Millimeter — Submillimeter Waves and the Rydberg Atoms

Electronic atomic transitions usually belong to the optical domain. In highly excited states however, they shift towards the far infrared and millimeter wave domain. Let us consider the very simple hydrogen energy spectrum formula: E = − R/n2 where E is the level energy measured from the ionization, R the Rydberg constant and n the principal quantum number. The distance between successive levels DE ∼ n-3 which is in the visible and infrared domain for small n becomes in the millimeter range for n ∼ 30.