Abstract
We resolve the real-time coherent rotational motion of isolated water molecules encapsulated in fullerene-C60 cages by time-domain terahertz (THz) spectroscopy. We employ single-cycle THz pulses to excite the low-frequency rotational motion of water and measure the subsequent coherent emission of electromagnetic waves by water molecules. At temperatures below ~ 100 K, C60 lattice vibrational damping is mitigated and the quantum dynamics of confined water are resolved with a markedly long rotational coherence, extended beyond 10 ps. The observed rotational transitions agree well with low-frequency rotational dynamics of single water molecules in the gas phase. However, some additional spectral features with their major contribution at ~2.26 THz are also observed which may indicate interaction between water rotation and the C60 lattice phonons. We also resolve the real-time change of the emission pattern of water after a sudden cooling to 4 K, signifying the conversion of ortho-water to para-water over the course of 10s hours. The observed long coherent rotational dynamics of isolated water molecules confined in C60 makes this system an attractive candidate for future quantum technology.
Highlights
We resolve the real-time coherent rotational motion of isolated water molecules encapsulated in fullerene-C60 cages by time-domain terahertz (THz) spectroscopy
The rotational dynamics of water has a complex pattern because of (i) the low symmetry of water molecules -causing the irregular spacing of its rotational energy levels- and (ii) the constraint that is imposed on its rotational quantum wave function due to the symmetry of the wave functions of its spin isomers
Our goal is to understand whether the encapsulated water in the highly symmetrical, homogenous and isolated environment of the inner space of C 60 is able to rotate coherently, and if so, what is the coherence time of its rotation? Since there is no hydrogen binding between neighboring encaged water molecules, can the rotational coherence of water be resolved at cryogenic temperatures? can we capture the conversion of the spin isomers of water through its real-time coherent rotational motions?
Summary
We resolve the real-time coherent rotational motion of isolated water molecules encapsulated in fullerene-C60 cages by time-domain terahertz (THz) spectroscopy. The rotational dynamics of water has a complex pattern because of (i) the low symmetry of water molecules -causing the irregular spacing of its rotational energy levels- and (ii) the constraint that is imposed on its rotational quantum wave function due to the symmetry of the wave functions of its spin isomers. The high symmetry of the cage allows water to rotate freely, with no perceptible hindering potential This conclusion has been corroborated by nuclear magnetic r esonance[29], molecular dynamics (MD) simulations[30] and multidimensional quantum calculations of the energy level structure of the encapsulated molecule[31,32]. The most prominent influence of the molecular environment on the lowest rotational states of the encapsulated water is the breaking of the threefold degeneracy of the orthowater ground state[23,24], which has been attributed to an electric quadrupolar field generated by neighboring fullerene molecules in the lattice[33,34,35]
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