Transitions New Results

Vibronic transitions have been discussed at length (127, 128), and much deeper than in Section II, A. These transitions have been observed for many t3rpes of optical centers. Here two special classes are discussed, viz., vibronic transitions in rare-earth ion spectra, and vibronic transitions involving Jahn-Teller vibrational modes (see Section VI). 

The transitions within a given 4/ configuration arc usually assumed to be purely electronic, that is, to be zero-phonon lines. This is a very good approximation. However, careful monitoring of the spectra yields weak features that must be due to vibronic transitions. In some cases these are even relatively intense. Caro et al. have reviewed several examples, related mainly to Nd and Pr (129). 

Recently the present author has reported extensive data on the vibronic transitions of Gd (in cooperation with Dr. L. H. Brixner) and of Eu (130, 131). These data will be reviewed here. Both ions show transitions that are very suitable to study the vibronic transition. For Gd this is the P7/2 emission transition and for Eu the Fo — D2 absorption (excitation) transition. In the spectra these transitions occur as lines that limit a spectral region without any electronic transition. For example, in the emissin spectrum of Gd the Pv/2  

This shows that the occurrence of vibronic transitions is a common phenomenon. The total vibronic intensity varies from less than 1% of the parent electronic transition to more than 10%. The examples mentioned show two different types of vibronic transitions, viz., those in which the relevant vibration occurs between (jd Euid the ligand (e.g., (jd-F) and those in which the relevant vibration takes place on the ligand (e.g., Gd-H20). The latter type is called a cooperative vibronic 

Emission spectrum of Cs2NaGdCl6 at 4.2 K. (After A. J. deVries, Thesis, Utrecht.) 

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B. The Strong-Coupling Case Vibronic Transitions New Results... 

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