Fonction Dielectrique Complete D'un Cristal Moleculaire: Polarisabilite Moleculaire et Absorption Infrarouge

In the special case of a molecular crystal, it has been shown that the background of the infrared absorption spectra is not only a function of the dielectric constant at high frequency: the pertinent formulation introduces the entity (ε) _fc in which the contribution of all the optical modes have been taken into account. An analytical expression for (ε) _fc is suggested and make possible the determination of the oscillators strengths with a good accuracy. Calculations have been performed for polycrystalline samples of H.M.T and H.M.T.D. This new parameter has no effect upon the damping constants and slightly affects the static dielectric constant. On the other hand, its influence is clearly established for the infrared reflexion spectra.

The relation between microscopic molecular polarizability α(ω) and macroscopic dielectric function ε(ω) has been given. From the analytical expression of ε(ω), both real and imaginary parts of α(ω) have been expressed in terms of frequency. All the characteristic points of them have been calculated for spectral ranges corresponding to molecular crystal infrared absorption. Application to the F₂ vi-brational modes of solid H.M.T has been given. In other respects the contribution of the nuclei to the static molecular polarizability has been estimated with a good accuracy.