Intermolecular interactions and the weakly bound precursor states of elementary physicochemical processes

This study concerns the importance of the precursor (or pre-reactive) state of elementary physicochemical processes whose basic features, as structure, stability, and trapping effect of reagents, are controlled by the balance of intermolecular forces that arise at long range and operate at intermediate and short separation distances. The detailed formulation of such forces, determining formation probability and dynamical evolution of the precursor state, is of relevance in molecular science and difficult to be treated by quantum chemistry. Such a problem has been tackled by us exploiting the phenomenological approach, which employs semi-empirical and empirical formulas to represent strength, range and angular dependence of the leading interaction components involved. In addition to the study of transport phenomena, part of the attention is addressed to chemi-ionization (or Penning ionization) reactions for which neutral reagents lead to atomic and/or molecular ions plus electrons as products. Chemi-ionizations are bimolecular processes occurring in several environments of interest, where a reagent is a species, formed in excited-metastable electronic states by collisions with energetic electrons or cosmic rays. For such reactions all crucial electronic rearrangements, affecting stability and evolution of the weakly bound precursor state, here coincident with the reaction transition state, are characterized with a high detail. The results of the present study are of interest for many other processes, whose precursor states and their relevant features are difficult to characterize, often masked by several other effects.