Doubly Charged Carbon Dioxide Produced by the Electron Impact with Molecular Clusters

Strong coulombic repulsion in small doubly charged molecular ions usually leads to fragmentations. Some of them, such as CO2++, could survive and be detected if they are stable or metastable in energetics, but how to produce these observable doubly charged ions is a puzzling issue. Here we investigate CO2++ production by electron-impact ionization with a supersonic molecular beam of CO2 under different nozzle pressures, using time-of-flight mass spectrometry measurements and ab initio calculations. The mass spectral profile of CO2++ varies slightly with the nozzle pressure, implying different mechanisms of the ion production. The calculations indicate that the ground state of CO2++ is 3A" with a nonlinear conformation, while the linear conformer is in the first excited state 1Δg. We further suggest that, besides CO2++ (1Δg) produced from CO2, CO2++ (3A") could be produced by the dissociation of doubly charged dimer (C2O4++) in a repulsive triplet state.