Kinetic Isotope Effect in Low-Energy Collisions between Hydrogen Isotopologues and Metastable Helium Atoms: Theoretical Calculations Including the Vibrational Excitation of the Molecule

We present very accurate theoretical results of Penning ionization rate coefficients of the excited metastable helium atoms (He-4(2(3)S) and He-3(2(3)S)) colliding with the hydrogen isotopologues (H-2, HD, D-2) in the ground and first excited rotational and vibrational states at subkelvin regime. The calculations are performed using the current best ab initio interaction energy surface, which takes into account the nonrigidity effects of the molecule. The results confirm a recently observed substantial quantum kinetic isotope effect (Nat. Chem. 2014, 6, 332-335) and reveal that the change of the rotational or vibrational state of the molecule can strongly enhance or suppress the reaction. Moreover, we demonstrate the mechanism of the appearance and disappearance of resonances in Penning ionization. The additional model computations, with the morphed interaction energy surface and mass, give better insight into the behavior of the resonances and thereby the reaction dynamics under study. Our theoretical findings are compared with all available measurements, and comprehensive data for prospective experiments are provided.

Automated summary

This study presents very accurate theoretical results of Penning ionization rate coefficients of excited metastable helium atoms colliding with hydrogen isotopologues in different states. The research confirms a significant quantum kinetic isotope effect and discusses the impact of molecular state changes on the reaction. Additionally, the mechanism of resonance appearance and disappearance in Penning ionization is demonstrated.