Vibrationally Resolved Inner-Shell Photoexcitation of the Molecular Anion C₂

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 $$ and $$ 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 ^- $$ and $$ core-excited levels.