Spectroscopy of excited states of carbon anions above the photodetachment threshold

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 2 sigma u (+)-X 2 pi 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 2 delta g-X 2 pi u and 2 sigma g(-)-X 2 pi 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.