Abstract. Supersonic beams of oxygen, nitrogen, and chlorine atoms and of metastable oxygen and nitrogen molecules produced from a high-pressure radiofrequency discharge beam source have been characterized by coupling velocity selection with magnetic analysis in the transmission mode. The present work leads to the determination of the relative populations of the electronic states of the species in the produced beams, showing that estimates of the populations from plasma temperatures or final translational temperatures could bring on incorrect conclusions.
Conspectus
Most chemical processes are triggered by electron
or charge transfer
phenomena (CT). An important class of processes involving CT are chemi-ionization
reactions. Such processes are very common in nature, involving neutral
species in ground or excited electronic states with sufficient energy
(X*) to yield ionic products, and are considered as the primary initial
step in flames. They are characterized by pronounced electronic rearrangements
that take place within the collisional complex (X···M)*
formed by approaching reagents, as shown by the following scheme,
where M is an atomic or molecular target: X* + M → (X···M)*
→ [(X+···M) ↔ (X···M+)]
e−
(X···M)+ + e– → final ions.
Despite
their important role in fundamental and applied research,
combustion, plasmas, and astrochemistry, a unifying description of
these basic processes is still lacking. This Account describes a new
general theoretical methodology that demonstrates, for the first time,
that chemi-ionization reactions are prototypes of gas phase oxidation
processes occurring via two different microscopic mechanisms whose
relative importance varies with collision energy, E
c, and separation distance, R. These
mechanisms are illustrated for simple collisions involving Ne*(3P2,0) and noble gases (Ng). In thermal and hyperthermal
collisions probing interactions at intermediate and short R, the transition state [(Ne···Ng)+]
e− is a molecular species described
as a molecular ion core with an orbiting Rydberg electron in which
the neon reagent behaves as a halogen atom (i.e., F) with high electron
affinity promoting chemical oxidation. Conversely, subthermal collisions
favor a different reaction mechanism: Ng chemi-ionization proceeds
through another transition state [Ne*······Ng],
a weakly bound diatomic-lengthened complex where Ne* reagent, behaving
as a Na atom, loses its metastability and stimulates an electron ejection
from M by a concerted emission–absorption of a “virtual”
photon. This is a physical radiative mechanism promoting an effective
photoionization. In the thermal regime of E
c, there is a competition between these two mechanisms. The proposed
method overcomes previous approaches for the following reasons: (1)
it is consistent with all assumptions invoked in previous theoretical
descriptions dating back to 1970; (2) it provides a simple and general
description able to reproduce the main experimental results from our
and other laboratories during last 40 years; (3) it demonstrates that
the two “exchange” and “radiative” mechanisms
are simultaneously present with relative weights that change with E
c (this viewpoint highlights the fact that the
“canonical” chemical oxidation process, dominant at
high E
c, changes its nature in the subthermal
regime to a direct photoionization process; therefore, it clarifies
differences between the cold chemistry of terrestrial and interstellar
environments and the energetic one of combustion and flames); (4)
the proposed method explicitly accounts for the influence of the degree
of valence orbital alignmen...
The investigation of chemi-ionization processes provides unique information on how the reaction dynamics depend on the energy and structure of the transition state which relate to the symmetry, relative orientation of reagent/product valence electron orbitals, and selectivity of electronic rearrangements. Here we propose a theoretical approach to formulate the optical potential for Ne*(3 P 2,0) noble gas atom chemi-ionizations as prototype oxidation processes. We include the selective role of atomic alignment and of the electron transfer mechanism. The state-to-state reaction probability is evaluated and a unifying description of the main experimental findings is obtained. Further, we reproduce the results of recent and advanced molecular beam experiments with a state selected Ne* beam. The selective role of electronic rearrangements within the transition state, quantified through the use of suitable operative relations, could cast light on many other chemical processes more difficult to characterize.
A weak halogen bond, together with charge transfer from a noble gas to Cl2, characterizes the intermolecular interaction between a noble gas atom and Cl2 in a collinear configuration.
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