Precursor state of chemi-ionization reactions and confinement of valence electrons by anisotropic intermolecular forces

THE EUROPEAN PHYSICAL JOURNAL D Eur. Phys. J. D (2021)75:94 https://doi.org/10.1140/epjd/s10053-021-00113-8 Regular Article - Atomic and Molecular Collisions Precursor state of chemi-ionization reactions and confinement of valence electrons by anisotropic intermolecular forces Stefano Falcinelli1,a , Franco Vecchiocattivi1, Simonetta Cavalli2 , and Fernando Pirani2 1 2 Dipartimento di Ingegneria Civile ed Ambientale, Università di Perugia, 06125 Perugia, Italy Dipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia, 06123 Perugia, Italy Received 29 October 2020 / Accepted 22 February 2021 © The Author(s) 2021 Abstract. Modifications in atomic alignment and in molecular alignment/orientation determine a different structure of the adduct, formed by collisions of reagents, which represents the precursor state of many elementary chemical–physical processes. The following evolution of the system is directly controlled by the confinement of interacting partners in such a precursor state. However, a deep characterization of these phenomena is still today not fully available, especially when weak intermolecular forces are operative, although the inquiry is of general relevance for the control of the stereodynamics of processes, occurring under a variety of conditions both in gas phase and at surface. In this paper recent advances in the knowledge of the selective role of atomic alignment and molecular orientation effects on the stereodynamics of chemi-ionization reactions will be presented and discussed. These advances represent a basic step along a path whose final target is the complete and internally consistent rationalization and revaluation of the experimental findings already obtained, and published, in our and in other laboratories on chemi-ionization reactions involving as reagent molecules which are of great relevance in several fields. The basic idea is to export important guidelines provided by a recent detailed study of chemi-ionization of noble gas atoms to more complex reactions involving molecules. The main focus of the present paper is on the quantum confinement effects of valence electrons within the reaction transition state. 1 Introduction Understanding the mode-specificity in reaction dynamics of open-shell atoms, free radicals, molecules, atomic and molecular ions, under hyper-thermal, thermal and sub-thermal conditions is a general topic of fundamental importance for catalysis, plasmas, photo-dynamics as well as interstellar and low-temperature chemistry. However, a pivotal target to assess the selectivity of the molecular dynamics under a variety of conditions is the knowledge of strength, anisotropy and radial dependence of the leading intermolecular interaction components which drive two-body collisions. A basic component of the intermolecular interaction is the charge transfer (CT). Despite CT affects a relevant variety of phenomena [1], its role is not fully understood, being difficult to characterize at the high level of detail often required. They include harpooning, coulomb explosion, proton transfer and the formation of intermolecular halogen and hydrogen bonds. CT can also affect structure and stability of the precursor state of several elementary chemical reactions, determining their following evolution towards the final products. a e-mail: author) (corresponding 0123456789().: V,-vol Chemi-ionization processes, also known as Penning or auto-ionization phenomena [2–4], represent barrier-less reactions promoted by collisions of a neutral reagent, exhibiting a sufficiently high internal energy content, with an atomic/molecular partner leading to the formation of ionic products. Such processes are driven by an anisotropic optical potential, whose real part controls approach of reactants and removing of products, while the imaginary part triggers the passage from neutral reactants to ionic products through a selective electronic rearrangement. The investigation at microscopic level of these reactions is important for fundamental research, to assess the coherent control of reactive events at low temperature and then to explore the quantum nature of matter [5], and also for the applied research, to exploit the soft ionization in the development of mass spectrometry technique [6] and to prepare particular ionic clusters [7]. However, detailed information on radial and angular dependences of the optical potential is rather limited since the quantitative relations between real and imaginary part of the optical potential and basic components of the intermolecular forces involved are still lacking. In particular, their strength is often smaller than few meV (a fraction of kJ/mol) and then they are very difficult to obtain by ab initio methods. 123 94 Page 2 of 9 Main focus of this paper is on the selective role of anisotropic CT, a fundamental component of the intermolecular interaction affecting both real and imaginary parts of the optical potential, on the stereo-dynamics of chemi-ionization processes promoted by collisions of an open shell atom, electronically excited in a high energetic metastable state, with another atomic/molecular partner. The reactions give rise to the spontaneous emission of electrons accompanied by the formation of parent ions, aggregate ions and fragmentation ions as final products. Particular attention is addressed to structure and stability of the adduct formed by collisions of reagents which represents the confined environment where all basic electronic rearrangements triggering the reactions occur. In particular, the measure of the energy dependence of electrons, emitted by single collision events and known as Penning Ionization Electron Spectra (PIES), represents a sort of spectroscopic investigation of the collision complex [8–10]: it provides direct information on the occurring electronic rearrangements, which directly correlates with structure and stability the precursor transition state of involved reactions. According to the different structure of the adduct, molecular ionization probability and emitted PIES are strongly dependent on symmetry and energy of the atomic or molecular orbital from which the electron is extracted and ends, and then on their spatial orientation within the collision complex [11–13]. Recent advances obtained in our laboratory and concerning the characterization of the selectivity of the electronic rearrangements which drive the stereodynamics of chemi-ionization reactions will be presented and discussed in the next sections. In particular, in the following we summarize basic details of the chemi-ionization stereo-dynamics, obtained in our laboratory on two prototype processes [11,12,14–18], in order to emphasize better the focus of our present research, indicating also important perspectives. 2 Structure of the collision complex and observables Figure 1 shows a metastable Ne∗ atom, whose external electron is excited in the 3s orbital and its ionic core exhibits the same electroni (...truncated)