Magneto-Optical Properties of Paramagnetic Superrotors

A. A. Milner, A. Korobenko, J. Floß, I. Sh. Averbukh, and V. Milner

Phys. Rev. Lett. 115, 033005 – Published 14 July 2015

Abstract

We study the dynamics of paramagnetic molecular superrotors in an external magnetic field. An optical centrifuge is used to create dense ensembles of oxygen molecules in ultrahigh rotational states. In is shown, for the first time, that the gas of rotating molecules becomes optically birefringent in the presence of a magnetic field. The discovered effect of “magneto-rotational birefringence” indicates the preferential alignment of molecular axes along the field direction. We provide an intuitive qualitative model, in which the influence of the applied magnetic field on the molecular orientation is mediated by the spin-rotation coupling. This model is supported by the direct imaging of the distribution of molecular axes, the demonstration of the magnetic reversal of the rotational Raman signal, and by numerical calculations.

Received 17 October 2014

DOI:https://doi.org/10.1103/PhysRevLett.115.033005

Authors & Affiliations

A. A. Milner¹, A. Korobenko¹, J. Floß², I. Sh. Averbukh², and V. Milner¹

¹Department of Physics and Astronomy, The University of British Columbia, Vancouver V6T 2K9, Canada
²Department of Chemical Physics, The Weizmann Institute of Science, Rehovot 76100, Israel

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Vol. 115, Iss. 3 — 17 July 2015

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Images

Figure 1
(a) Experimental setup. BS, beam splitter; DM, dichroic mirror; LP (LA), linear polarizer (analyzer) oriented at angle $θ_{p} (θ_{p} + 90 °)$ with respect to $\vec{B}$ ; DL, delay line; $L$ , lens; $M$ . two magnetic coils connected in a Helmholtz configuration. “ $O_{2}$ ” marks the pressure chamber filled with oxygen gas under pressure $P$ at room temperature. An optical centrifuge field is illustrated above the centrifuge shaper with $\vec{k}$ being the propagation direction and $\vec{E}$ the vector of linear polarization undergoing an accelerated rotation. (b) Geometry of the magnetic and optical fields used in this work. The cloud of centrifuged molecules is depicted as a dark ellipse.
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Figure 2
Spectrum of probe pulses transmitted through the ensemble of centrifuged oxygen molecules as a function of the applied magnetic field. All spectra have been recorded at the probe delay of $t = 1.14 ns$ and under 0.3 atm of gas pressure. A crossed circular (rather than linear) polarizer and analyzer were used here to detect a weak magneto-rotational Raman signal [note the change of vertical scale ( $\times 200$ ) at frequencies higher than 7 THz].
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Figure 3
(a),(b) Calculated angular distribution of the molecular axes for the rotational state with $N = 59$ at time $t = 0.9 ns$ in an external magnetic field of 0 and 0.32 T, respectively. (c) Birefringence signal (scaled to peak at 1) as a function of angle $θ_{p}$ between the polarization of probe pulses and the magnetic field direction. Black circles: data taken at 2 T, $t = 1.5 ns$ , and $N = 95$ . The red curve is a fit to $\cos^{2} (θ_{p})$ . (d) Experimentally measured distribution of molecular axes, imaged in the direction of the applied field. All parameters are the same as in (b).
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Figure 4
(a) Decay of the birefringence signal for different values of the rotational quantum number at $B = 2 T$ . All solid curves are generated by spline fitting the experimental data (shown with colored markers) and normalized to the peak at 1. The corresponding lifetimes are $85 \pm 10, 290 \pm 20, 660 \pm 50$ , and $610 \pm 50 ps atm$ for $N = 13, 33, 73$ , and 99, respectively. Colored dashed lines indicate the rate of rotational energy transfer due to the $N$ -changing collisions [32]. The black dashed line represents the asymptote of the rotational decoherence rate at the limit of ultrahigh angular momenta. (b) Dependence of the birefringence signal recorded at $t = 1 ns$ on the strength of the applied magnetic field.
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