Confined Water Molecules: THz spectroscopy of ortho- to para-water conversion Martin Dressel1,2, Sergey S. Zhukov2, Vasileios Balos3, Gabriela Hoffman4, Shamim Alom4, Mikhail Belyanchikov2, Mehmet Nebioglu1, Seulki Roh1, Artem Pronin1, George R. Bacanu4, Pavel Abramov2, Martin Wolf3, Malcolm H. Levitt4, Richard J. Whitby4, Boris Gorshunov2, Mohsen Sajadi3,5 1
1. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany 2 Moscow Institute of Physics and Technology, Moscow, Russia 3 Fritz-Haber-Institut der MPG, Berlin, Germany 4 School of Chemistry, University of Southampton, Southampton, United Kingdom 5 Department of Chemistry, University of Paderborn, Paderborn, Germany
Abstract—Using single cycle THz pulses, we launch coherent wave packets in the low-frequency rotational transitions of an ensemble of H2O@C60 and resolved the coherent emission of electromagnetic waves by the oriented dipoles. At 4 K, the rota-
tion of water attains a long coherent trajectory extended beyond 10 ps; indicating a transition from classical to quantum rotation of H2O. Also, a change of the pattern of the dipole emission is observed, from a mixture of ortho- and para-water to a more purified para emission after a waiting period of 10s of hours, indicating the inter-conversion of spin isomers of water at cryogenic temperatures.
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Fig. 1: Ortho-para spin conversion of water at T = 4 K. In the time window of 10 hours the ortho lines at ~0.52 THz and ~1.53 THz lose their amplitude (coded dark blue), while the para line at ~1 THz gains larger amplitude (coded red). The right panel shows the kinetics of the amplitude of the parawater (@ 1THz) and ortho-water (@ 0.52 THz). The solid lines are single exponential fits to the experimental data with ortho decay time of 11.22±0.14 hrs and para rise time of 13.5 ± 0.25 hrs..
I. INTRODUCTION
ESOLVING the real-time coherent rotational dynamics of water are of particular interest because the low symmetry of water molecules and the constraint that is imposed due to the symmetry of the wave functions of the two spin isomers: ortho-water with total nuclear spin of l = 1 and para-water with total nuclear spin of l = 0. Here we study water encapsulated in the highly symmetrical, homogenous and isolated environment of the inner space of C60. This enables us to study the rotational coherence of water at cryogenic temperatures and capture the conversion of the spin isomers of water through its realtime coherent rotational motions.
This manifests the real-time conversion of ortho-to-para spin isomers of water and demonstrate that the coherent rotation of entrapped water molecules lasts for about 15 ps inside its cage at 4 K, to the best of our knowledge the longest coherent rotational motion observed for confined molecules in nano-cages. III. SUMMARY We resolve the real-time coherent rotational motion of isolated water molecules encapsulated in fullerene-C60 cages by timedomain THz spectroscopy. 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. We also resolve the real-time change of the emission pattern of water after a sudden cooling to T = 4 K, signifying the conversion of orthowater 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.
II. RESULTS The H2O@C60 sample are excited by THz pulses with center frequency of ~1 THz, a temporal duration of 0.5 ps and bandwidth of 1.5 THz; the coherent emissions of water molecules are measured using short laser pulses with temporal resolution of ~20 fs using photo-conductive switching method. The lowest rotationnal states are almost unperturbed at low temperatures since the high symmetry of the fullerene cages allows water to rotate freely. The free-induction-decay (FID) emissions of the encapsulated water was measured over the course of ~30 hours. The sample was cooled to T = 20 K, and then the temperature rapidly lowered to 4 K. The change of the FID components over the course of 10 hours can be clearly seen in the Fourier spectra of the FID signals of the H2O@C60 in Fig. 1, where the two ortho transitions lose their strength, while the para component becomes brighter. The temporal evolution of the latter amplitude variation, taken from the ortho component 1 = 0.52 THz and the para component 2 = 1.0 THz given in the right panel.
REFERENCES S. S. Zhukov, V. Balos, G. Hoffman, S. Alom, M. Belyanchikov, M. Nebioglu, S. Roh, A. Pronin, G.R. Bacanu, P. Abramov, M. Wolf, M. Dressel, M.H. Levitt, R.J. Whitby, B. Gorshunov and M. Sajadi, Sci. Rep. 10, 18329 (2020).
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