Electronic Rearrangements and Angular Momentum Couplings in Quantum State-to-State Channels of Prototype Oxidation Processes

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 3 P J 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.

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

“…More in detail, C x accounts for the change of the angular momentum coupling of the open shell atom within the intermolecular electric field, whose strength and anisotropy varies with R and with the mutual orientation of both reagents. As in the atom–atom case 15 , 16 , where it also accounts for the system confinement in specific Hund’s cases 17 , represents the alignment degree of the half-filled orbital of the open shell Ne + ionic core within the collision complex. In particular, and define respectively states of 3 Σ and 3 Π symmetries in the C ∞v (linear) configuration of the system, that splits in 3 A 1 , 3 B 1 and 3 B 2 symmetries in the C 2v (perpendicular) configuration.…”

“…The proposed approach, exploiting the interdependence of both real and imaginary part of the optical potential on the leading components of the intermolecular forces involved, casted light on important stereodynamics effects, like the role played by the atomic orbital alignment of reagents and products during the reaction evolution. Elementary processes have been also described in terms of confinement and/or transition between Hund’s cases of rotating diatoms, formed by the interacting systems during each collision event 17 , and the selectivity of all state-to-state reaction channels has been properly identified 16 .…”

The topology of the reaction stereo-dynamics in chemi-ionizations

“…Chemi-ionization reactions (CHEMI) are bimolecular processes involving as reagents a highly energetic X * species, electronically excited in a metastable state, interacting with another partner atomic or molecular M (see for instance refs. 7 , 8 , 13 – 30 and references therein). Therefore, CHEMI can be so schematized: Where [X···M] + is an intermediate ionic collision complex formed by a spontaneous autoionization of the [X···M] * neutral excited PS which is the reaction transition state (TS) of such processes.…”

“…CHEMI are of great interest for fundamental and applied research 7 , 8 , 13 – 30 since they participate to the balance of elementary processes in several environments, where X * easily forms. They happen both at very low-temperature T (under conditions of the cold chemistry) 18 , 21 , as in interstellar medium, at intermediate T values, as in planetary atmospheres 25 , and at high T, as in flames and plasmas (see refs.…”

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

“…CHEMI are of great interest for fundamental and applied research 7,8,[13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] since they participate to the balance of elementary processes in several environments, where X * easily forms. They happen both at very low temperature T (under conditions of the cold chemistry), as in interstellar medium, at intermediate T values, as in planetary atmospheres, and at high T, as in flames and plasmas.…”