Rigorous quantum scattering calculations on ultracold molecular collisions in external fields present an outstanding computational problem due to strongly anisotropic atom-molecule interactions that depend on the relative orientation of the collision partners, as well as on their vibrational degrees of freedom. Here, we present the first numerically exact three-dimensional quantum scattering calculations on strongly anisotropic atom-molecule (Li $+$ CaH) collisions in an external magnetic field based on the parity-adapted total angular momentum representation and a new three-dimensional potential energy surface for the triplet Li-CaH collision complex developed using the unrestricted coupled-cluster method with single, double, and perturbative triple excitations and a large quadruple-zeta-type basis set. We find that while the full three-dimensional treatment is necessary for the accurate description of cold $\mathrm{Li}(M_S=1/2)+\mathrm{CaH}(v=0,N=0,M_S=1/2)$ collisions in a magnetic field, the magnetic resonance density and statistical properties of spin-polarized atom-molecule collisions are not strongly affected by vibrational degrees of freedom, justifying the rigid-rotor approximation used in previous calculations. We observe rapid, field-insensitive vibrational quenching in ultracold $\mathrm{Li}(M_S=1/2)+\mathrm{CaH}(v=1,N=0,M_S=1/2)$ collisions, leading to efficient collisional cooling of CaH vibrations.

• Received 28 April 2020
• Accepted 26 October 2020

DOI:https://doi.org/10.1103/PhysRevResearch.2.043294

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Published by the American Physical Society