The dicarbon bonding puzzle viewed with photoelectron imaging

Abstract

Bonding in the ground state of C[subscript 2] is still a matter of controversy, as reasonable arguments may be made for a dicarbon bond order of 2 , 3 , or 4. Here we report on photoelectron spectra of the C[superscript - over subscript 2] anion, measured at a range of wavelengths using a high-resolution photoelectron imaging spectrometer, which reveal both the ground X[superscript 1]Σ[superscript + over subscript g] and first-excited a[superscript 3]Π[subscript u] 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-like (3 σ[subscript g]) and p-like (1 π[subscript u]) orbitals, respectively, which is inconsistent with the predictions required for the high bond-order models of strongly sp-mixed orbitals. This result suggests that the dominant contribution to the dicarbon bonding involves a double-bonded configuration, with 2π bonds and no accompanying σ bond.