On the Infra‐Red Absorption of Crystals Due to Lattice Vibrations

Abstract

Abstract The theory of the infra‐red absorption and dispersion of crystals due to lattice vibrations is reinvestigated. Dipole moments up to third order and cubic and quartic anharmonicity in the displacements of the nuclei are considered. Using Green functions and the functional method of Martin and Schwinger a dispersion formula is derived in terms of renormalized phonons. The introduction of a renormalized linear dipole moment causes the oscillator strength to become a complex function of frequency. In the case of diatomic ionic crystals the frequency shift and lifetime of the dispersion oscillator can be obtained from the measured optical parameters.