Inelastic Collisions of Ultracold Heteronuclear Molecules in an Optical Trap

Ultracold RbCs molecules in high-lying vibrational levels of the $a^3{\Sigma }^{+}$ ground electronic state are confined in an optical trap. Inelastic collision rates of these molecules with both Rb and Cs atoms are determined for individual vibrational levels, across an order of magnitude of binding energies. The long-range dispersion coefficients for the collision process are calculated and used in a model that accurately reproduce the observed scattering rates.

Figure 1

Formation and detection processes for ultracold RbCs. (a) The PA laser excites colliding atom pairs into bound ${\mathrm{RbCs}}^{*}$ molecules, which (b) decay into a range of vibrational states of the $a^3\Sigma$ potential. (c) $a^3\Sigma (v)$ molecules are excited to level $i$, then (d) ionized and detected.

Figure 2

Typical molecular lifetime data. Here, the number of molecules in the $a^3{\Sigma }^{+}(v=v_0)$ state with binding energy $E_B=-5.0\pm 0.6{\mathrm{cm}}^{-1}$ is observed in the QUEST as a function of time. The presence of inelastic collisions between the atoms and molecules is evidenced by the dramatic reduction of the molecular lifetime when atoms are present. With no atoms present, we observe molecule lifetimes consistent with the background gas-limited lifetime seen for isolated atomic clouds in the trap.

Figure 3

Molecular trap-loss scattering rate constant $K$ vs binding energy $E_B$, for molecules in specific vibrational levels of the $a^3{\Sigma }^{+}$ state. Vibrational state label appears below each data point. The black (red) crosshatched box is the prediction of the model described in the text for collisions with Cs (Rb). The width of the boxes reflects the uncertainty in the collision temperature.

Figure 4

Numerically calculated scattering rate constant $K$ vs center-of-mass frame collision energy $E$ for atom-molecule and molecule-molecule collisions.