Experimental Investigation of Ultracold Atom-Molecule Collisions

Ultracold collisions between Cs atoms and ${\mathrm{Cs}}_2$ dimers in the electronic ground state are observed in an optically trapped gas of atoms and molecules. The ${\mathrm{Cs}}_2$ molecules are formed in the triplet ground state by cw photoassociation through the outer well of the $0_g^{-}(P_{3/2})$ excited electronic state. Inelastic atom-molecule collisions converting internal excitation into kinetic energy lead to a loss of ${\mathrm{Cs}}_2$ molecules from the dipole trap. Rate coefficients are determined for collisions involving Cs atoms in either the $F=3$ or $F=4$ hyperfine ground state, and ${\mathrm{Cs}}_2$ molecules in either highly vibrationally excited states ($v^{\prime }=32–47$) or in low vibrational states ($v^{\prime }=4–6$) of the $a{}^3\Sigma _u^{+}$ triplet ground state. The rate coefficients $\beta \sim {10}^{-10}{\mathrm{cm}}^3/\mathrm{s}$ are found to be largely independent of the vibrational and rotational excitation indicating unitary limited cross sections.

Figure 1

Storage time measurement for ${\mathrm{Cs}}_2(v^{\prime }=32–47)$ molecules with $\mathrm{Cs}(F=3)$ atoms in the dipole trap at a mean density of $n_{\mathrm{at}}=9.1\pm {0.4}_{\mathrm{stat}}\pm {2.4}_{\mathrm{syst}}\times {10}^{10}{\mathrm{cm}}^{-3}$ (black filled squares) and with only molecules in the trap (circles). The lines represent fits to an exponential decay. Inset: Time-of-flight spectrum showing the ${\mathrm{Cs}}^{+}$ and ${\mathrm{Cs}}_2^{+}$ signals.

Figure 2

Molecule loss rate $8\beta n_{\mathrm{at}}/\sqrt{27}+{\Gamma }_{\mathrm{mol}}$ vs $n_{\mathrm{at}}$ for collisions between $\mathrm{Cs}(F=3)$ and ${\mathrm{Cs}}_2(v^{\prime }=32–47)$ (solid squares) and ${\mathrm{Cs}}_2(v^{\prime }=4–6)$ (circles). The straight lines represent fits discussed in the text (solid: $v^{\prime }=32–47$, dashed: $v^{\prime }=4–6$).

Figure 3

Storage time measurement for ${\mathrm{Cs}}_2(v^{\prime }=32–47)$ in collisions with $\mathrm{Cs}(F=4)$. Equation (2) is fitted to the data to obtain the inelastic rate coefficient.