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: $$\sigma =I-\frac{R}{(n-\mathit{\Delta }{)}^2}$$ where $\sigma$ is the wavenumber, $I$ the ionization potential of the atom or molecular entity, $n$ a principal quantum number, $R$ the Rydberg constant, and $\mathit{\Delta }$ the quantum defect which differentiates between s, p, d, etc., orbitals. The notation used is, e.g. $\pi \to \mathrm{ns}$.