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arXiv:2109.07435 · New Journal of Physics

Complexes formed in collisions between ultracold alkali-metal diatomic molecules and atoms

Matthew D. Frye, Jeremy M. Hutson

Published Dec 1, 2021• 22 citations
arXivHTMLPDF

Abstract

Abstract We explore the properties of three-atom complexes of alkali-metal diatomic molecules with alkali-metal atoms, which may be formed in ultracold collisions. We estimate the densities of vibrational states at the energy of atom–diatom collisions, and find values ranging from 2.2 to 350 K−1. However, this density does not account for electronic near-degeneracy or electron and nuclear spins. We consider the fine and hyperfine structure expected for such complexes. The Fermi contact interaction between electron and nuclear spins can cause spin exchange between atomic and molecular spins. It can drive inelastic collisions, with resonances of three distinct types, each with a characteristic width and peak height in the inelastic rate coefficient. Some of these resonances are broad enough to overlap and produce a background loss rate that is approximately proportional to the number of outgoing inelastic channels. Spin exchange can increase the density of states from which laser-induced loss may occur.

DOI
10.1088/1367-2630/ac3ff8

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Abstract We explore the properties of three-atom complexes of alkali-metal diatomic molecules with alkali-metal atoms, which may be formed in ultracold collisions. We estimate the densities of vibrational states at the energy of atom–diatom collisions, and find values ranging from 2.2 to 350 K−1. However, this density does not account for electronic near-degeneracy or electron and nuclear spins. We consider the fine and hyperfine structure expected for such complexes. The Fermi contact interaction between electron and nuclear spins can cause spin exchange between atomic and molecular spins. It can drive inelastic collisions, with resonances of three distinct types, each with a characteristic width and peak height in the inelastic rate coefficient. Some of these resonances are broad enough to overlap and produce a background loss rate that is approximately proportional to the number of outgoing inelastic channels. Spin exchange can increase the density of states from which laser-induced loss may occur.
We consider the long-range states expected for complexes formed from an alkali-metal diatomic molecule in a singlet state and an alkali-metal atom. We explore the structure of the Hamiltonian for such systems, and the couplings between the six angular momenta that are present. We consider the patterns and densities of the long-range states, and the terms in the Hamiltonian that can cause Feshbach resonances when the states cross threshold as a function of magnetic field. We present a case study of 40K87Rb+87Rb^{40}\mathrm{K}^{87}\mathrm{Rb}+^{87}\mathrm{Rb}40K87Rb+87Rb. We show multiple types of resonance due to long-range states with rotational and/or hyperfine excitation, and consider the likelihood of them existing at low to moderate magnetic fields.
It has recently become possible to form molecules in ultracold gases of trapped alkali metal atoms. Once formed, the molecules may undergo elastic, inelastic and reactive collisions. Inelastic and reactive collisions are particularly important because they release kinetic energy and eject atoms and molecules from the trap. The theory needed to handle such collisions is presented and recent quantum dynamics calculations on ultracold atom–diatom collisions of spin-polarized Li + Li2, Na + Na2 and K + K2 are described. All these systems have potential energy surfaces on which barrierless atom exchange reactions can occur, and both inelastic and reactive rates are very fast (typically k inel > 10-10 cm3 s−1 in the Wigner regime).
We consider the long-range states expected for complexes formed from an alkali-metal diatomic molecule in a singlet state and an alkali-metal atom. We explore the structure of the Hamiltonian for such systems, and the couplings between the six angular momenta that are present. We consider the patterns and densities of the long-range states, and the terms in the Hamiltonian that can cause Feshbach resonances when the states cross threshold as a function of magnetic field. We present a case study of 40^{40}40K87^{87}87Rb+87^{87}87Rb. We show multiple types of resonance due to long-range states with rotational and/or hyperfine excitation, and consider the likelihood of them existing at low to moderate magnetic fields.
Collisions of atoms and molecules in external electromagnetic fields can depolarize electronic and nuclear spins, leading to decoherence of quantum superposition states. The fundamental microscopic processes giving rise to spin depolarization are spin exchange and spin relaxation. In this work, we present an analysis of spin exchange transitions in collision of alkali metal atoms with 3He. We use accurate ab initio calculations to evaluate the electron spin density of Li-He and the interaction energy of Rb-He. The Fermi contact interaction constants for the complexes of Na, K, and Rb atoms with 3He are extracted from the measured frequency shift enhancement factors using the ab initio interaction potentials. The spin density calculations also yield the hyperfine pressure shift of Li-He.
We study atom-molecule inelastic scattering in a trapped alkali metal gas at ultracold temperature. We derive the atom-molecule Hamiltonian in the presence of hyperfine interactions and introduce angular momentum coupling schemes suited to the description of the short-and long-range physics of the collision. We provide a first estimate of the relative importance of rotational, hyperfine-changing, and spin-changing transitions within a simple rigid-rotor model. We also investigate the occurrence of resonance phenomena near the atom-molecule energy threshold.
II. Inelastic atom -molecule collisions 2 A. Vibrational and rotational relaxation 2 1. Collisions at cold and ultracold temperatures 2 2. Shape resonances in molecular collisions 6 3. Feshbach resonances in molecular collisions 6 B. Quasi-resonant transitions 7 C. Atom -molecular ion collisions 8 III.Chemical Reactions at ultracold temperatures 9 A. Tunneling dominated reactions 9 1.Reactions at zero temperature 10 2. Feshbach resonances in reactive scattering 12 B. Barrierless reactions 14 1.Collision systems of three alkali metal atoms 14 2. Role of PES in determining ultracold reactions 16 3.Relaxation of vibrationally excited alkali metal dimers 18 4. Reactions of heteronuclear and isotopically substituted alkali-metal dimer systems 19 IV.Inelastic molecule -molecule collisions 20 A. Molecules in the ground vibrational state 20 B. Vibrationally inelastic transitions 22 V. Summary and outlook 24 References 24 I. A. Vibrational and rotational relaxation 1. Collisions at cold and ultracold temperatures