The dicarbon bonding puzzle viewed with photoelectron imaging.

Bonding in the ground state of C[Formula: see text] is still a matter of controversy, as reasonable arguments may be made for a dicarbon bond order of [Formula: see text], [Formula: see text], or [Formula: see text]. Here we report on photoelectron spectra of the C[Formula: see text] anion, measured at a range of wavelengths using a high-resolution photoelectron imaging spectrometer, which reveal both the ground [Formula: see text] and first-excited [Formula: see text] electronic states. These measurements yield electron angular anisotropies that identify the character of two orbitals: the diffuse detachment orbital of the anion and the highest occupied molecular orbital of the neutral. This work indicates that electron detachment occurs from predominantly [Formula: see text]-like ([Formula: see text]) and [Formula: see text]-like ([Formula: see text]) orbitals, respectively, which is inconsistent with the predictions required for the high bond-order models of strongly [Formula: see text]-mixed orbitals. This result suggests that the dominant contribution to the dicarbon bonding involves a double-bonded configuration, with 2[Formula: see text] bonds and no accompanying [Formula: see text] bond.