Yet it is a well-known fact of basic chemistry that atomic hydrogen is extremely unstable, and that it reacts violently to produce molecular hydrogen, which is extremely stable. Given a bottle of pure atomic hydrogen, one would expect an immediate energetic explosion, producing molecular hydrogen at a very high temperature. Because of the spin structure of the particle, it is easily detectable using a high frequency radio signal at 21-cm wavelength. The radio-frequency spectral line at 21 cm no longer exists, and the molecular hydrogen becomes totally invisible at that wavelength. However, the only two electrons are so tightly coupled, that they form a pair in which the electric field and the spin of the electrons are completely cancelled.
Molecular hydrogen possesses no permanent dipole.
Such a perfect coupling is unusual among diatomic molecules. When light passes through normal molecular gases, such as oxygen, nitrogen, and others, radiation excites the resulting electric dipole in the molecule, and some energy is scattered or absorbed. However, in the case of molecular hydrogen, there is no dipole moment, so that no radiation can be absorbed or emitted. By comparison, the spontaneous emission of light from the first rotational state of molecular hydrogen is practically impossible. One must reach the sixth state before the probability of the transition occurring becomes once a year.