On the interaction between the radiation field and ionic crystals

A symmetric treatment of the interaction between the radiation field and a diatomic ionic crystal with optical isotropy is presented; the explicit solutions of the problem can be readily obtained, which describe all the possible modes of vibration of the combined system of the radiation field and the ionic lattice. An unexpected conclusion is that the optical waves immediately below the infra-red dispersion frequency are in fact none other than what are normally considered as the transverse lattice vibrations of long wave-length in the theory of ionic crystals. It is shown that such waves consist mainly of mechanical vibrational energy rather than radiative energy.