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

Abstract

The detailed characterization of barrier-less chemi-ionization reactions provides unique-direct information on basic quantities determining the topology of their stereo-dynamics. The precursor state, formed by collisions of reagents, plays here an important role being coincident with the reaction transition state. All the features of such a state, as well as its structure and stability, are directly or indirectly controlled by intermolecular forces operative in each collision complex. Particular attention it must be focused on the selectivity of the orbital angular momentum, affecting the fate of each collision event at any collision energy. Moreover, the distance of the closest approach (turning point) of reagents, controls the relative weight of two different reaction mechanisms: i) An exchange mechanism triggered by strong chemical forces that act mainly at short separation distances, inducing an electron transfer between the reactants through a prototype oxidation process; ii) A radiative mechanism, caused by the combination of weak chemical and physical forces dominant at larger distances and which provokes an electron ejection via a pure photoionization event determined by the exchange of a “virtual” photon between the reacting partners. Obtained results are of great interest for many other elementary processes, difficult to characterize at the same level of detail. They suggest some limitations of the so called "capture models", usually exploited to describe many other barrier-less processes, like ion–molecule reactions, occurring in interstellar medium, planetary atmospheres and plasmas.