The long range intermolecular potential of H2-H2

Approximately 250 lines of the CaO c ³Σ⁺-a ³Π (0, 0) band have been resolved using sub-Doppler intermodulation spectroscopy and the principal molecular constants of the c ³Σ⁺ (2³Σ⁺) state determined. These results will be of particular value to those wishing to use laser induced fluorescence population diagnostics for all six Ω, e/f components of the a ³Π state. Accurate rotational linestrengths for all branches of the c ³Σ⁺-a ³Π (0,0) transition, computed from eigenvectors of the effective hamiltonians, are presented. Extreme localization of the CaO valence orbitals on either the Ca or the O centre has permitted simple and accurate estimates to be made of fine-structure-determining matrix elements and the locations of unobserved electronic states, including the use of results from CaF to describe the properties of Ca-centred orbitals in CaO. Isoconfigurational second-order spin-orbit arguments, applied to the observed value of the effective spin-spin constant (λ₀) for the c ³Σ⁺ ν = 0 state, lead to confirmed predictions of the previously unknown locations of the E ¹Σ^- (1¹Σ^-) and e ³Σ^- (1³Σ^-) states and of the value of λ₀(e ³Σ^-). Assignments are made of nearly all band features in the CaO orange band spectrum.