Algebraic approach to molecular rotation-vibration spectra. I. Diatomic molecules

An algebraic approach to molecular rotation-vibration spectra, similar to that used in the analysis of nuclear rotation-vibration spectra, is suggested. The spectrum generating algebra appropriate to diatomic molecules U(4) is constructed. Its two dynamical symmetries O(4) and U(3) are discussed and their relation to rigid and nonrigid structures is investigated. It is shown how other properties, such as intensities of dipole radiation and state-to-state excitation probabilities, can be evaluated.