Theory for the Infrared Absorption Intensities of the Lattice Vibrations of Molecular Solids

Abstract

A theory is developed for the infrared-transition dipole intensities of the lattice vibrations in molecular solids. For nonpolar molecules like N2 and CO2 the origin of the intensities lies in the nonzero dipole derivatives where the dipole is induced by the molecular quadrupole moment of the neighboring molecules in the lattice. The distortion moment is not taken into account and its neglect is justified on the basis of independent available experimental evidence. For molecular solids composed of polar molecules, the intensity of quasirotational lattice modes is calculated. The results are given in general form and, in addition, dipole-moment derivative sums are evaluated for lattices of factor-group symmetry Th6 appropriate for solid CO2 and approximately for the α phase of N2.