Dynamical Collectivity and Nuclear Quantum Effects on the Intermolecular Stretching Mode of Liquid Water

This study investigated the broadband terahertz and low-frequency Raman spectroscopy of liquid water (H2O, D2O, and (H2O)-O-18) over 2 decades of frequency to address long-standing challenges regarding the interpretation of the intermolecular stretching mode at around 5 THz. We experimentally demonstrated that the intermolecular stretching mode of liquid water obtained via terahertz spectroscopy is significantly redshifted and broadened compared with that via Raman. This result was rationalized by the enhanced dynamical collectivity probed by terahertz spectroscopy, although both have a common origin in the kinetic motion. Their temperature and isotope dependences emphasize the significance of oscillation mass in determining the intermolecular stretching lineshape, while quantum effects cannot be overlooked in both terahertz and low-frequency Raman spectra.

Automated summary

This study compared the broadband terahertz and low-frequency Raman spectroscopy of liquid water to address challenges regarding the interpretation of the intermolecular stretching mode. The experiment demonstrated a significant redshift and broadening in the intermolecular stretching mode obtained via terahertz spectroscopy compared to Raman, attributed to enhanced dynamical collectivity. The temperature and isotope dependences emphasize the role of oscillation mass in determining the intermolecular stretching lineshape, while quantum effects are important in both terahertz and low-frequency Raman spectra.