Cooperative vibronic transitions

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... 

Determination of the critical temperature after equation (1), T0 = En/Syl, is considered in the cooperative JT and PJT effects [2,3,8,9]. In particular, the origin of structural ferroelectric phase transitions as due to the PJT effect (the vibronic theory of ferroelectricity) was suggested first in the sixties [10] (see also Ref. [9]). JT structural phase transitions are reviewed in Ref. [8]. 

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