Observation of Feshbach-Like Resonances in Collisions between Ultracold Molecules

We observe magnetically tuned collision resonances for ultracold ${\mathrm{Cs}}_2$ molecules stored in a ${\mathrm{CO}}_2$-laser trap. By magnetically levitating the molecules against gravity, we precisely measure their magnetic moment. We find an avoided level crossing which allows us to transfer the molecules into another state. In the new state, two Feshbach-like collision resonances show up as strong inelastic loss features. We interpret these resonances as being induced by ${\mathrm{Cs}}_4$ bound states near the molecular scattering continuum. The tunability of the interactions between molecules opens up novel applications such as controlled chemical reactions and synthesis of ultracold complex molecules.

Figure 1

Molecular energy structure below the scattering continuum of two cesium atoms in the $|F=3,m_F=3⟩$ state. The energy of the dissociation threshold corresponds to $E_b=0$. The arrows mark the paths to the molecular states we explore, which include the creation of the molecules in $|\alpha ⟩$ via the atomic Feshbach resonance at 19.84 G [1, 19] and an avoided crossing to $|\beta ⟩$ at $\sim 13.6\mathrm{G}$. Included are only molecular states which can couple to the continuum via Feshbach couplings up to $g$-wave interaction ($l\le 4$, $m_f+m_l=6$ and $m_f\ge 2$).

Figure 2

Magnetic moment of the ${\mathrm{Cs}}_2$ molecules. The measured magnetic moment (solid circles) is compared to the NIST calculation (solid line). The fast change at $\sim 13.6\mathrm{G}$ is associated with an avoided crossing. In the inset, we derive the molecular binding energy (solid circles) by integrating the measured magnetic moment. Binding energies from the NIST calculation (solid lines) for both branches of the avoided crossing between state $|\alpha ⟩$ and state $|\beta ⟩$ are shown; see also Fig. 1.

Figure 3

Remaining fraction of optically trapped molecules after a storage time of 300 ms. Initially, there are 11 000 molecules at a peak density of $6\times {10}^{10}{\mathrm{cm}}^{-3}$ and a temperature of 250 nK. The dashed lines mark the background loss rates in state $|\alpha ⟩$ and in state $|\beta ⟩$. The two loss resonances for $|\beta ⟩$ are fit by a sum of two Lorentzian profiles (inset).

Figure 4

Time evolution of the molecule number in the ${\mathrm{CO}}_2$-laser trap for molecules in state $|\alpha ⟩$ at 15.4 G (open circles), in state $|\beta ⟩$ at 12.1 G (off resonance, solid circles) and at 12.7 G (on resonance, crosses). Fits based on two-body loss (dashed lines) work well for 15.4 and 12.1 G. A fit based on three-body loss (solid line) works better for 12.7 G.