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Physical Review A

Mesoscopic production of hyperpolarized N 2 15 O and H 2 O via optical excitation

Dimitris Sofikitis, Theodore Peter Rakitzis

Published Sep 10, 2015• 4 citations
Publisher

Abstract

We propose laser-based methods for the production of nuclear spin hyperpolarized molecules. Following pulsed, coherent excitation from ground-state molecules in a molecular beam to specific rotational $|J\phantom{\rule{0.16em}{0ex}}{m}_{J}\ensuremath{\rangle}$ states, the resulting hyperfine polarization beating produces desirable nuclear spin ${m}_{I}$ population distributions and ${m}_{I}\ensuremath{-}{m}_{J}$ correlations at specific later times. At this point, selective dissociation can be used to remove undesired spin states, or a strong magnetic field can be applied, which later can be removed adiabatically to transfer the population-enhanced nuclear spin configurations to the desired nuclear spin state. Finally, the polarization beating is stopped, and the molecules are stored by cryogenic trapping. We show how these methods can be used to produce the long-lived singlet state of $^{15}\mathrm{N}_{2}\mathrm{O}$ and ortho ${\mathrm{H}}_{2}\mathrm{O}$, which have been proposed as magnetic tracers. We discuss how large production rates between 1 and $100 \ensuremath{\mu}\mathrm{mol}$/s with singlet-state populations between 35% and 100% can be achieved.

DOI
10.1103/physreva.92.032507

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We propose a general technique to produce cold spin-polarized molecules in the electronic states of Σ symmetry, in which rotationally excited levels are first populated by coherent microwave excitation, and then allowed to spin flip and relax via collisional quenching, which populates a single final spin state. The steady-state spin polarization is maximized in the regime, where collisional slip-flipping transitions in the ground rotational manifold (N=0) are suppressed by a factor of ≥10 compared to those in the first rotationally excited manifold (N=1), as generally expected for Σ-state molecules at temperatures below the rotational spacing between the N=0 and N=1 manifolds. We theoretically demonstrate the high selectivity of the technique for ^{13}C^{16}O molecules immersed in a cold buffer gas of helium atoms, achieving a high degree (≥95%) of nuclear spin polarization at 1 K.