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 C 2 + cation starting from the ground electronic state X Σ g + 1 of the neutral is not possible, especially since the C 2 + ground state is of X + Σ g − 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 Σ g + 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 1 Σ g + ) = 12.440 (10) eV, E i ( X + 4 Σ g − ← a 3 Π 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 C 2 + cation in the gas phase is determined at 0 K, Δ f H 0 (0 K) ( C 2 + ( Π u 2 )) = 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. [ABSTRACT FROM AUTHOR]