Theory of infrared and Raman intensity of the lattice vibrations of molecular crystals. Application to solid ammonia and benzene

A theory of infrared and Raman intensity of lattice vibrations in molecular crystals is discussed. The effect of induction due to the molecular dipole and quadrupole fields is included. It is shown that both rotational and translational modes can contribute to the infrared and Raman intensity. Explicit expressions for the calculation of the dipole and polarizability derivatives with respect to the crystal normal coordinates are worked out. The theory is applied to polycrystalline ammonia and benzene and to single crystal benzene, and a good agreement is obtained. The influence of the eigenvectors on the calculated intensities is discussed.