Interference in LO-Phonon-Assisted Absorption

In this paper we calculated by second-order perturbation theory the optical absorption caused by the simultaneous creation of a Wannier exciton and a LO phonon. In insulators with $E_B\gg \hslash {\omega }_l$, a striking exhibition of quantum-mechanical interference is predicted for $\hslash \omega \approx E_{\mathrm{xn}}$. Excepting for this interference effect, there are no sharp phonon peaks in the alkali halides if one assumes a linear exciton-phonon interaction. Peaks are predicted, but they are extremely broad and $\hslash \omega l$ can be placed energetically as a function of $\overrightarrow{\mathrm{K}}$ anywhere in the Brillouin zone for $\hslash \omega >E_{\mathrm{xn}}+\hslash \omega l$, by using presently available experimental and theoretical parameters.