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

Collisional alignment and molecular rotation control the chemi-ionization of individual conformers of hydroquinone with metastable neon

The relationship between the shape of a molecule and its chemical reactivity is a central tenet in chemistry. However, the influence of the molecular geometry on reactivity can be subtle and result from several opposing effects. Here, using a crossed-molecular-beam experiment in which individual rotational quantum states of specific conformers of a molecule are separated, we study the chemi-ionization reaction of hydroquinone with metastable neon atoms. We show that collision-induced alignment of the reaction partners caused by geometry-dependent long-range forces influences reaction pathways, which is, however, countered by molecular rotation. The present work provides insights into the conformation-specific stereodynamics of complex polyatomic systems and illustrates the capability of advanced molecule-control techniques to unravel these effects.

Isomeric and rotational effects in the chemi-ionisation of 1,2-dibromoethene with metastable neon atoms

The specific geometry of a molecule can have a pronounced influence on its chemical reactivity. However, experimental data on reactions of individual molecular isomers are still sparse because they are often difficult to separate and frequently interconvert into one another under ambient conditions. Here, we employ a novel crossed-beam experiment featuring an electrostatically controlled molecular beam combined with a source for radicals and metastables to spatially separate the cis and trans stereoisomers as well as individual rotational states of 1,2-dibromoethene and study their specific reactivities in the chemi-ionisation reaction with excited neon atoms. The experiments reveal pronounced isomeric and rotational specificities in the rates and product branching ratios of the reaction. The present study underlines the importance and combined role of molecular geometry and of rotational motion in the dynamics of chemi-ionisation reactions.

General treatment for stereo-dynamics of state-to-state chemi-ionization reactions

The investigation of chemi-ionization processes provides unique information on how the reaction dynamics depend on the energy and structure of the transition state which relate to the symmetry, relative orientation of reagent/product valence electron orbitals, and selectivity of electronic rearrangements. Here we propose a theoretical approach to formulate the optical potential for Ne*(³P_2,0) noble gas atom chemi-ionizations as prototype oxidation processes. We include the selective role of atomic alignment and of the electron transfer mechanism. The state-to-state reaction probability is evaluated and a unifying description of the main experimental findings is obtained. Further, we reproduce the results of recent and advanced molecular beam experiments with a state selected Ne* beam.The selective role of electronic rearrangements within the transition state, quantified through the use of suitable operative relations, could cast light on many other chemical processes more difficult to characterize.

Leading Interaction Components in the Structure and Reactivity of Noble Gases Compounds

The nature, strength, range and role of the bonds in adducts of noble gas atoms with both neutral and ionic partners have been investigated by exploiting a fine-tuned integrated phenomenological–theoretical approach. The identification of the leading interaction components in the noble gases adducts and their modeling allows the encompassing of the transitions from pure noncovalent to covalent bound aggregates and to rationalize the anomalous behavior (deviations from noncovalent type interaction) pointed out in peculiar cases. Selected adducts affected by a weak chemical bond, as those promoting the formation of the intermolecular halogen bond, are also properly rationalized. The behavior of noble gas atoms excited in their long-life metastable states, showing a strongly enhanced reactivity, has been also enclosed in the present investigation.

Imaging Recoil Ions from Optical Collisions between Ultracold, Metastable Neon Isotopes

We present an experimental scheme that combines the well-established method of velocity-map imaging with a cold trapped metastable neon target. The device is used for obtaining the branching ratios and recoil-ion energy distributions for the penning ionization process in optical collisions of ultracold metastable neon. The potential depth of the highly excited dimer potential is extracted and compared with theoretical calculations. The simplicity to construct, characterize, and apply such a device makes it a unique tool for the low-energy nuclear physics community, enabling opportunities for precision measurements in nuclear decays of cold, trapped, short-lived radioactive isotopes.

A New Insight on Stereo-Dynamics of Penning Ionization Reactions

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^*(³P_{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 H₂O 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^*(³P_{2, 0}) reagent and two different spin-orbit states of the ionic M⁺(²P_{3/2, 1/2}) products (M = Kr, Xe). The obtained results emphasized the reactivity change of ³P₀ atoms with respect to ³P₂, in producing ions in ²P_3/2 and ²P_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 ³P₂ and ³P₀ sublevels.

The electron couplings in the transition states: The stereodynamics of state to state autoionization processes

Measurements of the kinetic energy distribution of electrons, emitted in collision between Ne^*(³P_2,0) and Kr(¹S₀) and Xe(¹S₀), have been performed in a crossed molecular beam apparatus which employs a mass spectrometer and a hemispherical electron analyzer as detectors. The analysis of the obtained experimental results provides new insights on electronic rearrangements and electronic angular momentum coupling effects that determine relevant properties of the transition state of autoionization processes, and that we have found useful to classify as adiabatic and non-adiabatic effects. In particular, while the adiabatic effects control sequence, energy, and symmetry of quantum states accessible to both reagents and products in the probed collision energy range, the non-adiabatic ones trigger the passage from entrance to exit channels. The obtained results are important not only to compact previous theoretical schemes of autoionization reactions in a unified representation but also to cast light on the role of electronic rearrangements within the transition state of many other types of chemical processes that are more difficult to characterize.

Adiabatic and Nonadiabatic Effects in the Transition States of State to State Autoionization Processes

The energy distribution of electrons, emitted from collisions between Ne^{*}(^{3}P_{2,0}) and Kr(^{1}S_{0}), have been measured under high resolution conditions in a crossed molecular beam apparatus containing a hemispherical electron analyzer as detector. The experimental results provide new insights on the electronic adiabatic and nonadiabatic effects in the stereodynamics of state to state atomic and molecular collisions, controlling relevant properties of the transition state of autoionization processes. In particular, while the adiabatic effects determine sequence, energy, and symmetry of quantum states accessible both to reagents and products, the nonadiabatic effects trigger the passage from entrance to exit channels.

Stereodynamics of Ne(3P2) reacting with Ar, Kr, Xe, and N2

Stereodynamics experiments of Ne(³P₂) reacting with Ar, Kr, Xe, and N₂ leading to Penning and associative ionization have been performed in a crossed molecular beam apparatus. A curved magnetic hexapole was used to state-select and polarize Ne(³P₂) atoms which were then oriented in a rotatable magnetic field and crossed with a beam of Ar, Kr, Xe, or N₂. The ratio of associative to Penning ionization was recorded as a function of the magnetic field direction for collision energies between 320 cm^-1 and 500 cm^-1. Reactivities are obtained for individual states that differ only in Ω, the projection of the neon total angular momentum vector on the inter-particle axis. The results are rationalized on the basis of a model involving a long-range and a short-range reaction mechanism. Substantially lower probability for associative ionization was observed for N₂, suggesting that predissociation plays a critical role in the overall reaction pathway.

The role of charge transfer in the stability and reactivity of chemical systems from experimental findings

Phenomena are described within a unifying picture, by isolating charge/electron transfer as an interaction component triggering chemical reactivity.

Stereoselectivity in Autoionization Reactions of Hydrogenated Molecules by Metastable Noble Gas Atoms: The Role of Electronic Couplings

Focus in the present paper is on the analysis of total and partial ionization cross sections, measured in absolute value as a function of the collision energy, representative of the probability of ionic product formation in selected electronic states in Ne*-H2 O, H2 S, and NH3 collisions. In order to characterize the imaginary part of the optical potential, related to electronic couplings, we generalize a methodology to obtain direct information on the opacity function of these reactions. Such a methodology has been recently exploited to test the real part of the optical potential (S. Falcinelli et al., Chem. Eur. J., 2016, 22, 764-771). Depending on the balance of noncovalent contributions, the real part controls the approach of neutral reactants, the removal of ionic products, and the structure of the transition state. Strength, range, and stereoselectivity of electronic couplings, triggering these and many other reactions, are directly obtained from the present investigation.

Measurements of Ionization Cross Sections by Molecular Beam Experiments: Information Content on the Imaginary Part of the Optical Potential

In this work, we present and analyze in detail new and recent ionization cross section and mass spectrum determinations, collected in the case of He*, Ne*-H2O, -H2S, and -NH3 ionizing collisions. These sets of data, obtained under the same experimental conditions, are relevant to identify differences in the autoionization stereodynamics of the three hydrogenated molecules and on the selective role of the imaginary part of the optical potential. We demonstrate that in these autoionization processes hydrogen and halogen bonds are competing because they are controlling both real and imaginary components of the optical potential that drives the complete reaction dynamics. In particular, we found that both components critically depend on the angular and radial approach between the reagent partners in determining the collision dynamics.