The effect of electron-phonon interactions on optical absorption spectra
requires a special treatment in materials with strong electron-hole
interactions. We conceptualize these effects as exciton-phonon coupling.
Through phonon absorption and emission, the optically accessible excitons are
scattered into dark finite-momentum exciton states. We derive a practical
expression for the exciton-phonon self-energy that relates to the temperature
dependence of the optical transitions and their broadening. This expression
differs qualitatively from previous approximated expressions found in
literature