Introduction

Ozone (O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\vphantom{0}_3$$\end{document}) is a triatomic molecule of central importance in the chemistry and physics of the Earth and other planetary atmospheres1. In the stratosphere, ozone forms a protective layer that absorbs much of the energetic ultraviolet (UV) radiation of the sun, preventing biologically harmful wavelengths from reaching the Earth’s surface and thus protecting living organisms including mankind from DNA and tissue damage. In contrast, high concentrations of ozone in the lower atmosphere (troposphere) constitute a major air pollutant, causing oxidative stress in plants and animals and contributing to respiratory and cardiovascular disease in humans. The dual environmental role of ozone has rendered it a key topic in atmospheric science research for many decades. Following the discovery of the Antarctic ozone hole2, intense research efforts have been devoted to understanding both the depletion mechanisms of stratospheric ozone and the formation of tropospheric ozone. These concerns are reflected in international agreements such as the Kyoto3 (1997) and Gothenburg protocols4 (1999, revised in 2012), which aim to mitigate the adverse effects of ozone imbalance on climate and health.

From a fundamental scientific perspective, detailed knowledge of ozone’s electronic structure, energetics, and reactivity across its various states of charge and excitation is essential for modeling atmospheric and astrochemical processes. The properties of neutral ozone have been extensively investigated using a range of spectroscopic techniques, providing accurate information on its vibrational structure, potential energy surfaces, and symmetry properties. As can be found in chemistry textbooks, the neutral ozone molecule adopts a bent geometry of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\textrm{C}_{2v}$$\end{document} symmetry, and its ground-state electronic configuration is given by:

\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$(1a_1)^2(1b_2)^2(2a_1)^2(3a_1)^2(2b_2)^2(1b_1)^2(4a_1)^2(5a_1)^2(3b_2)^2(1a_2)^2(4b_2)^2(6a_1)^2 \hspace{0.5cm} \mathrm {X^1A_1}$$\end{document}

Singly positively charged ozone (O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\vphantom{0}_3^+$$\end{document}) has also been the subject of considerable attention. Its energy levels and vibrational structure have been characterized by vacuum ultraviolet (VUV) photoelectron spectroscopy5–9, threshold photoelectron spectroscopy10, high-resolution pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE)techniques11 and theoretical methods12,13. These studies have elucidated the fine structure of the lowest cationic states and revealed strong vibronic coupling and state mixing, reflecting the rich multi-configurational nature of ozone. Core-level photoelectron spectroscopy has further provided site-specific binding energies for the terminal and central oxygen atoms14, offering insight into the localization of charge and the dissociation dynamics following core excitation.

By contrast, doubly ionized ozone (O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\vphantom{0}_3^{2+}$$\end{document}) remains much less well understood. From a theoretical standpoint, the removal of two electrons from this highly correlated triatomic system is expected to result in multiple electronic states with complex potential energy surfaces and strong coupling between electronic and nuclear degrees of freedom. Experimentally, only a few studies have addressed the double ionization of ozone so far, most notably those based on electron-impact ionization combined with ion–ion coincidence detection15. These experiments identified a threshold of 34.4 eV for forming the lowest-lying dicationic state, which was observed to dissociate into O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\vphantom{0}^+$$\end{document} and O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\vphantom{0}_2^+$$\end{document} fragments. Computations suggest that this threshold corresponds to the adiabatic double ionization energy of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm {O_3}$$\end{document} (of 34.7–35.0 eV), whereas the vertical double ionization energy is found higher (at _∼_35–36 eV) because of unfavourable Franck-Condon factors upon doubly ionizing ozone (from bent-to-linear structures)16–18. Besides, the electron-impact dissociation of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm {O_3^+}$$\end{document} is shown to produce \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm {O^+}$$\end{document} + \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm {O_2^+}$$\end{document} and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm {O^+}$$\end{document} + \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm {O^+}$$\end{document} + O fragments in addition to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm {O^+}$$\end{document} + \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm {O_2}$$\end{document} and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm {O^+}$$\end{document} + O + O channels. The former pathways are most likely due to the unimolecular decomposition of the intermediate \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm {O_3^{2+}}$$\end{document} dication formed upon ionizing the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm {O_3^{+}}$$\end{document}cation19,20. So far, the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm {O_3^{2+}}$$\end{document} parent ion has not been observed, being too short lived to reach the detector. For explanation, Price and co-workers showed that the dicationic potential energy surfaces of singlet – triplet spin cross in the Franck – Condon region upon doubly ionizing \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm {O_3}$$\end{document}. The triplet is dissociative and leads to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm {O_2^{+}}$$\end{document} + \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm {O^{+}}$$\end{document} products and thus prevents measurement of long-lived \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm {O_3^{2+}}$$\end{document} ions by impact ionization of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm {O_3}$$\end{document}16. While such measurements have provided already some valuable energetic benchmarks, the valence double ionization electron spectrum of ozone has not previously been measured, and little is known about its photon-induced fragmentation dynamics or the stability of its excited dicationic electronic states.

In the present work, we extend substantially the experimental and theoretical characterization of ozone further into the realm of double ionization. Using He II-α, He II-β and higher energy vacuum ultraviolet radiation and a versatile multiple charged-particle correlation detection method, we have recorded the first single-photon valence double ionization electron spectrum of O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\vphantom{0}_3$$\end{document}. The choice of He II-β radiation is particularly relevant, as helium emission is a dominant component of the solar spectrum, rendering these results directly applicable to photoionization and ion–molecule processes in the Earth’s ionosphere and in other planetary atmospheres. The measured double ionization energies of O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\vphantom{0}_3$$\end{document} provide new benchmarks for theory and enable an improved understanding of the energy available for the dissociation and chemical reactivity of ionic O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\vphantom{0}_3$$\end{document}. For interpretation of the experimental observations, we mapped the lowest potential energy surfaces of ozone dication using post-Hartree-Fock multiconfiguration interaction approaches. We also computed with high accuracy the energetics of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm {O_3^{2+}}$$\end{document} and its dissociation channels. With these results, we aim to elevate our fundamental understanding of ozone’s electronic structure well beyond single-ionization and to contribute to a more comprehensive picture of the energy available for ion-molecule reactions of dicationic O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\vphantom{0}_3$$\end{document} in the Earth’s ionosphere and extraterrestrial planetary atmospheres.