NONLINEAR MECHANISM OF MULTIPHONON RELAXATION OF THE ENERGY OF ELECTRONIC EXCITATION IN OPTICAL-CRYSTALS DOPED WITH RARE-EARTH IONS

A review of the nonlinear theory of multiphonon relaxation (MR) in the crystal-field approximation and the super-weak electron-phonon coupli ng limit is presented, The analysis of the measured multiphonon relaxa tion rate dependence on the number of phonons involved in the nonradia tive transition in a LiYF4:Er3+ crystal was done considering both the Coulomb interaction between the rare-earth ions and the ligands and th e corrections to the Coulomb interaction resulting from the spatial di stribution of the ligand electronic charge, the energy of exchange int eractions and the contributions from the charge transfer states.