RELAXATION, HOMOGENEOUS BROADENING, AND FREQUENCY-SHIFTS - APPLICATION TO ELECTRONIC-TRANSITIONS IN SOLIDS DOPED WITH RARE-EARTH IONS

We first recall the results of relaxation theory regarding homogeneous broadening and frequency shift of a transition. We then apply these v ery general results to the electron-phonon interaction case reconsider ing, in particular, the broadening and shift due to acoustic phonons i n the Debye approximation and their mutual relationship. The experimen tal data corresponding to the F-7(1) <-- F-7(0) transition of the CaF2 :Sm2+ system are reported and interpreted in the light of our theoreti cal results. In this case, previous theories do not allow us to distin guish between Raman processes involving acoustic or optical phonons. O ur theory however demonstrates that acoustic phonons do not play the m ajor role here and that the broadening and shift are due to coupling w ith the TO phonons.