One-photon rare earth optical transitions: recent theoretical developments
‘The puzzle of rare earth spectra in solids’, a term first coined by van Vleck, was solved 30 years ago by Judd and Ofelt. The rich variety of sharp optical absorption and emission lines characteristic of 4f N →4f N lanthanide transitions are parity forbidden in the free ions. However, in the quantitative Judd-Ofelt theory, observation of these transitions is explained through simple configuration mixing caused by a noncentrosymmetric crystal field, allowing opposite-parity excited configurations to become slightly mixed into 4f N.
Solid State Luminescence: Theory, materials, and devices
Burdick, Gary and Downer, M, "One-photon rare earth optical transitions: recent theoretical developments" (1993). Faculty Publications. 51.