High-resolution pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopic study of the two lowest electronic states of the ozone cation $O3+$

The pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectrum of jet-cooled $O3$ has been recorded in the range $101 000–104 000 cm−1.$ The origins of the $X̃ 1A1→X̃+ 2A1$ and $X̃ 1A1→Ã+ 2B2$ transitions could be determined from the rotational structure of the bands, the photoionization selection rules, the photoionization efficiency curve, and comparison with ab initio calculations. The first adiabatic ionization energy of $O3$ was measured to be $101 020.5(5) cm−1$ [12.524 95(6) eV] and the energy difference between the $X̃+ 2A1 (0,0,0)$ and $Ã+ 2B2 (0,0,0)$ states was determined to be $ΔT0=1089.7(4) cm−1.$ Whereas the $X̃→X̃+$ band consists of an intense and regular progression in the bending $(ν2)$ mode observed up to $v2+=4,$ only the origin of the $X̃→Ã+$ band was observed. The analysis of the rotational structure in each band led to the derivation of the $r0$ structure of $O3+$ in the $X̃+$ $[C2v,r0=1.25(2) Å,α0=131.5(9)°]$ and $Ã+[C2v,r0=1.37(5) Å,α0=111.3(38)°]$ states. The appearance of the spectrum, which is regular up to $102 300 cm−1,$ changes abruptly at $≈102 500 cm−1,$ a position above which the spectral density increases markedly and the rotational structure of the bands collapses. On the basis of ab initio calculations, this behavior is attributed to the onset of large-amplitude motions spreading through several local minima all the way to large internuclear distances. The ab initio calculations are consistent with earlier results in predicting a seam of conical intersections between the $X̃+$ and $Ã+$ states $≈2600 cm−1$ above the cationic ground state and demonstrate the existence of potential minima at large internuclear distances that are connected to the main minima of the $X̃+$ and $Ã+$ states through low-lying barriers.