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Abstract

Carbon 1s core-hole excitation of the molecular anion C₂^- has been experimentally studied at high resolution by employing the photon-ion merged-beams technique at a synchrotron light source. The experimental cross section for photo–double-detachment shows a pronounced vibrational structure associated with 1σ_u ->3σ_g and 1σ_g ->1π_u core excitations of the C₂^- ground level and first excited level, respectively. A detailed Franck-Condon analysis reveals a strong contraction of the C₂^- molecular anion by 0.2 Å upon this core photoexcitation. The associated change of the molecule's moment of inertia leads to a noticeable rotational broadening of the observed vibrational spectral features. This broadening is accounted for in the present analysis which provides the spectroscopic parameters of the C₂^- 1σ_u^-13σ_g^{2 2}∑_u⁺ and
1σ_g^-13σ_g^{2 2}∑_g⁺ core-excited levels.