Quantum kinetic isotope effect in He(2³S)-H₂/HD/D₂ collision pairs at subkelvin temperatures

We demonstrate very accurate theoretical results of cold reaction rate coefficients of excited metastable helium and H₂/HD/D₂, confirming the recently observed strong quantum kinetic isotope effect in subkelvin collisions [1]. The calculations are carried out using our newly developed high-quality ab initio interaction energy surface [2]. This surface includes information about the nonrigidity effects of the molecule that play an important role in low-energy molecular anisotropic collisions. The obtained rate of the Penning ionization process can differ even by an order of magnitude, depending on the used isotopologue at certain collision energy. We demonstrate the origin of observed resonances and the mechanism of their appearing and disappearing. Since our unique calculations include, besides the rotational excitation, the vibrational excitation of the molecule, we are able to show that the low-energy resonances (below 1 K) can be effectively controlled by the rovibrational state of the molecule. The change of the rotational and vibrational quantum numbers of the molecule can strongly enhance or suppress the Penning ionization reaction like a quantum switch. 

[1] E. Lavert-Ofir et al., Nature Chem. 6, 332 (2014)

[2] M. Pawlak et al., J. Chem. Theory Comput. 16, 2450 (2020)