How the reactivity depends on the angular approach between reagents: stereo-dynamics of Penning ionization involving hydrogenated molecules

Gas phase chemiionization processes of water, hydrogen sulfide, and ammonia molecules, induced by excited metastable noble gas atoms (Ng*), known as Penning ionizations, play an important role in several phenomena occurring in low energy ionized plasmas, electric discharges, planetary atmospheres, and interstellar environments [1,2]. In the present contribution we report on ammonia molecules ionized by collisions with neon and helium metastable atoms. The investigation has been carried out by analyzing the total ionization cross section as a function of the collision energy for different approaching angles between Ng* and NH3 reagents, as well as the energy of emitted electron spectra. For such advanced analysis we have exploited a semiclassical treatment of the collision dynamics together with potential energy surfaces whose formulation, given in terms of fundamental physical properties of the involved partners, is provided in an analytical form, useful to represent the system in the whole space of configurations. Therefore, such a potential formulation appears suitable for quantitative molecular dynamics calculations that allow to control the angle of approach between the reagent particles, and therefore to perform a detailed stereo-dynamical study of Penning ionization reactions. The final results obtained from this investigation show the evidence that ionization process is controlled by the competition between the formation of a hydrogen bond like complex, when the rare gas excited atom approaches the molecules towards the hydrogen atom side, and the formation of a halogen bond like complex, when the approach occurs towards the lone pair side. In the latter configuration the ionization is stimulated by a pronounced electron exchange probability between a filled orbital of the molecule and the inner shell hole of the ionic core of Ng* atom.