Charge inversion of ground-state and metastable-state C2+ cations formed from electroionised C2H2 and C2N2, and a re-evaluation of the carbon dimerʹs ionisation energy Original Research Article

Charge-inversion, translational-energy spectrometry has been used to probe the electronic-state compositions of C2+ ion beams formed from electroionised acetylene and cyanogen, and to determine state-specific ionisation energies for C2. It was found that C2+ ions were formed predominantly in the 4Σg− ground state when electrons of about 23 eV energy were used to ionise acetylene. Below this threshold, and down to about 19 eV electron energy, the C2+ ions formed from C2H2 were found to be in the metastable 2Πu state, while above 23 eV the beams were of mixed-state composition. In contrast, ionisation of cyanogen by electrons with energies from about 18 to 100 eV apparently produced beams containing only C2+ (2Πu) ions. From the energy-loss positions of observed spectral peaks, the energies for ionisation of C2(X̃1Σg+) into the 4Σg− and 2Σu states were deduced to be 11.4(1) ± 0.30 and 12.3(3) ± 0.30 eV, respectively.