If hydrogen is diffused into a calcium fluoride crystal that contains a significant number of rare earth ions, R³⁺, substituted for Ca²⁺ in the crystal lattice, an H^- ion may occupy a site close to a Re³⁺ ion. When this happens the light ion vibrates independently of the heavier lattice, and there is a narrow absorption in the infrared, corresponding to the energy of the transition from the ground state, to the first excited state, of the quantized harmonic oscillator.

If Deuterium and Tritium ions are substituted for hydrogen, the frequency of the harmonic oscillator decreases in proportion to one over the square root of the mass, and the absorptions in the infrared shift in wavelength accordingly.