Anharmonic Dynamics in Liquid Water Explored With Time-Resolved Broadband Multidimensional IR Spectroscopy
Water molecules play an essential and active dynamical role in aqueous processes such as chemical reactions, charge transport, and protein folding. These dynamics, largely dictated by water’s extended hydrogen-bond network, have proven difficult to study directly due to the sub-picosecond timescales involved. Fortunately, the liquid’s nuclear motions are encoded in its IR absorption spectrum, which spans several orders of magnitude in frequency. Two-dimensional infrared spectroscopy (2D IR) is a nonlinear time-resolved technique that allows us to track frequency correlations in the IR spectrum with femtosecond time resolution, and it has proven itself useful in studying condensed phase molecular dynamics. Using a novel plasma-based broadband-IR source, we have extended our experiment to measure IR correlations over 2000 cm‑1 of bandwidth, allowing us to track water’s motions throughout the entire mid-IR.
We typically interpret vibrational spectra water, the s 2O) and heavy water (D2O), which show different dynamics and qualitatively different modes. This is entirely unexpected for a harmonic system and is a result of the extreme anharmonicity induced by the hydrogen-bonding interaction. Our results point to water playing an even more important role in aqueous phenomena than previously thought, and this role varies substantially whether we consider H2O or D2O.