In the liquid phase, water molecules are connected to one another via hydrogen bonds. Fluctuations in the water network occur on the sub-ps and ps timescale. THz absorption measurements (1 THz = 1012 Hz = 1 ps-1) "see" these dynamic reorientations of dipole moments. This makes THz spectroscopy a sensitive tool to probe solute-induced changes in the collective water network motions.
Ultracold Chemistry: How do molecules aggregate at ultracold temperatures?
Do chemical reactions exist at temperatures barely above absolute zero?
Microsolvation: How many water molecules are necessary to separate ion pairs in acids?