Chemi-Ionization Reactions and Basic Stereodynamical Effects in Collisions of Atom-Molecule Reagents.

Stefano Falcinelli,Fernando Pirani,Franco Vecchiocattivi,James M Farrar

doi:10.1021/acs.jpca.1c00688

Stefano Falcinelli, Fernando Pirani + Show 2 more

Open Access

https://doi.org/10.1021/acs.jpca.1c00688

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Abstract

A new theoretical method, developed by our laboratory to describe the microscopic dynamics of gas-phase elementary chemi-ionization reactions, has been applied recently to study prototype atom–atom processes involving reactions between electronically excited metastable Ne*(3P2,0) and heavier noble gas atoms. Important aspects of electronic rearrangement selectivity have been emphasized that suggested the existence of two fundamental microscopic reaction mechanisms. The distinct mechanisms, which are controlled by intermolecular forces of chemical and noncovalent nature respectively, emerge under different conditions, and their balance depends on the collision energy regime investigated. The present paper provides the first step for the extension of the method to cases involving molecules of increasing complexity, whose chemi-ionization reactions are of relevance in several fields of basic and applied researches. The focus is here on the reactions of Ne* with simple inorganic molecules as Cl2 and NH3, and the application of the method discloses relevant features of the reaction microscopic evolution. In particular, this study shows that the balance of two fundamental reaction mechanisms depends not only on the collision energy and on the relative orientation of reagents but also on the orbital angular momentum of each collision complex. The additional insights so emphasized are of general relevance to assess in detail the stereodynamics of many other elementary processes.

Highlights

It is important to stress that all stereodynamical effects emphasized for the chemi-ionization reactions (CHEMI) of Cl2 and NH3 must be considered averaged over all fine structure states accessible to the open-shell Ne*(3PJ) reagent, identified for atom−atom reactions by J and Ω quantum numbers
As demonstrated for atom−atom CHEMIs,[6,7] both real and imaginary parts of the optical potential are depending on J and Ω, and this determines the opening of different state-to-state reaction channels
While N2, from a phenomenological point of view, behaves to CO and CH4, with a total ionization cross section that increases with Ecoll, under thermal collision energies the cross section of O2 is at least a factor 3 larger with respect to that of N2, and it decreases with Ecoll as for Cl2 and C2H2

Summary

Introduction

If CHEMI involving Ng atoms are important, especially from the point of view of basic research, those involving molecules are of a more general interest, especially in highlighting the role of electronic transfer, that is, the redox nature of this type of process.[6,7] they control the balance of phenomena occurring in interstellar environments, in combustion and flames, where CHEMI are considered as the primary initial step,[14,15] in molecular plasmas and nuclear fusion They govern interstellar chemistry and planetary ionospheres[16−18] affecting the transmission of radio and satellite signals.[18] These reactions are implicated in softionization mass spectrometry techniques,[19,20] since the controlled internal degrees excitation of the molecular ionic products limits the number of fragmentation channels

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