Electron – phonon interactions in CsCdBr 3 : Yb 3 1

Pronounced electron–phonon coupling is observed for the F7/2↔F5/2 4 f transitions of Yb doped into CsCdBr3. A comparison of the Raman spectrum and the luminescence excitation sideband accompanying the F7/2(0)→F5/2(28) 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 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 21 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 F7/2(0)→F5/2(28) absorption of Yb, 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 D process, rather than an M process, induces the vibronic intensity. Huang–Rhys factors of S local50.01060.002 and Sbulk50.1560.03 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. © 1997 American Institute of Physics. @S0021-9606~97!00637-5#