VIBRONIC TRANSITIONS OF RARE-EARTH IONS

An overview of recent research on vibronic transitions of intraconfigu rational 4f(n) transitions in rare earth ions is given. The strong var iation in vibronic coupling strengths is shown to depend on the type o f rare earth ion and the chemical nature of the ligands. In the discus sion on the mechanism of vibronic coupling, new evidence for the impor tance of DELTA process vibronics (or Franck-Condon phonon replicas) ne xt to the usually considered M process vibronics (or vibrationally ind uced forced electric dipole transitions) is presented. Concentration e nhancement of vibronic transitions in the excitation spectra of Pr3+ i n several covalent host lattices is reported. A model based on superex change interaction between Pr3+ ions over distances of about 11 angstr om can account for the observed concentration enhancement.