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arXiv:2404.07416 · Physical Review Letters

Highly Spin-Polarized Molecules via Collisional Microwave Pumping

Rebekah Hermsmeier, Timur V. Tscherbul

Published Oct 21, 2024• 0 citations
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Abstract

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.

DOI
10.1103/physrevlett.133.173001

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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.
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.
A scheme is proposed for making highly rotationally excited diatomic molecules (“super rotors”) in their ground vibrational and electronic state, e.g., 6Li2X 1Σg+ (v=0,J⩾115) where the rotational energy exceeds the bond strength (E(0,J)−E(0,0)⩾D00). Such levels, while strictly speaking quasibound, have very long tunneling lifetimes (>1011 s for J⩽130), and should have very interesting and unique collisional properties, especially at low temperature. The rotation of the molecules is “spun up” by sequential irradiation by R branch photons in the A 1Σu+–X 1Σg+ bands starting with cold molecules at low J. Spontaneous emission to other vibrational levels is overcome by using a pump laser and its multiple Raman sidebands as in previous work on “spinning down” diatomics.
We have calculated cross sections for vibrational quenching in collisions of O217(X 3∑g−) with He3 at cold and ultracold temperatures, explicitly accounting for the electronic spin degree of freedom. The observed population of the final spin-rotational levels can be interpreted on the basis of useful “propensity rules” governing the intensity of the collision-induced transitions. In addition, it is found that the orientation of the molecular spin is reasonably robust against collisional relaxation, even in vibrationally excited states. This finding implies that magnetic trapping of vibrationally excited, but rotationally and translationally cold, molecules may be possible.
Recent experiments on formation of translationally cold ground state molecules, their subsequent broadband vibrational cooling and study of rotations are presented together with the related modeling. We produce cold molecules by photoassociating pairs of cold cesium atoms that can decay into ground state molecules in different vibrational levels. Then we laser cool the vibrational degree of freedom by selecting a single target vibrational level. Our method is based on repeated optical pumping by laser light with a spectrum broad enough to excite all populated vibrational levels but limited in its frequency bandwidth with a spatial light modulator. In such a way we are able to eliminate transitions from the selected level, in which molecules accumulate. Results for vibrational cooling into the v = 0, v = 1, v = 2 and v = 7 level of the singlet X 1Σ g ground electronic state are presented. Depletion spectroscopy is performed to study the rotational distribution of the created molecules. In the theoretical modeling of the process we are able to reproduce our results and investigate the prospects for a ‘complete’ cooling of molecules, including also their rotational degree of freedom.
We explore the effects of microwave laser fields on spin-changing transitions in collisions of polar molecules in a $^{2}\ensuremath{\Sigma}$ electronic state with atoms in the presence of a static magnetic field. We use the dressed-state formalism of quantum optics to describe the interaction of the radiation field with the collision complex and perform rigorous scattering calculations to study spin-changing transitions in collisions of CaH molecules with He atoms at temperatures below 1 K. Our calculations demonstrate that collision-induced spin relaxation of $^{2}\ensuremath{\Sigma}$ molecules is enhanced near avoided crossings between different Zeeman levels. We show that the positions of the avoided crossings can be modified by varying the frequency and the intensity of the microwave laser field and discuss the mechanisms of microwave field control of spin-dependent interactions in cold open-shell molecules.
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.