A New Insight on Stereo-Dynamics of Penning Ionization Reactions

Abstract

Recent developments in the experimental study of Penning ionization reactions are presented here to cast light on basic aspects of the stereo-dynamics of the microscopic mechanisms involved. They concern the dependence of the reaction probability on the relative orientation of the atomic and molecular orbitals of reagents and products. The focus is on collisions between metastable Ne * ( 3 P 2, 0 ) atoms with other noble gas atoms or molecules, for which play a crucial role both the inner open-shell structure of Ne * and the HOMO orbitals of the partner. Their mutual orientation with respect to the intermolecular axis controls the characteristics of the intermolecular potential, which drives the collision dynamics and the reaction probability. The investigation of ionization processes of water, the prototype of hydrogenated molecules, suggested that the ground state of water ion is produced when Ne * approaches H 2 O perpendicularly to its plane. Conversely, collisions addressed toward the lone pair, aligned along the water C 2v symmetry axis, generates electronically excited water ions. However, obtained results refer to a statistical/random orientation of the open shell ionic core of Ne * . Recently, the attention focused on the ionization of Kr or Xe by Ne * , for which we have been able to characterize the dependence on the collision energy of the branching ratio between probabilities of spin orbit resolved elementary processes. The combined analysis of measured PIES spectra suggested the occurrence of contributions from four different reaction channels, assigned to two distinct spin-orbit states of the Ne * ( 3 P 2, 0 ) reagent and two different spin-orbit states of the ionic M + ( 2 P 3/2, 1/2 ) products (M = Kr, Xe). The obtained results emphasized the reactivity change of 3 P 0 atoms with respect to 3 P 2 , in producing ions in 2 P 3/2 and 2 P 1/2 sublevels, as a function of the collision energy. These findings have been assumed to arise from a critical balance of adiabatic and non-adiabatic effects that control formation and electronic rearrangement of the collision complex, respectively. From these results we are able to characterize for the first time, according to our knowledge, the state to state reaction probability for the ionization of Kr and Xe by Ne * in both 3 P 2 and 3 P 0 sublevels.

Theoretical and Computational Analysis at a Quantum State Level of Autoionization Processes in Astrochemistry

“…This provided an internally consistent rationalization of most relevant experimental findings, characterized with different techniques in various laboratories, that includes total and partial ionization cross sections, BRs and PIES measurements [ 53 – 56 ]. The energy dependence of BRs between selected reaction channels must also relate with the PIES spectra for systems measured in our laboratory [ 5 , 39 , 40 ] as those reported in Fig. 2 where, as an example, are showed only those for .…”

“…Obtained results suggest also how to extend the methodology to autoionization reactions involving molecules [ 7 , 9 , 59 ], which are of great interest in several fields, including the balance of phenomena occurring in interstellar environments and planetary atmospheres. In addition, also electron-molecule impacts are of crucial relevance in many applications of molecular plasmas [ 5 ].…”

“…In our theoretical approach, the two V and Г components are, for the first time, considered as interdependent [ 5 , 45 ], whose magnitude is related to the internuclear distance, R, between X and Y, their relative orientation and the manifold of quantum states attainable for the system along the collision.…”

“…The focus of this paper is to point out a theoretical and computational methodology based on the selective role of anisotropic CT on the stereo-dynamics of autoionization processes, also known as Penning or chemi-ionization phenomena [ 4 , 5 ]. Such elementary reactions can be represented according to Eq.…”

Exciton energy transfer reveals spectral signatures of excited states in clusters

“…7 It has been known since the 1980s, that pure rare gas clusters can lead to exciton formation with their properties explored explicitly, 3,8,9 and that composite rare gas complexes can generate processes reminiscent of Penning ionization. 10,11 The appeal of using these systems is that a molecular-level picture of energy transfer and ionization could be followed from small clusters hence testable by theory to the bulk material that is relevant for applications. Specifically, molecules embedded or solvated in argon clusters shed light on the dynamics and mechanisms of many chemical and physical processes.…”