Collisions Between Ultracold Molecules and Atoms in a Magnetic Trap

TL;DR

This study investigates inelastic collisions between ultracold CaF molecules and Rb atoms in a magnetic trap, revealing state-dependent loss rates and providing insights into collision dynamics at ultracold temperatures.

Contribution

It provides the first measurements of inelastic collision rates between ultracold CaF molecules and Rb atoms, highlighting the impact of rotational states on collision outcomes.

Findings

Inelastic collision rate coefficient for excited molecules: (6.6 ± 1.5) × 10^{-11} cm^3/s.

No inelastic loss observed for molecules in the ground rotational state.

Upper bound on spin relaxation rate coefficient for ground state molecules: < 5.8 × 10^{-12} cm^3/s.

Abstract

We prepare mixtures of ultracold CaF molecules and Rb atoms in a magnetic trap and study their inelastic collisions. When the atoms are prepared in the spin-stretched state and the molecules in the spin-stretched component of the first rotationally excited state, they collide inelastically with a rate coefficient of $k_2 = (6.6 \pm 1.5) \times 10^{-11}$ cm$^{3}$/s at temperatures near 100~$\mu$K. We attribute this to rotation-changing collisions. When the molecules are in the ground rotational state we see no inelastic loss and set an upper bound on the spin relaxation rate coefficient of $k_2 < 5.8 \times 10^{-12}$ cm$^{3}$/s with 95% confidence. We compare these measurements to the results of a single-channel loss model based on quantum defect theory. The comparison suggests a short-range loss parameter close to unity for rotationally excited molecules, but below 0.04 for molecules in the rotational ground state.