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

Stefano Falcinelli, Franco Vecchiocattivi, and Fernando Pirani

Phys. Rev. Lett. 121, 163403 – Published 16 October 2018

Abstract

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.

Received 20 July 2018

DOI:https://doi.org/10.1103/PhysRevLett.121.163403

Physics Subject Headings (PhySH)

Atomic & molecular collisions Chemical reactions Electronic excitation & ionization Interatomic & molecular potentials

Atomic, Molecular & Optical

Authors & Affiliations

Stefano Falcinelli^1,*, Franco Vecchiocattivi¹, and Fernando Pirani²

¹Dipartimento di Ingegneria Civile ed Ambientale, Università di Perugia, Via G. Duranti 93, 06125 Perugia, Italy
²Dipartimento di Chimica, Biologia e Biotecnologie, Università di Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy

^*Corresponding author. stefano.falcinelli@unipg.it

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Issue

Vol. 121, Iss. 16 — 19 October 2018

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Images

Figure 1
A schematic view of the apparatus: the primary beam of metastable ${Ne}^{*} ({{}^{3}P}_{2, 0})$ atoms, produced by an electron bombardment supersonic seeded source, with a relative population of ${{}^{3}P}_{0} / {{}^{3}P}_{2} = 0.20 \pm 0.02$ , crosses at right angles the secondary beam of Kr atoms. The mass spectrometry technique has been not directly used in the PIES measurements (for details see Supplemental Material [11]).
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Figure 2
Upper panel: ${Ne}^{*} − Kr$ PIES measured at a collision energy of 0.050 eV with a resolution of about 0.025 eV. The contributions of four different reaction channels (see text and Supplemental Material [11]) are resolved. Vertical continuous lines represent the peak positions, predicted for the ionization of Kr by Ne(I) photons. The shift of 0.05–0.08 eV of the maxima of Gaussian functions, used to fit the PIES data, with respect to the vertical lines, depends on the structure and stability of the intermediate transition state. Lower panel: Adiabatic potential energy curves, providing the dependence of the interaction on the internuclear distance $R$ for all quantum states accessible to the system (the $| 2, 1 ⟩$ state, not indicated, is very similar to $| 2, 0 ⟩$ ). $R_{0}$ is the distance, where atom-atom and ion-atom interactions have the same weight. The potential parameters are the same of Ref. [18], only a fine tuning of $R_{0}$ has been performed. All details of ${Ne}^{+} − Kr$ and $Ne − {Kr}^{+}$ systems, crucial to define the role of CT coupling, the interaction anisotropy and the triggering of autoionization reactions, are plotted. Atomic quantum states $| J, Ω ⟩$ , associated to the potential energy curves at large and intermediate internuclear distances, correlate with molecular states, here indicated as ${}^{2}Σ$ and ${}^{2}Π$ , emerging at short distances.
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Figure 3
Upper panel: PIES for ${Ne}^{*} − Kr$ measured as a function of the collision energy with a resolution of 0.080 eV. Contributions of the four channels, described by four Gaussian functions whose combination reproduces the measured peak shape and position, are also plotted. Vertical lines allow us to estimate a shift of the peak positions of 0.1–0.2 eV (see caption of Fig. 2). Lower panel: Height peak ratios (black points), provided by Gaussian functions that fit PIESs measured as a function of the collision energy. Open circles are the results of a treatment used to extract information on the relative probability of the four reaction channels (see text and Supplemental Material [11]).
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Figure 4
Upper panel: A cartoon of direct and indirect mechanisms of autoionization processes promoted by nonadiabatic effects. Lower panel: Obtained $A_{Σ − Π} / A_{Σ - Σ}$ coupling term ratios, plotted as a function of the collision energy. Open and black circles refer to two different sequences of $Σ$ and $Π$ character (see Table S2 of the Supplemental Material [11]) of the quantum states accessible to the collision complex.
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