The phonon sideband of the 6890-Å transition of ${\mathrm{Sm}}^{++}$ in KBr is studied. New high-resolution experiments at 7°K are presented, and a theory of the sideband shape is given. It is assumed that the electric field from the phonons is responsible for the sideband, and a detailed shape is calculated by the use of the phonons of pure KBr as determined by neutron measurements. The electric field due to the phonons is calculated by the Ewald method. With the exception of a few peaks that can be attributed to local modes, all the important features of the observed sideband agree with the curve given by the model.

• Received 7 August 1967

DOI:https://doi.org/10.1103/PhysRev.164.345