High-Resolution Spectroscopy of the X-A Transition of the Carbon Monoxide Dication CO$^{2+}$

Abstract

We report rovibronic spectra of the A $^3Σ^+$($v'=0-2$) - X $^3Π_Ω(v=0)$ rovibronic transitions ($|Ω|=0, 1$ and 2) of the CO$^{2+}$ doubly-charged molecular ion. Spectra were recorded at high resolution ($\sim 5$~cm$^{-1}$) in a fast beam of CO$^{2+}$ molecules by detecting the Coulomb explosion of the molecules upon excitation to the A state. Measurements were guided by \textit{ab initio} calculations which then assisted the assignment of the observed spectral features. Our results resolve the spin-orbit splittings of the ground vibronic state X $^3Π_Ω(v=0)$, but not the rotational structure of the bands due to spectral congestion, and provide spectroscopic information on CO$^{2+}$ with unprecedented resolution. In doing so they expand our knowledge of this benchmark doubly charged molecular ion and expand the short list of doubly charged molecules studied at high resolution.

Figures (8)

• Figure 1: Potential electronic curves of CO^2+ that illustrate the peculiar topology commonly encountered for low-lying electronic states in diatomic dications. Both X$^3\Pi$ and $^3\Sigma^+$ electronic states dissociate into C$^+$($^2$P)+O$^+$($^4$S) fragments and the $^3\Sigma^-$ electronic state dissociates into C$^+$($^2$P)+O$^+$($^2$D) fragments.
• Figure 2: Scheme of the spectroscopy and detection regions. Fast CO$^{2+}$ ions are photodissociated by a near-collinear laser beam and the C$^+$ and O$^+$ fragments are detected in coincidence by a pair of position sensitive detectors (PSD) allowing simultaneous determination of the impact positions (POS) and arrival times using a time-to-digital converter (TDC)
• Figure 3: Distribution of C$^+$-O$^+$ coincidences as a function of total kinetic energy release. The sharp peak near 8 eV corresponds to the photodissociation of CO$^{2+}$ molecules in the X state, while the broad distribution corresponds to spontaneous dissociation events along the beam flight toward the detectors. The blue area shows the event-selection window, the gray area the background events used for normalization.
• Figure 4: Potential energy curves of CO^2+ computed at the CASSCF-MRCI/aug-cc-pCV6Z level of theory. Electronic states important to the discussion are pointed out.
• Figure 5: Spin-orbit coupling between the lowest-lying electronic states of CO^2+.