Vibrationally Resolved Inner-Shell Photoexcitation of the Molecular Anion C${}_2^{-}$

Carbon $1s$ core-hole excitation of the molecular anion C${}_2^{-}$ 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 $1{\sigma }_u\to 3{\sigma }_g$ and $1{\sigma }_g\to 1{\pi }_u$ core excitations of the C${}_2^{-}$ ground level and first excited level, respectively. A detailed Franck-Condon analysis reveals a strong contraction of the C${}_2^{-}$ 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 C${}_2^{-}$ $1{\sigma }_u^{-1}3{\sigma }_g^2{}^2{\mathrm{\Sigma }}_u^{+}$ and $1{\sigma }_g^{-1}3{\sigma }_g^2{}^2{\mathrm{\Sigma }}_g^{+}$ core-excited levels.