Rydberg energy (hcR∞)

13.6 eV is the energy that must be paid to tear the electron off a hydrogen atom in its ground state, and the ladder every hydrogen level hangs from: E_n = −13.606/n² eV. Because it is comparable to chemical bond energies and to the photon energies of near-ultraviolet light, it explains why UV ionises air and visible light does not, and why the interstellar medium is neutral except where hot stars pump out photons above 13.6 eV, carving the ionised Strömgren spheres visible as emission nebulae.

Careful with names: the Rydberg energy hcR∞ is 13.6057 eV, exactly half the Hartree energy used as the atomic unit in quantum chemistry. Codes that report "energies in Rydbergs" and codes that report "energies in Hartrees" differ by a factor of two, which is one of the most common unit errors in electronic-structure work. And 13.6 eV is the value for infinite nuclear mass — real hydrogen ionises at 13.5984 eV once the reduced-mass correction is applied.