Neutron Inelastic Scattering by Coupled Defect-Phonon Modes in KCl-CN

A clear splitting, present only at low temperatures, is observed where the unperturbed phonon curve crosses the C${\mathrm{N}}^{-}$ $T_{1\mathrm{u}}$ and $T_{2\mathrm{u}}$ levels in a KCl crystal doped with 6×${10}^{19}$ C${\mathrm{N}}^{-}$ ${\mathrm{cm}}^{-3\mathrm{}}$. Calculated dispersion curves require coupling constants which agree within experimental uncertainty with those determined ultrasonically. The absence of the splitting at high temperatures can be explained because the phonon gain and loss processes in the interaction with the C${\mathrm{N}}^{-}$ level cancel almost completely.