One-colour (∼220 nm) resonance-enhanced (S1 − S0) multi-photon dissociation of acetylene: probe of the C2 A1 Πu − X1 Σ+g band by frequency-modulation spectroscopy

Diabatic Valence-Hole States in the C2 Molecule: "Putting Humpty Dumpty Together Again"

Despite the long history of spectroscopic studies of the C₂ molecule, fundamental questions about its chemical bonding are still being hotly debated. The complex electronic structure of C₂ is a consequence of its dense manifold of near-degenerate, low-lying electronic states. A global multi-state diabatic model is proposed here to disentangle the numerous configuration interactions that occur within four symmetry manifolds of excited states of C₂ (¹Π_g, ³Π_g, ¹Σ_u⁺ , and ³Σ_u⁺ ). The key concept of our model is the existence of two "valence-hole" configurations, 2σg22σu11πu33σg2 for ^1,3Π_g states and 2σg22σu11πu43σg1 for ^1,3Σ_u⁺ states, that are derived from 3σ_g ← 2σ_u electron promotion. The lowest-energy state from each of the four C₂ symmetry species is dominated by this type of valence-hole configuration at its equilibrium internuclear separation. As a result of their large binding energy (nominal bond order of 3) and correlation with the 2s²2p² + 2s2p³ separated-atom configurations, the presence of these valence-hole configurations has a profound impact on the global electronic structure and unimolecular dynamics of C₂.