Collisions of ultracold ²³Na⁸⁷Rb molecules with controlled chemical reactivities

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The collision of molecules at ultracold temperatures is of great importance for understanding the chemical interactions at the quantum regime. While much theoretical work has been devoted to this, experimental data are only available sparsely mainly due to the difficulty in producing ground-state molecules at ultracold temperatures. We report here the creation of optically trapped samples of ground-state bosonic sodium-rubidium molecules with precisely controlled internal states and, enabled by this, a detailed study on the inelastic loss with and without the NaRb + NaRb → Na₂ + Rb₂ chemical reaction. Contrary to intuitive expectations, we observed very similar loss and heating, regardless of the chemical reactivities. In addition, as evidenced by the reducing loss rate constants with increasing temperatures, we found that these collisions are already outside the Wigner region even though the sample temperatures are sub-microkelvin. Our measurement agrees semi-quantitatively with models based on long-range interactions, but calls for a deeper understanding on the short-range physics for a more complete interpretation.