Complex Formation in Three-Body Reactions of Cl^- with H₂

Three-body reaction rates of Cl^- with H₂ to form the weakly bound complex Cl^-(H₂) are measured between 10 and 26 K in a linear radio-frequency wire trap. Formation of larger clusters of the form Cl^-(H₂)₂ are also observed. The three-body (or termolecular) rate coefficients follow the form aT^-1, with a = 1.12(2) × 10^-29 cm⁶ K s^-1. Reverse reactions to repopulate the Cl^- parent ion were measured, even though the binding energy of the complex makes bimolecular dissociative collisions energetically inaccessible at low temperatures. The back-reaction was found to be proportional to the cube of the hydrogen density, suggesting that the dissociation mechanism depends on multiple collisions. Comparisons of the rate coefficients measured in a 16-pole wire trap and a 22-pole trap demonstrate significantly lower ion temperatures in the wire trap.