Copious physical, chemical and thermodynamic properties make water a unique material. For instance, it is known that confined water in nano-cages behaves differently from bulk water. Recent studies even indicate on quantum behavior and incipient ferroelectricity of water in nano-cages. To further study the behavior of confined water molecules, we use H2O@C60 system: encapsulated single water molecule in fullerene-C60 and study the distinct rotational dynamics of water’s spin isomers at cryogenic temperatures. We employ single-cycle terahertz (THz) pulses to coherently excite the low-frequency rotational motion of ortho- and para-water. The excitation leads to the slight orientation of water’s permanent dipoles towards the field polarization and consequently to the emission of electromagnetic waves, which we resolve via the field-free electro-optic sampling technique. We discuss our results on the real-time conversion of ortho- to para-water at 4 K and further show the direct impact of temperature on rotational degrees of freedom of entrapped water inside its cage.

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