Selectivity of weak intermolecular forces and precursor state of elementary oxidation reactions, a new insight on Ne* + N2 chemiionization

Abstract

This paper reports on the collision dynamics of N2 with metastable Ne* promoting chemiionizations, prototype of barrier-less oxidation reactions of great interest for fundamental and applied research. Extending guidelines presented in previous papers for the atom–atom case, an innovative treatment of the reaction stereodynamics involving molecules in a quantum state-to-state resolution conditions is proposed that emphasizes the role of structure and stability of the precursor that is here the reaction transition state. A critical test of such treatment, carried out exploiting a new formulation both of real and imaginary parts of the optical potential driving the reaction dynamics, is represented by the detailed-combined description of all relevant findings, provided by high resolution molecular beam scattering experiments carried out in our and other laboratories. The present analysis casts light on basic electronic rearrangements of such prototype oxidation reaction which are expected to be of fundamental interest for many other reactions involving open shell atoms and free radicals.

Basic features of Ne*–HX (X = Cl, Br) chemi-ionization reactions

“…The reactions discussed in this study have been experimentally investigated using beams of Ne*( 3 P J ) atoms with a statistical population of J = 2, 0 fine levels. Generalizing the detailed behaviour recently emphasized for Ne*( 3 P J ) + N 2 CHEMI, 18 it can be expected that under the collision energy conditions here adopted, essentially confined in the thermal range, the atoms in the two different spin orbit levels J behave similarly because a compensation between direct and indirect mechanism features. It has been demonstrated 18 that, under hyper-thermal conditions, J = 0 atoms become more reactive since promoting a more efficient direct mechanism.…”

“…Recently, extending/generalizing some guidelines previously suggested, 16 we demonstrated that Ne* + atom 3 and Ne* + apolar diatomic molecule 17,18 CHEMI occur through two different-complementary mechanisms, whose relative role is modulated by the collision energy. In particular, as stressed above, the direct mechanism, dominant at short separation distances (high collision energy), involves an effective electron transfer between reagents controlled by strong chemical forces.…”

“…5 and 6. In particular, as confirmed by the detailed experimental investigation of other systems, 3,17,18 the indirect mechanism, controlled by the critical balance of weak interaction components of physical and chemical nature, dominates at large separation distances, while the direct mechanism stimulated by chemical forces, like the charge transfer, emerges at intermediate and short separation distances. Moreover, the formation of the ionic products in their ground state involves a precursor state having a perpendicular geometry, while the one promoting the formation of ionic products in their first excited electronic state preferentially assumes a collinear configuration.…”

“…Moreover, the formation of the ionic products in their ground state involves a precursor state having a perpendicular geometry, while the one promoting the formation of ionic products in their first excited electronic state preferentially assumes a collinear configuration. It is important to note that, as recently demonstrated for Ne* + N 2 reactions, 18 the perpendicular approach is the proper one giving associated NeN 2 + ion formation, since, under such configuration, pre-dissociation effects due to the formation of vibration excited molecular ions are ineffective.…”

“…On the other hand, the indirect mechanism (low collision energy), stimulated by a critical balance between weak forces of chemical and physical nature, dominates at large separation distances. Therefore, following the previous general guidelines accounting for the specific nature of reagents and products, 3,17,18 important indications on the stereodynamics of present reactions can be obtained.…”