Photodetachment spectroscopy of C₂⁻ autodetaching resonances

The cross section for photodetachment of C₂⁻ is investigated in the photon energy range 14 000–20 000 cm⁻¹ (1.75–2.5 eV). Sharp resonances due to autodetachment are observed at photon energies corresponding to transitions between high vibrational levels of the C₂⁻ X ²Σ_g⁺ state and high vibrational levels of the C₂⁻ B ²Σ_u⁺ states. The resonances are narrower than 6 GHz, and those arising from the v=5 level of the B state are about 1/10 as strong as those arising from the v=6 and higher levels. In addition, the signal from the allowed direct photodetachment of C₂⁻ X state, which would produce a smooth background cross section, is not observed, indicating that it is less than the 10³ peak to background contrast ratio. A rotational analysis of the nine bands of the C₂⁻ B–X transition observed in this study, coupled with previous measurements provides a new, more precise set of spectroscopic constants for these states. These constants are then used to generate RKR potential curves for the B and X states of C₂⁻. Based on all C₂⁻ data, the only consistent interpretation of the observations is that the electron affinity of C₂ is bounded by 3.374 eV ?EA(C₂)?3.408 eV. The autodetachment rate into (C₂ a ³Π_u+e) is deduced to be much faster than the rate into (C₂ X ¹Σ_g⁺+e), probably as a result of strongly R dependent configuration interaction in the C₂⁻ B state. The weakness in the direct photodetachment process is attributed to poor vibrational overlap between the initial and final states.