Spectroscopy of excited states of carbon anions above the photodetachment threshold

Abstract

Electronic transitions of C₃⁻ and C₅⁻ to states lying above the electron affinity of the neutral (EA) have been recorded in the gas phase by laser photodetachment spectroscopy. The excited states are identified by comparison with absorption spectra for the mass-selected ions deposited in neon matrices and with ab initio calculations. The C ²Σ_u⁺–X ²Π_g transition and two higher energy band systems are observed for C₃⁻, corresponding to excitation energies more than 1.5 eV above the EA. In the case of C₅⁻ the strongest features, at about 0.6 eV above the EA, are attributed to close lying ²Δ_g–X ²Π_u and ²Σ_g⁻–X ²Π_u transitions. The dominant configurations in these states identify them as long-lived Feshbach resonances. Lifetimes for these resonances in C₃⁻ are estimated to be between 200 fs and 3 ps from the band widths.