European Conference on Trapped Ions (ECTI)

Quantum tunneling reactions play an important role in chemistry when classical pathways are energetically forbidden [1]. Binary collisions of atomic with molecular hydrogen belong to the most fundamental molecular systems and are simple enough to be theoretically investigated using first-principle calculations. The rate of the tunneling reaction H${}_2$ + D${}^{-}\to$ H${}^{-}$ + HD has been calculated [2] but has until now lacked verification. Here we present high-sensitivity measurements of the reaction rate carried out in a cryogenic 22-pole ion trap. 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. Our measured value agrees with quantum tunneling calculations, serving as a benchmark for molecular theory and advancing the understanding of fundamental collision processes [3].

Further work has focused on inelastic collisions of C${}_2^{-}$, which has been proposed as a candidate for laser cooling due to the existence of multiple stable electronic states. We have demonstrated vibrational state control of C${}_2^{-}$ via a novel scheme that uses optical pumping in conjunction with inelastic collisions of H${}_2$ and measured the vibrational quenching rate [4]. Additionally, we precisely determined the proposed laser-cooling transitions of C${}_2^{-}$. We resolve the spin-rotation splittings and use it to perform accurate thermometry in our newly-developed wire trap [5].

[1] McMahon, R. J. Chemical reactions involving quantum tunneling. Science 299, 833–834 (2003).
[2] Yuen, C. H., Ayouz, M., Endres, E. S., Lakhmanskaya, O., Wester, R., Kokoouline, V. Quantum-tunneling isotope-exchange reaction H${}_2$ + D${}^{-}\to$ HD + H${}^{-}$. Phys. Rev. A 97, 022705 (2018).
[3] Wild, R., Nötzold, M., Simpson, M., Tran, T. D., Wester, R. Tunnelling measured in a very slow ion–molecule reaction. Nature (2023).
[4] Nötzold, M., Wild, R., Lochmann, C., Rahim, T., Melath, S. P., Mant, B., Franz, J., Gianturco, F., Wester, R. (in preparation)
[5] Nötzold, M., Wild, R., Lochmann, C., Wester, R. Spectroscopy and ion thermometry of C${}_2^{-}$ using laser-cooling transitions, Phys. Rev. A 106, 023111 (2022).