Ultracold Rb-OH Collisions and Prospects for Sympathetic Cooling

We compute ab initio cross sections for cold collisions of Rb atoms with OH radicals. We predict collision rate constants of order ${10}^{-11}{\mathrm{cm}}^3/\mathrm{s}$ at temperatures in the range 10–100 mK at which molecules have already been produced. However, we also find that in these collisions the molecules have a strong propensity for changing their internal state, which could make sympathetic cooling of OH in a Rb buffer gas problematic in magnetostatic or electrostatic traps.

Figure 1

Rb-OH collision cross sections vs energy. Solid (dashed) lines represent elastic (inelastic) cross sections. In (a) results are shown for the incident channel $|{}^2\Pi _{3/2},f_d{m}_{f_d}=22,f{⟩}_{\mathrm{OH}}$ $|f_a{m}_{f_a}=22{⟩}_{\mathrm{Rb}}$, where both the atom and diatom are weak-magnetic-field seeking, and the OH is also weak-electric-field seeking. In (b) the incident channel is shown $|{}^2\Pi _{3/2},f_d{m}_{f_d}=11,e{⟩}_{\mathrm{OH}}$ $|f_a{m}_{f_a}=1-1{⟩}_{\mathrm{Rb}}$. Again, both species are in weak-magnetic-field seeking states, but the OH is now in a strong-electric-field seeking state. The points indicate the Langevin cross section, Eq. (2).