Knowledge Bank

The far infrared optical properties of KCl and KBr have been measured at room temperature by using the technique of Asymmetric Fourier Transform Spectroscopy. In reflection, this technique permits one to measure experimentally both the frequency dependent amplitude and phase of the radiation reflected from the crystal. From these data one can calculate both the real and imaginary parts of the complex index of refraction. It is found that both KCl and KBr exhibit structure due to lattice anharmonicities. We have theoretically calclated the frequency dependent proper self energy matrix for the $k=0$ phonon mode of KCl. This was done by using the eigenvalues and eigenvectors for 1000 wave vectors lying within the first Brillouin zone that were calculated by Karo and Hardy in their work on the lattice dynamics of $KCl.1$ The proper self energy matrix was then used to calculate directly the complex index of refraction. The agreement between experiment and theory is satisfactory.