ECAMP 15

Doubly charged molecular ions are elusive species that, despite their role in plasmas [1], have rarely been spectroscopically characterized. We report the first study by laser spectroscopy of the X($^3\Pi_\Omega$)-A($^3\Sigma^+$) transition of the CO⁺⁺ ion. Experimentally, a fast beam of CO⁺⁺ ions was produced from a plasma ion source. In the ground electronic state, all vibrational levels except the ground state and the first excited state are short lived. Therefore, despite the high temperature of the ion source, only those ions with $v=0,1$ are present in the beam. The beam is then intersected, at a small angle, by a laser beam resonant with rovibrational transitions from the X to the A state. In the A state, the ions rapidly predissociate and Coulomb explode into C⁺ and O⁺ fragments whose kinetic energies are known. Excitation spectra are recorded in a background-free manner by scanning the laser while detecting, in coincidence, the C⁺ and O⁺ fragments with the appropriate energy. Experimental investigations are assisted by accurate ab-initio calculations of the potential-energy curves of twelve low-lying states of CO⁺⁺ with the multi-reference configuration interaction method. The nonadiabatic spin-orbit couplings are also calculated and used, together with the potential-energy curves, to determine the energies of the rovibrational levels and their lifetimes with a complex-scaling-based method.

With this approach we measured the spectra of the X($v'=0,1$)$-$A($v=0-2$) vibronic transitions. The resolution achieved ($\sim 5$ cm$^{-1}$) represents a 15-fold improvement compared to previous studies [2]. The rotational envelopes of the vibronic spectra confirm the theoretical values of the rotational constants, and the spin-orbit splitting of the X state is fully resolved and characterized for the first time. We also extract from our spectra and calculations an accurate set of spectroscopic constants of the X and A states of CO⁺⁺. These results provide a benchmark for understanding doubly-charged molecular ions of atmospheric relevance.

[1] Thissen et al., Phys. Chem. Chem. Phys. 41, 18264 (2011)
[2] Penent et al., Phys. Rev. Lett. 81, 3619 (1998)