Knowledge Bank

In a Rydberg state of a very light diatomic molecule, like $H2$ or $He2$, the Rydberg electron is characterized by a nearly integral value, l, of the orbital angular momentum quantum number. Rydberg series can therefore be characterized by quantum numbers n, l and $\lambda$ and each series (i.e. fixed l and $\lambda$) has a nearly constant quantum defect $\delta$. In a heavier molecule the molecular core can cause a mixing of some pairs of $n/\lambda$ Rydberg series. The NO molecule, for which the best experimental data are available, is discussed as an example. A quantitative treatment of the interaction between levels in the series $(n+1)s\sigma$ and $nd\sigma$ leads to ``deperturbed'' Rydberg series which are closely similar to those of an alkali atom.