Conclusion

Abstract

As a rule, only the absorption is measured in the first stage of investigating excitons by optical techniques, without analyzing the problem of the shape of the absorption lines. This is a natural procedure, provided that the subject of research is the detection of more or less distinct excited levels in crystals or the elucidation of the corresponding series laws, etc. If the crystal can be regarded as being optically isotropic, the situation here is quite similar to that for determining the atomic energy levels in gases. This analogy can also be extended to an anisotropic medium if we replace the ordinary gas by an “oriented gas”, i.e., an assembly of independent molecules with fixed directions of certain axes (e.g., the directions of normal vibrations of an anisotropic oscillator). Since, in the case of weak absorption, the position of the absorption lines determines the frequencies Ω(k) of the Coulomb excitons, these are precisely the frequencies that are measured in the experiments we referred to.