To see C₂: Single-photon ionization of the dicarbon molecule

The C₂ carbon cluster is found in a large variety of environments including flames, electric discharges, and astrophysical media. Due to spin-selection rules, assessing a complete overview of the dense vibronic landscape of the C₂ ⁺ cation starting from the ground electronic state X Σg+1 of the neutral is not possible, especially since the C₂ ⁺ ground state is of X⁺ Σg-4 symmetry. In this work, a flow-tube reactor source is employed to generate the neutral C₂ in a mixture of both the lowest singlet X Σg+1 and triplet a ³Π_u electronic states. We have investigated the vibronic transitions in the vicinity of the first adiabatic ionization potential via one-photon ionization with vacuum ultraviolet synchrotron radiation coupled with electron/ion double imaging techniques. Using ab initio calculations and Franck-Condon simulations, three electronic transitions are identified and their adiabatic ionization energy is determined E_i(a^{+  2}Π_u←X ¹Σ_g ⁺)=12.440(10) eV, E_i(X^{+  4}Σ_g ^-←a ³Π_u)=11.795(10) eV, and E_i(a⁺²Π_u ← a³Π_u) = 12.361(10) eV. From the three origin bands, the following energy differences are extracted: ΔE(a - X) = 0.079(10) eV and ΔE(a⁺ - X⁺) = 0.567(10) eV. The adiabatic ionization potential corresponding to the forbidden one-photon transition X⁺ ← X is derived and amounts to 11.873(10) eV, in very good agreement with the most recent measurement by Krechkivska et al. [J. Chem. Phys. 144, 144305 (2016)]. The enthalpy of formation of the doublet ground state C₂ ⁺ cation in the gas phase is determined at 0 K, Δ_fH⁰(0K)(C₂ ⁺(Πu2))=2019.9(10) kJ mol^-1. In addition, we report the first experimental ion yield of C₂ for which only a simple estimate was used up to now in the photochemistry models of astrophysical media due to the lack of experimental data.