Photodetachment spectroscopy of C2− autodetaching resonances

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Abstract
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 2Σg+ state and high vibrational levels of the C2− B 2Σu+ 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 3Πu+e) is deduced to be much faster than the rate into (C2 X 1Σg++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.