The homogeneous linewidth is calculated for resonant optical transitions in glasses. The width is due to modulation of the optical levels caused by coupling to the diagonal as well as off-diagonal elements of the two-level modes. The main contribution arises from diagonal modulation of abundant but weakly coupled two-level modes. For a flat density of states of two-level modes and a dipole-quadrupole coupling a large $T^{1.75}$ behavior is found in good agreement with recent data.

• Received 3 September 1981

DOI:https://doi.org/10.1103/PhysRevLett.48.688