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arXiv:2212.05363 · The Journal of Physical Chemistry A

Nuclear Spin Relaxation in Cold Atom–Molecule Collisions

Rebekah Hermsmeier, Xiaodong Xing, Timur V. Tscherbul

Published May 16, 2023• 6 citations
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Abstract

We explore the quantum dynamics of nuclear spin relaxation in cold collisions of 1 Σ + molecules with structureless atoms in an external magnetic field. To this end, we develop a rigorous coupled-channel methodology, which accounts for rotational and nuclear spin degrees of freedom of 1 Σ + molecules and their interaction with an external magnetic field as well as anisotropic atom–molecule interactions. We apply the methodology to study the collisional relaxation of the nuclear spin sublevels of 13 CO molecules immersed in a cold buffer gas of 4 He atoms. We find that nuclear spin relaxation in the ground rotational manifold ( N = 0) of 13 CO occurs extremely slowly due to the absence of direct couplings between the nuclear spin sublevels. The rates of collisional transitions between the rotationally excited ( N = 1) nuclear spin states of 13 CO are generally much higher due to the direct nuclear spin–rotation coupling between the states. These transitions obey selection rules, which depend on the values of space-fixed projections of rotational and nuclear spin angular momenta ( M N and M I ) for the initial and final molecular states. For some initial states, we also observe a strong magnetic field dependence, which can be understood by using the first Born approximation. We use our calculated nuclear spin relaxation rates to investigate the thermalization of a single nuclear spin state of 13 CO( N = 0) immersed in a cold buffer gas of 4 He. The calculated nuclear spin relaxation times ( T 1 ≃ 1 s at T = 1 K at a He density of 10 –14 cm –3 ) display a steep temperature dependence decreasing rapidly at elevated temperatures due to the increased population of rotationally excited states, which undergo nuclear spin relaxation at a much faster rate. Thus, long relaxation times of N = 0 nuclear spin states in cold collisions with buffer gas atoms can be maintained only at sufficiently low temperatures ( k B T ≪ 2 B e ), where B e is the rotational constant.

DOI
10.1021/acs.jpca.2c08646

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We report a combined experimental and theoretical study of collision-induced dipolar relaxation in a cold spin-polarized gas of atomic nitrogen (N). We use buffer gas cooling to create trapped samples of 14N^{14}\mathrm{N}14N and 15N^{15}\mathrm{N}15N atoms with densities $(5\ifmmode\pm\else\textpm\fi{}2)\ifmmode\times\else\texttimes\fi{}{10}^{12} {\mathrm{cm}}^{\ensuremath{-}3}$ and measure their magnetic relaxation rates at milli-Kelvin temperatures. These measurements, together with rigorous quantum scattering calculations based on accurate ab initio interaction potentials for the ${}^{7}{\ensuremath{\Sigma}}_{u}^{+}$ electronic state of N2{\mathrm{N}}_{2}N2​ demonstrate that dipolar relaxation in N+N\mathrm{N}+\mathrm{N}N+N collisions occurs at a slow rate of $~{10}^{\ensuremath{-}13} {\mathrm{cm}}^{3}$/s over a wide range of temperatures (1 mK to 1 K) and magnetic fields (10 mT to 2 T). The calculated dipolar relaxation rates are insensitive to small variations of the interaction potential and to the magnitude of the spin-exchange interaction, enabling the accurate calibration of the measured N atom density. We find consistency between the calculated and experimentally determined rates. Our results suggest that N atoms are promising candidates for future experiments on sympathetic cooling of molecules.
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