Double-ionization energies of acetonitrile (methyl cyanide) to singlet and triplet electronic states of the dication were measured and calculated. In the experimental part of the investigation, double-electron-capture reactions of fast-moving singly charged positive ions with acetonitrile molecules were investigated. Double-ionization energies were determined from the translational energies of the negative ions generated. Because of spin conservation in the reactions, triplet states of CH3CN2+ were populated when using OH+ projectile ions, and singlet states when using H+ projectile ions. The double-ionization energies measured were compared with values calculated using two different computational methods. Ab initio MP2J3-21G predictions of the double-ionization energies to the lowest singlet and triplet states agreed well with the measured values of 34.3 ± 0.2 and 33.3 ± 0.3 eV, respectively. Double-ionization energies to higher-lying states were calculated using an ab initio Green's function method. In general, the theoretically predicted results agreed well with those measured, thus identifying the electronic transitions which take place in the double-electron-capture reactions.

Double-ionization energies of the fluorobenzene molecule to singlet and triplet electronic states of its dication have been measured using double-charge-transfer spectroscopy. The values obtained are compared with those calculated using a semi-empirical form of the multiple-scattering Xα computational method. The calculated data show that the density of electronic states of C6H5F2+ increases markedly as the energy increases. However, the first six measured energies to triplet states, and the first five to singlet states, closely match the energies of predicted electronic transitions, thereby verifying the association of these measured energies with double ionization to specific states of C6H5F2+.