3-BODY ENERGY-TRANSFER PROCESSES OF LANTHANIDE IONS IN CRYSTALS

New three-body energy transfer mechanisms are summarized in this artic le. The importance of the phonon-assisted three-body cooperative proce ss and of the cooperative radiative energy transfer process is indicat ed. The role of stepwise versus cooperative upconversion is re-examine d. The inapplicability of the closure approximation in quantifying ind irect multipolar transfer is pointed out. The extension of second-orde r three-body energy transfer, the ''six-dipole'' process, is formulate d.