Analytical Potential Energy Formulation for a New Theoretical Approach in Penning Ionization

Abstract

The analysis of recent Penning ionization electron spectra as a function of the collision energy for both Ne*-Kr and Ne*-Xe autoionizing reactions allowed the development of a new general theoretical approach able to fully describe the stereodynamics of the Penning ionization reactions at a state to state level. Details on such a general and original approach based on the dependence of the reaction probability on the relative orientation of the atomic and molecular orbitals of reagents and products, are given. The mutual orientation of the collisional partners with respect to the intermolecular axis of the intermediate \([{{\rm Ne}}{\hbox{---}}{{\rm Rg}}]^* \) (with Rg = Kr or Xe) excited collision complex (i.e. the transition state of studied reactions) controls the characteristics of the intermolecular potential, which is formulated in a new analytical form whose details are presented and discussed. Obtained results refer to a statistical/random orientation of the open shell ionic core of Ne*, and in the two cases of Ne*-Kr and Ne*-Xe autoionizing collisions, we were able to reproduce and characterize the dependence on the collision energy of the experimental branching ratio between probabilities of spin-orbit resolved elementary processes already published. Such findings result from anisotropy effects connected to atomic orbital orientation/alignment, and their full understanding is a crucial point to describe the dependence of the stereo-dynamics on the electronic structure of the \([{{\rm Ne}}{\hbox{---}}{{\rm Rg}}]^* \) transition state. In this way, we are able to fully characterize the state to state reaction probability for the Penning ionization reactions involving Kr and Xe atoms with ionizing Ne* atoms in either 3P2 and 3P0 sublevels. This original methodology can be applied also to Penning ionization processes involving molecular targets, and in principle is able to point out the basic role of electronic rearrangements inside the transition state of various types of chemical reactions at thermal and sub-thermal collision energies which are of interest in astrochemical environments, being a much more arduous problem in order to be completely characterized.