Multiphonon sideband intensities in rare earth ions in crystal

Vibronic sidebands may originate from two main classes of processes which have been called Δ- and M-processes. We will consider multiphonon transitions in the M-process case; vibronics are induced by the joint action of the electromagnetic field and the high-order optical anharmonicity. The contributions to multiphonon sidebands due to point-charge, inhomogeneous-dielectric and polarized-ligand mechanisms are considered. As a result, all mechanisms considered by Judd for the one photon–one phonon transitions are extended to the multiphonon case.