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From Gyroscopic to Thermal Motion: A Crossover in the Dynamics of Molecular Superrotors

A. A. Milner, A. Korobenko, K. Rezaiezadeh, and V. Milner
Phys. Rev. X 5, 031041 – Published 23 September 2015
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Abstract

Localized heating of a gas by intense laser pulses leads to interesting acoustic, hydrodynamic, and optical effects with numerous applications in science and technology, including controlled wave guiding and remote atmosphere sensing. Rotational excitation of molecules can serve as the energy source for raising the gas temperature. Here, we study the dynamics of energy transfer from the molecular rotation to heat. By optically imaging a cloud of molecular superrotors, created with an optical centrifuge, we experimentally identify two separate and qualitatively different stages of its evolution. The first nonequilibrium “gyroscopic” stage is characterized by the modified optical properties of the centrifuged gas—its refractive index and optical birefringence, owing to the ultrafast directional molecular rotation, which survives tens of collisions. The loss of rotational directionality is found to overlap with the release of rotational energy to heat, which triggers the second stage of thermal expansion. The crossover between anisotropic rotational and isotropic thermal regimes is in agreement with recent theoretical predictions and our hydrodynamic calculations.

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  • Received 10 June 2015

DOI:https://doi.org/10.1103/PhysRevX.5.031041

This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Synopsis

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Gyroscopic Molecules

Published 23 September 2015

Fast-rotating molecules spun up by a laser pulse maintain their alignment despite collisions.

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Authors & Affiliations

A. A. Milner, A. Korobenko, K. Rezaiezadeh, and V. Milner

  • Department of Physics & Astronomy, The University of British Columbia, Vancouver, Canada V6T 1Z1

Popular Summary

The optical properties of dense gases such as ambient air are important for telecommunication, remote sensing, and design of new sources of radiation. Recent research has revealed how to control these properties: Spinning the gas molecules up with intense laser pulses. The deposited rotational energy is converted to heat through collisions, which increase the temperature of the gas and change its refractive index. Although the end result of such collisional thermalization comes as no surprise, the transition from a nonequilibrium ensemble of synchronously rotating molecules to their final thermal state is far from understood. Here, we analyze the exact dynamics of this transition using oxygen molecules, and we study how and when the optical properties of a rotationally excited gas change during the thermalization process.

We employ a gas cell filled with O2 at a pressure slightly less than atmospheric pressure. We expose the molecules to shaped laser pulses, known as an “optical centrifuge,” which creates an ensemble of molecular superrotors. The molecular superrotors, spinning at ultrafast frequencies like miniature gyroscopes, preserve the directionality of their rotation throughout multiple collisions. During this “gyroscopic stage” of rotational dynamics, the refractive index of the centrifuged gas changes and gives rise to strong optical birefringence and the appearance of a refractive “gyroscopic channel.” As the superrotors lose their rotational directionality, the disappearance of the gyroscopic channel is accompanied by the formation of a thermal channel. The gyroscopic-to-thermal crossover is observed and characterized here for the first time. We record the gas density with a temporal resolution of tens of nanoseconds, and we study how rotational energy is translated into heat.

Our experimental analysis of the gyroscopic stage and its transition to the thermal phase is a step toward exploring an exciting new area of ultrafast rotational control of the optical, acoustical, and hydrodynamical properties of gases.

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Vol. 5, Iss. 3 — July - September 2015

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