Collisions between Ultracold Molecules and Atoms in a Magnetic Trap

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 $k_2=(6.6\pm 1.5)\times {10}^{-11}{\mathrm{cm}}^3/\mathrm{s}$ at temperatures near $100\mu \mathrm{K}$. 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 $k_2<5.8\times {10}^{-12}{\mathrm{cm}}^3/\mathrm{s}$ 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.

Figure 1

Fraction of molecules, $r_{\mathrm{CaF}}(t)/r_{\mathrm{CaF}}(0)$, remaining in the magnetic trap after a hold time $t$, with Rb (open points) and without Rb (filled points). The molecules are prepared in the $|1,2,2⟩$ state. Each point is the average and standard deviation of six measurements. For the single species data the line is a fit to an exponential decay model. For the two-species data the line is a fit to Eq. (2). Inset: histogram of ${\mathrm{\Gamma }}_{\mathrm{Rb}\text{−}\mathrm{CaF}}$ estimates using the method described in [43], together with a fit to a normal distribution.

Figure 2

Points: collisional loss rate, ${\mathrm{\Gamma }}_{\mathrm{Rb}\text{−}\mathrm{CaF}}$, as a function of effective density, $\zeta$, for molecules in the $|1,2,2⟩$ state. Line: straight line fit to the data. Insets: radial density distributions, $c_s(x)=\int N_s{f}_s(x,y,z)dydz$, of the atoms and molecules for small and large $\zeta$, with Gaussian fits.

Figure 3

Loss of molecules from the magnetic trap in the presence of $1.2\times {10}^9\mathrm{atoms}$, for molecules in the $|1,2,2⟩$ state (open points) and the $|0,1,1⟩$ state (filled points). Solid lines: fits to Eq. (2). Also shown is the loss of molecules in $|0,1,1⟩$ in the absence of Rb (open triangles) together with a single exponential fit (dashed line). Inset: histogram of ${\mathrm{\Gamma }}_{\mathrm{Rb}\text{−}\mathrm{CaF}}$ estimates, together with fits to normal distributions.

Figure 4

Thermally averaged loss rate coefficient $k_2$ from the single-channel model as a function of loss parameter $y$ and short-range phase shift ${\delta }^{\mathrm{s}}$. The mean and standard error of the experimentally measured $k_2$ for molecules in $|1,2,2⟩$ are shown as black lines. The 95% upper bound for $k_2$ for molecules in $|0,1,1⟩$ is shown as a dashed red line.