Multireference configuration interaction calculations on the low lying singly excited states of linear C3, C5, and C7 clusters are reported. The results indicate that the 1Σ+u state of C3 has a term energy above 8.0 eV. The calculated vertical term energies of the low lying electronic states of these carbon chains are consistent with the photoelectron spectra of their parent negative ions. The predicted term energies are also compatible with the matrix spectra of carbon vapor. An assignment of strong spectral features in those spectra at ≊1950, 2227, and 2471 Å to the 1Σ+u←X̃ 1Σ+g transition in C5, C7, and C9, respectively, is proposed. A possible relation of carbon chains to diffuse interstellar bands is discussed.
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The potential energy curves of the low-lying electronic states of two diatomic (+2) ions A P + and SiF2+ have been calculated by using a multireference CI method with an extended Gaussian basis set. The X ' F state of A l p + is predicted to be thermodynamically stable by 1.89 eV, with a barrier to dissociation of 3.29 eV. SiF2+ has two thermodynamically stable electronic states, X 2 F bound by 1.89 eV and l2lI bound by 0.68 eV. These dications can therefore be added to a growing list of such dications which are thermodynamically stable in their ground electronic states. The nature of the bond in these species is described and the spectroscopic parameters are reported. Some possible ways of observing the ions are discussed.
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