Single-Photon Double Ionization of Ozone: Experiment and Theory

Technical Summary: Single-Photon Double Ionization of Ozone: Experiment and Theory

Problem Statement
While the neutral ozone molecule ($O_3$) and its singly charged cation ($O_3^+$) have been extensively characterized regarding their electronic structure, vibrational properties, and reactivity, the doubly ionized ozone dication ($O_3^{2+}$) remains poorly understood. Theoretical models suggest that removing two electrons from this highly correlated triatomic system results in complex potential energy surfaces (PESs) with strong coupling between electronic and nuclear degrees of freedom. Previous experimental efforts, primarily based on electron-impact ionization, have identified a threshold for the lowest-lying dicationic state but have failed to observe the parent $O_3^{2+}$ ion, attributing its absence to rapid dissociation driven by singlet-triplet spin crossings in the Franck-Condon region. Crucially, the valence double ionization electron spectrum of ozone had not been measured, leaving a gap in understanding the photon-induced fragmentation dynamics and the stability of excited dicationic states.

Methodology
The study employs a combined experimental and theoretical approach to characterize the single-photon valence double ionization of ozone.

• Experimental Setup: Experiments were conducted at the University of Gothenburg and the MAX IV Laboratory using time-of-flight photoelectron–photoelectron (TOF-PEPECO) and photoelectron–photoelectron–photoion–photoion coincidence (TOF-PEPEPIPICO) techniques based on magnetic bottles.

  • Radiation Sources: Measurements utilized He II-$\alpha$ (40.8 eV), He II-$\beta$ (48.4 eV), and synchrotron radiation at 56.0 eV.
  • Detection: The TOF-PEPECO setup recorded electron-electron coincidences to determine double ionization energies. The TOF-PEPEPIPICO setup enabled four-fold coincidence detection (two electrons and two ions), allowing for the correlation of specific double ionization energies with distinct fragmentation channels ($O^+ + O_2^+$ and $O^+ + O^+ + O$).
  • Sample Handling: Ozone was generated and adsorbed onto silica gel beads at 195 K to ensure purity, minimizing background contaminants like $O_2$, $N_2$, and $H_2O$.

• Theoretical Framework: Ab initio computations were performed to interpret experimental features.

  • Electronic Structure: The lowest potential energy surfaces of $O_3^{2+}$ were mapped using post-Hartree-Fock multi-configurational interaction methods (MRCI and MRCI-F12) based on state-averaged complete active-space self-consistent field (SA-CASSCF) wavefunctions.
  • Energetics: Adiabatic (ADIE) and vertical (VDIE) double ionization energies were calculated using a composite scheme: (R)CCSD(T)-F12b/aug-cc-pVTZ for geometry optimization and (R)CCSD(T)/aug-cc-pVTZ with core-valence, scalar relativistic, and zero-point vibrational energy corrections for single-point energy calculations.
  • Dynamics: One-dimensional cuts of the 3D PESs were analyzed along bond stretching and bending coordinates to identify dissociation pathways, spin-orbit couplings, and Renner-Teller effects.

Key Contributions and Results

1. First Valence Double Ionization Spectrum: The authors report the first single-photon valence double ionization electron spectrum of $O_3$. The spectrum reveals three primary features:

   • A band at a vertical ionization energy (VDIE) of 35.5 eV.
   • A broader feature around 36.5 eV.
   • A sharp band at 38.0 eV.

2. Assignment of Electronic States:

   • The lowest theoretical singlet state ($X^1\Sigma_g^+$) is linear and lies outside the Franck-Condon region of the bent neutral ozone, predicting a weak, broad feature.
   • The first experimental band (35.5 eV) is assigned to the lowest triplet state ($A^3B_2$), which possesses a bent geometry similar to neutral ozone, resulting in favorable Franck-Condon factors. The calculated VDIE for this state (35.6 eV) aligns with the experimental observation.
   • The high density of singlet and triplet states in the 36–42 eV range leads to spectral congestion, mediated by non-adiabatic and spin-orbit interactions.

3. Fragmentation Dynamics and Absence of Parent Ion:

   • No stable $O_3^{2+}$ parent ion was detected in mass spectra, confirming that all accessible dicationic states are short-lived.
   • $O^+ + O_2^+$ Channel: This channel opens at ~35.5 eV. Analysis of kinetic energy release (KER) suggests the production of $O_2^+$ in its ground state ($X^2\Pi_g$) and $O^+$ in an excited state ($^2D$), or high vibrational levels of the ground state.
   • $O^+ + O^+ + O$ Channel: This three-body fragmentation channel opens at a higher energy (~36.4 eV) and peaks above 38.5 eV. The KER is estimated at ~2.8 eV. Theoretical analysis indicates that this pathway involves the simultaneous elongation of both O-O bonds, leading to ground-state $O^+$ fragments, with the neutral oxygen atom acting as a spectator or departing prior to the final breakup.

4. Role of Excited States: The study demonstrates that dissociative double ionization produces not only ground-state fragments but also electronically excited $O^+$ ions and potentially excited oxygen atoms. The presence of spin-orbit crossings and avoided crossings between singlet and triplet states facilitates predissociation, preventing the observation of long-lived dications.

Significance
The paper establishes a comprehensive framework for understanding valence double ionization in ozone, bridging the gap between single-ionization physics and the complex dynamics of multiply charged molecules. By providing the first experimental electron spectrum and high-accuracy theoretical benchmarks, the work clarifies the fragmentation mechanisms of $O_3^{2+}$.

The authors highlight that the observation of electronically excited oxygen fragments is particularly significant for atmospheric and astrochemical modeling. These species can initiate highly reactive chemical pathways in planetary atmospheres, potentially serving as an ionization-driven analogue to the well-known photodissociation production of $O(^1D)$. The results offer critical data for modeling ion-molecule reactions in the Earth's ionosphere and other planetary environments where intense ionizing radiation is present.