Atoms and molecules have ionization energy thresholds associated with the energy required to remove an electron from the ionic core. (The Rydberg formula describes the energy level series of a Rydberg atom.) Rydberg series describe the energy levels associated with almost removing an electron from the ionic core. Each Rydberg series converges on an ionization energy threshold associated with a particular ionic core configuration. These quantized Rydberg energy levels can be associated with the quasiclassical Bohr atomic picture. The closer you get to the ionization threshold energy, the more "near threshold Rydberg states" there are. As the electron is promoted to higher energy levels, the spatial excursion of the electron from the ionic core increases and the system is more like the Bohr model picture. (That is, the ionic core interaction with the Rydberg looks more like the interaction between the proton and the electron in the hydrogen atom. This can be refined by including a correction in the Rydberg formula associated with the presence of the ionic core called the quantum defect.) One way to visualize this system classically is to think of the electron as comet far removed from the solar system that represents the ionic core. The angular momentum of a comet determines if it has a highly elliptical orbit that is more likely to interact with the core of the system or a more nearly circular orbit that is much less likely to interact with core. This is also true for the orbital angular momentum of a Rydberg state electron interacting with an ionic core.