An electronic transition described approximately as promotion of an electron from a 'bonding' orbital to a Rydberg orbital. Spectral bands corresponding to Rydberg transitions approximately fit the Rydberg formula: where is the wavenumber, the ionization potential of the atom or molecular entity, a principal quantum number, the Rydberg constant, and the quantum defect which differentiates between s, p, d, etc., orbitals. The notation used is, e.g. .