Electron–phonon interactions in CsCdBr3:Yb3+

Abstract

Pronounced electron–phonon coupling is observed for the F7/22↔F5/22 4f transitions of Yb3+ doped into CsCdBr3. A comparison of the Raman spectrum and the luminescence excitation sideband accompanying the F7/22(0)→2F5/2(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 F7/22(0)→2F5/2(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 Slocal=0.010±0.002 and Sbulk=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.