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.

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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.

Molecular‐beam scattering experiments and theoretical calculations prove the nature, strength, and selectivity of the halogen bonds (XB) in the interaction of halogen molecules with the series of noble gas (Ng) atoms. The XB, accompanied by charge transfer from the Ng to the halogen, is shown to take place in, and measurably stabilize, the collinear conformation of the adducts, which thus becomes (in contrast to what happens for other Ng‐molecule systems) approximately as bound as the T‐shaped form. It is also shown how and why XB is inhibited when the halogen molecule is in the 3Πu excited state. A general potential formulation fitting the experimental observables, based on few physically essential parameters, is proposed to describe the interaction accurately and is validated by ab initio computations.

Coronene is one of the basic polycyclic aromatic hydrocarbons (PAHs) used to test the reliabilty of a multidimensional potential energy surface (PES) and to assess its influence on the formation dynamics of PAH clusters with defined physical and chemical properties. We report an analytical potential energy surface for modeling the coronene− coronene interaction, whose parameters were fine-tuned on dispersion-corrected DFT estimations performed within the generalized gradient PBE approximation, that is suitable for describing molecular aggregates involving aromatic species. This model was used to build a potential function for coronene clusters (Cor n ) that was then applied in a detailed global geometry optimization study with an evolutionary algorithm. A large variety of low-energy structures were obtained for the Cor n (n = 2−15) clusters ranging from columnar-type to two-stacked in a handshake association motifs. Moreover, it was found that a transition from a single-stack columnar regime to other more complex shapes occurs at n = 6, whereas previous results based on a simpler coarse-grained potential pointed to a transition at n = 8. Geometry reoptimizations were also performed at the DFT level for the most representative low-energy structures of Cor n (n = 3−6), which confirmed the reliability of the present findings.

A new analytical potential energy surface is proposed to investigate, by semiclassical molecular dynamics calculations, the scattering of O 2 molecules in well-defined initial roto−vibrational (v i , j i ) states from graphite under a variety of conditions of applied interest. The reaction dynamics appears to be dominated by the coupling between translational and rotational internal degrees of freedom of molecule, that, at low-medium collision energies, can be also triggered by the energy exchange with the surface phonons. The final states (v f , j f ) of backscattered molecules are characterized and carefully analyzed. Most important results are the following:(1) after the interaction with the surface, molecules are backscattered mainly in a direction very close to the specular one; (2) v i is preserved, except for high initial vibrational states; (3) the surface temperature plays a minor role; and (4) the final j f states exhibit non-Boltzmann distributions with the main peak nearby j f = j i and a secondary maximum at very high j f . Moreover, the features of rotational distributions suggest a close correlation between the initial rotational configuration of impinging molecules and the final state achieved after the scattering. These findings, complementary to those from molecular beam experiments, cast light on relevant selectivities in elastic and inelastic collision events that control the stereodynamics of several elementary processes occurring both in gaseous and condensed phases for low energy (as those meet in the interstellar medium) as well as for high energy (as those of interest for aerospace applications).