is the lowest order technique capable of dissecting the low-frequency vibrational line shape, providing a sensitivity long enjoyed in NMR and electronic photon echo methods to unravel dense nuclear spin and electronic absorption spectra.; The great promise of fifth-order Raman spectroscopy is accompanied by considerable experimental challenges. Separating the inherently small fifth-order signal from several competing lower-order processes required the implementation of many state-of-the-art advances in nonlinear spectroscopy. The key technological development was the use of a diffractive optic beam splitter to create the six laser beams necessary for signal generation and optical heterodyne detection. A two-color pulse sequence arranged in a particularly effective phase matching beam geometry was found to provide significant direct fifth-order Raman signal levels while suppressing lower-order contaminants.; The principal finding of this work is that a modal description of liquids—that low-frequency motions can be described by independent harmonic oscillators—is incomplete. Due to the high sensitivity of the fifth-order response to anharmonicities, this work represents the first direct probe of the breakdown of the common modal description of liquid dynamics. Comparison to recent molecular dynamics simulations suggests that the dynamics leading to the fifth-order response is intimately related to the precise microscopic description of the many-body interactions in the liquid. Provided the fifth-order Raman response can be measured in interesting structured liquids, such as water, this work indicates that the extraordinary sensitivity to molecular details of this novel spectroscopic approach will substantially enhance our understanding of important dynamics in liquids. |
