Quantum-state-dependent chemistry of ultracold ${}^6\mathrm{Li}{}_2$ dimers

Starting from an ultracold ensemble of ${}^6{\mathrm{Li}}_2$ Feshbach molecules, we produce deeply bound triplet $a\left(1{}^3{\mathrm{\Sigma }}_u{}^{+}\right)$ molecules in a single quantum state by stimulated Raman adiabatic passage. The ensemble lifetime is found to be limited by two-body molecule-molecule collisions that depend on both the vibrational and rotational states. The loss rate observed is near the universal rate for the $|v=0,5,8;N=0,2\rangle$ states and, remarkably, below universality for the $|v=9,N=0\rangle$ state. We also observe a strong rotational state dependence of the $v=9$ reaction rate. Consistent with theoretical predictions, we observe that molecules in the absolute lowest triplet level are unstable and likely decay due to trimer formation.

Figure 1

Sketch of the ${}^6{\mathrm{Li}}_2$ molecular potentials [60]. The roles of the probe and Stokes laser fields are shown using arrows and highlight the four vibrational levels ($v=0$, 5, 8, and 9) we study. The inset shows that the probe and Stokes lasers are overlapped and collinear with the second arm of the crossed optical dipole trap used to confine the molecular ensemble.

Figure 2

Peak ensemble density of triplet ${}^6{\mathrm{Li}}_2$ dimers in the lowest Zeeman level of the $|v=0,N=0\rangle$ (circles), $|v=9,N=0\rangle$ (squares), and $|v=9,N=2\rangle$ (diamonds) levels in the $a\left(1{}^3{\mathrm{\Sigma }}_u{}^{+}\right)$ potential as a function of hold time inferred from the number of Feshbach molecules imaged after the reverse STIRAP pulse. Each experimental data point consists of five averages. The magnetic field was held fixed at 700 G for all of these measurements. The solid lines depict a fit to a molecular-dynamics simulation of the ensemble decay. The fitted two-body loss coefficients are provided in Table 1.

Figure 3

(a) Comparison of FM dipole oscillations at 112 mW (red squares), 129 mW (black circles), and 143 mW (blue diamonds) in the single-arm trap. The fitted trap frequencies are $(574\pm 2), (622\pm 4)$, and $(651\pm 1)\mathrm{Hz}$, respectively. This figure demonstrates that trap frequency differences of less than $5\%$ can be clearly resolved. (b) The FM (black circles) and DBM (red squares) dipole oscillations at 129 mW for the $|v=9,N=0\rangle$ state. The fitted trap frequencies are $(622\pm 4)$ and $(633\pm 5)\mathrm{Hz}$. The good overlap of the periods out to 8 ms demonstrates that the dynamical polarizability of the FM state and DBM are almost equal for this state. The powers required to produce matched trap frequencies for all states discussed in this paper are found in Table 2. (c) Fractional change of the two-body loss parameter $\beta$ extracted from DBM decay curves (for the $|v=9,N=0\rangle$ state) when the trap frequencies for the FM state (${\omega }_{\mathrm{FM}}$) and DBM (${\omega }_{\mathrm{DBM}}$) are intentionally mismatched. The black line indicates the line of best fit, yielding a slope of $1.95\pm 0.17$. This is used to quantify the error in $\beta$ introduced by the uncertainty in the matching of the FM and DBM trap frequencies.