The stereodynamics of the Penning ionization of water by metastable neon atoms

Abstract

The stereodynamics of the Penning ionization of water molecules by collision with metastable neon atoms, occurring in the thermal energy range, is of great relevance for the understanding of fundamental aspects of the physical chemistry of water. This process has been studied by analyzing the energy spectrum of the emitted electrons previously obtained in our laboratory in a crossed beam experiment [B. G. Brunetti, P. Candori, D. Cappelletti, S. Falcinelli, F. Pirani, D. Stranges, and F. Vecchiocattivi, Chem. Phys. Lett. 539-540, 19 (2012)]. For the spectrum analysis, a novel semiclassical method is proposed, that assumes ionization events as mostly occurring in the vicinities of the collision turning points. The potential energy driving the system in the relevant configurations of the entrance and exit channels, used in the spectrum simulation, has been formulated by the use of a semiempirical method. The analysis puts clearly in evidence how different approaches of the metastable atom to the water molecule lead to ions in different electronic states. In particular, it provides the angular acceptance cones where the selectivity of the process leading to the specific formation of each one of the two energetically possible ionic product states of H2O(+) emerges. It is shown how the ground state ion is formed when neon metastable atoms approach water mainly perpendicularly to the molecular plane, while the first excited electronic state is formed when the approach occurs preferentially along the C2v axis, on the oxygen side. An explanation is proposed for the observed vibrational excitation of the product ions.

Dynamics of gas phase Ne* + NH3 and Ne* + ND3 Penning ionisation at low temperatures

“…In the case of the Ne * -water complex, for example, the well along the O-lone pair -Ne * coordinate is more than 400 meV deep, while there is barely any minimum along the O-H-Ne * coordinate. 60 Similarly, our own calculations predict a minimum of 860 meV in the N-lone-pair-Ne * coordinate, whereas along the N-H-Ne * coordinate a more shallow minimum is expected. Accordingly, one might expect the branching ratio to be strongly in favor of formation of NH can be assumed to be independent of collision energy because the variation of the collision energy is small compared to the relevant internal energies.…”

The Possible Role of Penning Ionization Processes in Planetary Atmospheres

“…We have recently studied the ionization of water molecules by Penning ionization coupled with both Mass Spectrometry (MS) and Electron Spectroscopy (PIES) techniques [22][23][24]. The analysis of the obtained results suggests the selective role of a quite pronounced anisotropic attraction that controls the stereodynamics in the entrance channels of the analyzed systems.…”

“…In this respect, the study of the Penning ionization of water and hydrogen sulfide by metastable rare gas atoms (He , respectively, could help in explaining the possible routes of formation for these ionic species in Mercury's exosphere. The importance of these processes and, in particular, of Penning ionization phenomena of hydrogenated molecules by He * and Ne * metastable atoms has been widely discussed in [22][23][24], to which we refer for an overview of the experimental and theoretical works performed since the 1970s by several research groups, starting with the pioneering works by Čermák and Yencha [25] and by Brion and Yee [26].…”

“…This procedure was able to clearly point out how different approaches between the metastable atom and water molecules promote the formation of ions in different electronic states. In particular, it provides the angular acceptance cones where the stereo-selectivity of the processes leading to the specific formation of each one of the two energetically possible electronic states of H2O + final product ions [24] (the X 2 B1 ground state and the first A 2 A1 excited ones) mainly manifests.…”

“…In particular, we rationalized our experimental findings, taking into account the critical balancing between molecular orientation effects in the intermolecular interaction field and the ionization probability. In analyzing our PIES spectra, a novel semiclassical method was proposed: (i) it assumes ionization events as mostly occurring in the vicinities of the collision turning points (where the main part of the collision time is accumulated because the relative velocity tends to zero); and (ii) the potential energy driving the system in the relevant configurations of the entrance and exit channels, employed in the spectra simulation, has been represented by the use of a semiempirical method and given a proper analytical form [24]. This procedure was able to clearly point out how different approaches between the metastable atom and water molecules promote the formation of ions in different electronic states.…”