Sergey S. Zhukov, Vasileios Balos, Gabriela Hoffman, Shamim Alom, Mikhail Belyanchikov, Mehmet Nebioglu, Seulki Roh, Artem Pronin, George R. Bacanu, Pavel Abramov, Martin Wolf, Martin Dressel, Malcolm H. Levitt, Richard J. Whitby, Boris Gorshunov, and Mohsen Sajadi :
? ( ), (2020), pp. ;
arXiv:2006.02687 [physics.chem-ph] (2020), pp. 9;
DOI: arXiv:2006.02687 [physics.chem-ph]
Encapsulation of a single water molecule in fullerene-C60 via chemical surgery provides a unique opportunity to study the distinct rotational dynamics of the water spin isomers at cryogenic temperatures. Here, we employ single-cycle terahertz (THz) pulses to coherently excite the low-frequency rotational motion of ortho- and para-water, encapsulated in fullerene-C60. The THz pulse slightly orients the water electric dipole moments along the field polarization leading to the subsequent emission of electromagnetic waves, which we resolve via the field-free electro-optic sampling technique. At temperatures above ~100 K, the rotation of water in its cage is overdamped and no emission is resolved. At lower temperatures, the water rotation gains a long coherence decay time, allowing observation of the coherent emission for 10-15 ps after the initial excitation. We observe the real-time change of the emission pattern after cooling to 4 K, corresponding to the conversion of a mixture of ortho-water to para-water over the course of 10 hours.
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