Two-dimensional infrared-Raman spectroscopy as a probe of water's tetrahedrality

Direct spectroscopic probes of the impact of structure on dynamical processes in liquids remain scarce. Here, the authors use molecular dynamics simulations to show that the correlation between vibrational coupling and the local tetrahedral structure of liquid water can be studied via hybrid terahertz- and infrared-Raman spectroscopy. Two-dimensional spectroscopic techniques combining terahertz (THz), infrared (IR), and visible pulses offer a wealth of information about coupling among vibrational modes in molecular liquids, thus providing a promising probe of their local structure. However, the capabilities of these spectroscopies are still largely unexplored due to experimental limitations and inherently weak nonlinear signals. Here, through a combination of equilibrium-nonequilibrium molecular dynamics (MD) and a tailored spectrum decomposition scheme, we identify a relationship between the tetrahedral order of liquid water and its two-dimensional IR-IR-Raman (IIR) spectrum. The structure-spectrum relationship can explain the temperature dependence of the spectral features corresponding to the anharmonic coupling between low-frequency intermolecular and high-frequency intramolecular vibrational modes of water. In light of these results, we propose new experiments and discuss the implications for the study of tetrahedrality of liquid water.