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 in terms of weakly anharmonic nuclear potentials. With water, the situation is not so simple. The strong anisotropic hydrogen-bonds formed in water result in drastic deviation from harmonic behavior. In this talk, I will describe our efforts to understand the molecular dynamics of liquid water using broadband 2D IR spectroscopy.
The data show rich and complex dynamics driven by the strong mixing between inter- and intra-molecular modes. This mixing gives rise to excitonic O–H stretching vibrations whose fluctuations result in ultrafast relaxation. The relaxation is determined solely by the nature of the excited vibrations as evidenced by the fact that it is completely insensitive to temperature. We observe non-adiabatic effects in which low-frequency modes are driven in concert with high-frequency ones; as a result, the water spectrum appears to have risen in temperature almost immediately after excitation despite being in a highly non-equilibrium state. Some of the most dramatic behavior is seen in the comparison between light (H2O) 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.