Rotational Coherence of Encapsulated Ortho and Para Water in Fullerene-C60

TL;DR

This study uses terahertz pulses to investigate the rotational coherence of water molecules encapsulated in fullerene-C60, revealing temperature-dependent dynamics and ortho-para conversion over time.

Contribution

It demonstrates a novel method to coherently excite and observe the rotational dynamics of encapsulated water spin isomers at cryogenic temperatures.

Findings

Long coherence times observed below 100 K

Real-time emission pattern changes at 4 K

Conversion of ortho- to para-water over 10 hours

Abstract

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.