Fracton contribution to the optical linewidth in glasses

We use the previous formalism of Lyo [Phys. Rev. Lett. 48, 688 (1982)] for optical linewidths of paramagnetic impurities in glasses to include the effects of fractal excitations (fractons). We find the optical-linewidth temperature dependence for interactions between the two-level systems and the optically active center of dipole-quadrupole and quadrupole-quadrupole character to be T^1+(--d4) and T^1+(2--d5), respectively, where --d is the spectral dimensionality of Alexander and Orbach. Use of the percolating network value for --d, 4/3, predicts a temperature dependence T1.33 for the former coupling in reasonable agreement with very recent experimental results of V\"olker and collaborators in organic glasses.