Energy transfer and migration in highly forbidden transitions of lanthanide ion doped crystals

Abstract

Förster–Dexter theory for resonant energy transfer is extended to higher order and applied to explain the rates of energy transfer and migration processes in highly forbidden transitions for some solid-state lanthanide (Ln) ion systems for which experimental results are available. The second-order two-body energy transfer mechanism involves two inter-ion correlated dipole electrostatic interactions, i.e. dipole dipole–dipole dipole (dd–dd) energy transfer, also termed Axe–Axe energy transfer in view of the similarity of the theoretical formalism with that for two-photon transitions. Each of the dipolar transitions consists of a transition from the 4f n configuration to an opposite-parity configuration, taken to be 4f n−1 5d. dd–dd energy transfer is a short-range ( R −12) interaction so that it is most important in systems with short donor Ln–acceptor Ln separations. The energy transfer formalism is extended to include spin-forbidden transitions at one or two sites, the so-called Axe–Judd–Pooler (Axe–JP) and JP–JP energy transfer. In some cases the dd–dd mechanism is the dominant energy transfer process, as exemplified herein for energy migration in the 5 D 0 state of Sm 2+ in SrF 2, and also in the 5 D 0 state of Eu 3+ in Cs 2NaEuCl 6.