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

Mariusz Pawlak,Piotr S Żuchowski,Piotr Jankowski

doi:10.1021/acs.jctc.0c01122

Mariusz Pawlak, Piotr S Żuchowski + Show 1 more

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https://doi.org/10.1021/acs.jctc.0c01122

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Abstract

We present very accurate theoretical results of Penning ionization rate coefficients of the excited metastable helium atoms (4He(23S) and 3He(23S)) colliding with the hydrogen isotopologues (H2, HD, D2) 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.

Highlights

A chemical reaction is a fundamental process in chemistry
In follow-up studies, they focused on the isotope effect,[2] where the hydrogen molecule was substituted by hydrogen deuteride (HD) and deuterium (D2)
This paper shows the very accurate theoretical results of Penning ionization (PI) rate coefficients for all possible combinations of 4He and 3He in the metastable 23S state colliding with the hydrogen isotopologues (H2, HD, D2) in the two lowest rotational and vibrational states at subkelvin temperatures

Summary

INTRODUCTION

A chemical reaction is a fundamental process in chemistry. External control of its dynamics is crucial to understand many physical and chemical phenomena, especially at the quantum level. Narevicius and co-workers investigated the Penning ionization (PI) reactions by colliding excited metastable helium atoms 4He* (≡ 4He(23S)), contained in one beam, with ground-state molecular hydrogen H2, contained in another beam They observed a few shape resonances by measuring reaction rate coefficients, systematically decreasing collision energy from a few tens of kelvins down to millikelvins. The main motivation of our present studies was to confirm the strong isotope effect in cold chemistry reactions based on our recently developed, very accurate interaction energy surface.[38] This ab initio surface includes nonrigidity effects of the molecule, which play an important role in low-energy molecular anisotropic collisions.[38] this surface allows us to predict resonance structures for experimentally uncharted cases. It enables us better control over reactions in the subkelvin regime

THEORY

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