The orange arc bands of CaO. Analysis of a D,d 1,3Δ–a 3Π system

Abstract

The orange and green arc bands of CaO are shown to arise from Ca⁺ centered (4p,3d)‐4s transitions analogous to the CaF A ²Π–X ²Σ⁺ and B ²Σ⁺–X ²Σ⁺ systems. The tremendous complexity of these Ca⁺O⁻ systems is a consequence of the sixfold degeneracy of the O⁻ p‐hole, as compared to the nondegenerate F⁻(¹S). The 0, 0 and 1, 1 bands of a D,d ^1,3Δ–a ³Π transition are recorded by intermodulation spectroscopy and assigned (vibration, rotation, absolute parity) by nonstandard techniques. The D ¹Δ state is found to lie below the isoconfigurational d ³Δ₁ state and D∼d spin‐orbit perturbation causes the d ³Δ state to become irregular (³Δ₂≳³Δ₁≳³Δ₃). Two perturbation effects provide the key to the orbital interpretation of the orange and green arc bands: lambda doubling in a ³Π indicates that a π⁴σσ³Σ⁺ state lies 1500 cm⁻¹ above a ³Π; D, d ^1,3Δ∼B ¹Π perturbations suggest recurrence of this 1500 cm⁻¹ interconfigurational splitting which is the O⁻pσ/pπ splitting induced by the Ca⁺ ligand.