Tunnelling measured in a very slow ion-molecule reaction.

Quantum tunnelling reactions play an important role in chemistry when classical pathways are energetically forbidden 1 , be it in gas-phase reactions, surface diffusion or liquid-phase chemistry. In general, such tunnelling reactions are challenging to calculate theoretically, given the high dimensionality of the quantum dynamics, and also very difficult to identify experimentally 2-4 . Hydrogenic systems, however, allow for accurate first-principles calculations. In this way the rate of the gas-phase proton-transfer tunnelling reaction of hydrogen molecules with deuterium anions, H 2  + D -  → H -  + HD, has been calculated 5 , but has so far lacked experimental verification. Here we present high-sensitivity measurements of the reaction rate carried out in a cryogenic 22-pole ion trap. We observe an extremely low rate constant of (5.2 ± 1.6) × 10 -20  cm 3  s - 1 . This measured value agrees with quantum tunnelling calculations, serving as a benchmark for molecular theory and advancing the understanding of fundamental collision processes. A deviation of the reaction rate from linear scaling, which is observed at high H 2 densities, can be traced back to previously unobserved heating dynamics in radiofrequency ion traps.