The dicarbon bonding puzzle viewed with photoelectron imaging

Bonding in the ground state of C$${}_{2}$$2 is still a matter of controversy, as reasonable arguments may be made for a dicarbon bond order of $$2$$2, $$3$$3, or $$4$$4. Here we report on photoelectron spectra of the C$${}_{2}^{-}$$2− anion, measured at a range of wavelengths using a high-resolution photoelectron imaging spectrometer, which reveal both the ground $${X}^{1}{\Sigma}_{\mathrm{g}}^{+}$$X1Σg+ and first-excited $${a}^{3}{\Pi}_{{\mathrm{u}}}$$a3Πu 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 $$s$$s-like ($$3{\sigma}_{\mathrm{g}}$$3σg) and $$p$$p-like ($$1{\pi }_{{\mathrm{u}}}$$1πu) orbitals, respectively, which is inconsistent with the predictions required for the high bond-order models of strongly $$sp$$sp-mixed orbitals. This result suggests that the dominant contribution to the dicarbon bonding involves a double-bonded configuration, with 2$$\pi$$π bonds and no accompanying $$\sigma$$σ bond.