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

Abstract

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 1σu → 3σg and 1σg → 1πu 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 1σ −1 u 3σ 2 g 2 Σ + u and 1σ −1 g 3σ 2 g 2 Σ + g core-excited levels.