Ultracold atom-molecular ion systems: a photo-asisted spin-dependent polyatomic chemical reaction

In recent years, rapid progress in cooling and trapping of hybrid atom-ion systems has led to studies of atom-ion processes. These range from charge transfer, recombination, internal state conversion, to the possibility of changing cross sections and corresponding rates with tunable Feshbach resonances. A recent experiment from the group of Eric Hudson at UCLA explored how ultracold Ca atoms interact with a trapped polyatomic molecular ion, namely BaOCH$_3^+$. We computed stationary points of the potential energy surface (PES) for this complex system, and reaction paths for different spin states, and found that a large barrier prevents the reaction to form BaOCa$^+$ and CH$_3$ in the singlet channel. We also found a barrier corresponding to a transition state for the triplet channel corresponding asymptotically to Ca($^3P$)\,+\,BaOCH$_3^+$, but low enough to allow reactions. We computed reaction rates based on a Langevin model, taking into account the different $J$ values of the initial $^3P_J$ state of Ca, and found that this photo-assisted reaction depends not only on the singlet/triplet spin state, but also on the fine structure of the initial reactants.