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

Abstract

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.

Three-Body Collisions Driving the Ion–Molecule Reaction C2– + H2 at Low Temperatures

“…Figure shows the 3B reaction rate coefficients determined from the density-dependent ion loss rate as a function of temperature. The temperature dependence of the reaction rate is obtained through a power law fit of the form a · ( T / T 0 ) b where T 0 = 20 K. The 10 K point was exempted from the fit, as here the kinetic temperature of the ions might differ from the trap temperature as we previously observed in this trap for the 3B rate of Cl – with H 2 . We attribute this low-temperature behavior to heating effects due to patch potentials in the trapping fields as well as heating of the ions due to micromotion on the edges of the trap, which are typical characteristics for multipole ion traps. − As such, we cannot exclude that this point might deviate from the actual reaction rate.…”

“…This falls close to other previously reported anion-molecule 3B reaction rates. For example, at 20 K OH – plus H 2 has a rate of k 3 ≃ 3 × 10 –29 cm 6 /s and Cl – plus H 2 a rate of k 3 ≃ 6 × 10 –31 cm 6 /s. , Note, however, that for Cl – -(H 2 ), the temperature dependence follows T –1 .…”

“…Observing slow ternary reaction rates requires long interaction times and high gas densities . In the case of ion-neutral reactions, particularly when molecular species are involved, multipole ion traps have proven to be an ideal tool that can help fulfill these requirements. − In combined ion-atom traps, 3B collisions with ions can also be studied at ultracold temperatures . From the theoretical side, various attempts have been made to universally describe all-neutral and ion-atom-atom collisions on a fundamental level. , Collisions involving molecular partners, however, significantly increase the internal degrees of freedom involved in the reaction and, as such, make it more difficult to accurately describe the observed rate constants through classical, semiclassical, or quantum chemical calculations. ,, For these termolecular rate coefficients, the matching theory still has to rely heavily on experimental data to benchmark the calculations.…”

“… 12 In the case of ion-neutral reactions, particularly when molecular species are involved, multipole ion traps have proven to be an ideal tool that can help fulfill these requirements. 13 − 17 In combined ion-atom traps, 3B collisions with ions can also be studied at ultracold temperatures. 18 From the theoretical side, various attempts have been made to universally describe all-neutral and ion-atom-atom collisions on a fundamental level.…”

Dynamics of a buffer-gas-loaded, deep optical trap for molecules

“…Crossed or merged molecular beams have become established techniques used to interrogate cold collisions over short interaction times [20][21][22]. Buffer-gas cooling has enabled the production of a broad spectrum of molecular samples at temperatures of order 1 K [23], which has allowed for cold spectroscopic experiments [24,25], as well as trapping low-field-seeking paramagnetic molecules in magnetic traps [26][27][28] and molecular ions in ion traps [29,30]. Polar molecules like ND 3 [31], OH [32], CH 3 F [32], and CH 2 O [33] have been loaded from lowfield-seeking states in buffer-gas beams into low-density electrostatic traps.…”