Method for measuring the contribution of the nonlinear dipole moment to multiphonon absorption

A method is proposed by which one may determine the separate contributions of (a) the anharmonic interionic forces, and (b) the nonlinear electric dipole moment (the nonlinear dependence on ionic displacements) to the integral over infrared frequencies of the imaginary part ${\epsilon }_2\left(\omega \right)$ of a material's dielectric function (times the frequency $\omega$). The results can be used to determine parameters needed to calculate other features of multiphonon absorption. The method does not depend on the nature or magnitude of the anharmonic forces, but does require a model for the quadratic term in the dipole moment. The parameters in this model are then determined from measurements of ${\epsilon }_2\left(\omega \right)$ at several temperatures. We use the method to show from existing data the multiphonon contribution to the integral of $\omega {\epsilon }_2\left(\omega \right)$ for cubic LiF arises predominantly from anharmonic forces.