Vacuum ultraviolet photoionization of C3
Résumé
Photoionization efficiency (PIE) curves for C3 molecules produced by laser ablation are measured from 11.0 to 13.5 eV with tunable vacuum ultraviolet undulator radiation. A step in the PIE curve versus photon energy, obtained with N2 as the carrier gas, supports the conclusion of very effective cooling of C3 to its linear 1Σg+ ground state. The second step observed in the PIE curve versus photon energy could be the first experimental evidence of the C3+(2Σg+) excited state. The experimental results, complemented by ab initio calculations, suggest a state-to-state vertical ionization energy of 11.70 ± 0.05 eV between the C3(X̃1Σg+) and the C3+(X̃2Σu+) states. An ionization energy of 11.61 ± 0.07 eV between the neutral and ionic ground states of C3 is deduced using the data together with our calculations. Accurate ab initio calculations are performed for both linear and bent geometries on the lowest doublet electronic states of C3+ using Configuration Interaction (CI) approaches and large basis sets. These calculations confirm that C3+ is bent in its electronic ground state, which is separated by a small potential barrier from the 2Σu+ minimum. The gradual increase at the onset of the PIE curve suggests a geometry change between the ground neutral and cationic states. The energies between several doublet states of the ion are theoretically determined to be 0.81, 1.49, and 1.98 eV between the 2Σu+ and the 2Σg+, 2Πu, 2Πg excited states of C3+, respectively.