Electron-phonon interactions in CsCdBr3:Yb3+

Pronounced electron-phonon coupling is observed for the F-2(7/2)<->F-2(5/2) 4f transitions of Yb3+ doped into CsCdBr3. A comparison of the Raman spectrum and the luminescence excitation sideband accompanying the F-2(7/2)(0)-->F-2(5/2)(2') crystal-field transition reveals vibrational properties of the [YbBr6] coordination unit that differ markedly from those of the CsCdBr3 host. In particular, the vibronic transition associated with the totally symmetric [YbBr6] stretching mode appears as a very weak feature at 191 cm(-1) in the Raman spectrum, whereas the totally symmetric [CdBr6] stretching mode of the CsCdBr3 bulk, which appears as a strong feature at 162.5 cm(-1) in the Raman spectrum, is only weakly discernible In the sideband. This is direct evidence for a large contribution from [YbBr6] local modes and a small contribution from bulk modes to the vibronic intensity. The intensity of the local mode is enhanced by approximately a factor of 2 in the Raman spectrum when the laser is tuned into resonance with the F-2(7/2)(0)-->F-2(5/2)(2') absorption of Yb3+, providing direct confirmation of its assignment. The observation of the first and second members of a Franck-Condon progression for both the local and the bulk totally symmetric modes indicates that a Delta process, rather than an M process, induces the vibronic intensity. Huang-Rhys factors of S-local=0.010+/-0.002 and S-bulk=0.15+/-0.03 were determined from the data, and reflect quite different electron-phonon coupling strengths. These results suggest that multiphonon relaxation of excited electronic states proceeds by the excitation of local modes of [YbBr6] followed by energy transfer to bulk modes of the lattice, possibly through a nonlinear coupling mechanism which is discussed briefly. (C) 1997 American Institute of Physics.