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

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*( 3 P 2,0 ) 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 Cl 2 and NH 3 , 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.

The topology of the reaction stereo-dynamics in chemi-ionizations and ion chemistry

“…In the case of ammonia, calculated σ has been separated in partial σ p components, promoted by the most representative configurations of the precursor state opening the transition to the ionic product in the two different electronic states. 7,36 Actually, as detailed in Ref. 8, the most quantitative treatment of CHEMI involving polar hydrogenated molecules, as ammonia, should include the average on the acceptance angular cone of each reaction channel.…”

“…θ and ϕ are the polar orientation angles of the molecule, as defined in Figure 1 for NH3. 7,8,35,36 The Ecoll value has been fixed to 1.0 meV and represented by a horizontal dashed line. For both Ne * -Ar and Ne * -N2 the V component is that asymptotically correlating with Ne( 3 P0) fine level of the reagent, 26,35 while for Ne * -NH3, the plotted V components account for the formation of PS in two basic geometries, opening different CHEMI reactivity.…”

“…Recently, the optical potentials were formulated, in an internally consistent way and in analytical form, for Ne * ( 3 P J ) + Ar, 26 Ne * ( 3 P J ) + N 2 35 and Ne * ( 3 P) + NH 3 . 7,36 Their formulation, including angular and radial dependences for both real and imaginary components, permits to control such prototype CHEMI at increasing complexity of the M reagent. The proposed optical potentials, reproducing most of experimental findings, obtained in our and in other laboratories, are here exploited to cast light on the basic points of CHEMI stereodynamics stressed above.…”

“…For both Ne * -Ar and Ne * -N2 the V component is that asymptotically correlating with Ne( 3 P0) fine level of the reagent, 26,35 while for Ne * -NH3, the plotted V components account for the formation of PS in two basic geometries, opening different CHEMI reactivity. 36 The semi-classical relation, 𝑏 ≃…”

“…In the present study, exploiting an accurate formulation of the intermolecular interaction potentials, previously obtained, and tested on the experimental findings, 7,8,25,26,35,36 important molecular dynamics simulations have been performed, useful to properly address the following aspects of general interest:…”