PHOTOELECTRON-SPECTRA OF C2- AND C2H-

The photoelectron spectra of C2- and C2H- have been measured at a photon energy of 3.531 eV. The electron affinities of dicarbon, EA(C2) = 3.269 +/- 0.006 eV and ethynyl radical, EA(C2H) = 2.969 +/- 0.006 eV, are determined. The dissociation energy of dicarbon anion, D0(C2-) = 187.2 +/- 2.5 kcal mol-1, is derived from the experimental electron affinities of C2 and C and the literature value for the dissociation energy of neutral dicarbon, D0(C2) = 141.0 +/- 2.5 kcal mol-1. The CC stretch fundamental and CCH bending vibrational levels up to nu = 3 are observed in C2H(approximately X2-SIGMA+). The fundamental frequencies for C2H-(approximately X1-SIGMA+) are 1800 +/- 20 cm-1 (CC stretch) and 505 +/- 20 cm-1 (CCH bend). The photoelectron spectrum of ethynyl exhibits strong non-Franck-Condon transitions, induced by vibronic coupling, to odd vibrational levels in the CCH bend. These transitions also exhibit different photoelectron angular distributions than the Franck-Condon allowed vibrational transitions. The C2H(approximately A2-PI) excited state was not observed within the experimental energy range, implying a lower limit of 3400 cm-1 for its term energy.