Abstract
An innovative theoretical method to describe the microscopic dynamics of chemi-ionization reactions as prototype oxidation processes driven by selective electronic rearrangements has been recently published. It was developed and applied to reactions of Ne* atoms excited in their metastable 3PJ state, and here, its physical background is extensively described in order to provide a clear description of the microscopic phenomenon underlying the chemical reactivity of the oxidative processes under study. It overcomes theoretical models previously proposed and reproduces experimental results obtained in different laboratories. Two basic reaction mechanisms have been identified: (i) at low collision energies, a weakly bounded transition state is formed which spontaneously ionizes through a radiative physical mechanism (photoionization); (ii) in the hyperthermal regime, an elementary oxidation process occurs. In this paper, the selectivity of the electronic rearrangements triggering the two mechanisms has been related to the angular momentum couplings by Hund’s cases, casting further light on fundamental aspects of the reaction stereodynamics of general interest. The obtained results allow peculiar characteristics and differences of the terrestrial oxidizing chemistry compared to that of astrochemical environments to be highlighted.
Highlights
Most chemical reactions occur via a multistep mechanism, and often, they are triggered by charge transfer (CT) phenomena
We can mention: i) the harpooning processes, as evidenced in the oxidation reactions between a metal atom and a species with high electron affinity[1−3]; ii) the Coulomb explosions of molecular dications[4,5]; iii) the formation of excimer compounds which are involved in powerful UV lasers[6,7]; iv) the chemi-ionization reactions (ChemI).[8−10] Many studies focused on individual aspects of these phenomena, but still today, there is not a unifying description of them
This is due to the fact that, for the (Ne···noble gas (Ng))+ ionic core, the states correlating with Ne+ are those at higher energy, while states leading to Ne−Ng+ are the lowest ones
Summary
Most chemical reactions occur via a multistep mechanism, and often, they are triggered by charge (electron) transfer (CT) phenomena. ChemI are elementary fast reactions stimulated by electronic rearrangements taking place within the collision complex (X··· M)* (i.e., the transition state (TS) formed by an excited X* atom and an atomic/molecular species M as reagents). Their dynamical evolution is directly affected by the selectivity of CT, as shown below: e−(CT)
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