Two-dimensional terahertz spectroscopy on water isotopes

Water has been a central figure of research for centuries due to its importance in all life and its exceptional properties including hydrogen bonding strength which gives rise to exotic effects like negative volume of melting. Deuterated waters as the isotopic twins of light water are often used in comparative studies. Despite their similar physical properties, there are still many instances where these species behave quite differently from each other in chemical systems and biology. Thus, yet to be fully explored is how water and deuterated water molecules interact with each other on a microscopic level. We attempted to improve on that by applying two-dimensional nonlinear terahertz spectroscopy to study coherent rotational interactions in H₂O, D₂O, and HOD. Consistent with previous results on pure H₂O, we observed nonlinear intermolecular interaction signals for various rotational transitions in all water isotopes with clear dependences on the concentrations and nuclear statistics. The results provide new insights into metastable water complexes that are invisible to most spectroscopic measurements