Why is Condensed Oxygen Blue?

Abstract

The blue color of condensed oxygen is mostly derived from double transitions. This paper presents a theoretical calculation of the intensity of the double transition (a 1Δg) (a 1Δg)←(X 3Σg−) (X 3Σg−), using a model based on a pair of oxygen molecules at a fixed separation of 3.81 Å. The intensity enhancement is assumed to be derived from the mixing (a 1Δg) (a 1Δg)∼∼(X 3Σg−) (B 3Σu−) and (a 1Δg) (1Δu)∼∼(X 3Σg−) (X 3Σg−). Matrix elements for these interactions are calculated using a π-electron approximation for the pair system. Good molecular wavefunctions are used for all but the perturbing B 3Σu− state, which is approximated in terms of ground-state orbitals. The largest contribution to the matrix elements arises from large intramolecular terms multiplied by intermolecular overlap integrals. The strength of the interaction depends not only on the intermolecular separation of the two oxygen molecules but also, as expected, on the relative orientation. Matrix elements are calculated for different orientations, and the angular dependence is fit to an analytical expression. The theory therefore not only predicts an intensity dependence on density but also one on phase at constant density. Agreement between theory and available experimental results is satisfactory considering the nature of the approximations, and indicates the essential validity of the over-all theoretical approach to this interesting intensity enhancement problem. A qualitative interpretation is also suggested of the anomalous (1-0) / (0-0) intensity ratio in the double transition on the basis of a dependence of intermolecular electronic interaction on vibrational amplitude.