Electron-phonon interactions in CsCdBr₃=Yb³⁺

Pronounced electron-phonon coupling is observed for the ²F_7/2↔²F_5/2 4f transitions of Yb³⁺ doped into CsCdBr₃. A comparison of the Raman spectrum and the luminescence excitation sideband accompanying the ²F_7/2(0)→²F_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 ²F_7/2(0)→²F_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 Δ 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.