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.