Jahn-Teller effect, band

FIGURE 21. The He(I) photoelectron spectrum of tetra-t-butyltetrahedrane. The twin peaks of low intensity at threshold are assigned to the le orbitals of the rings split by the dynamic Jahn-Teller effect. Bands of higher energy are overlapped by ionizations of the t-butyl groups... 

The vibrational progression in the Se4+ and Te4+ emission bands indicates clearly the Jahn-Teller effect in the excited state the coupling with the eg vibrational mode points to a tetragonal distortion of the excited state, According to Schmidtke et al. [18], the magnitude of this tetragonal distortion is Az= — 2Ax= — 2Ay=0.lA, a considerable distortion indeed. 

Yu. E. Perlin and B. S. Tsukerblat, Optical bands and polarization dichroism of Jahn-Teller Centers, Dynamical Jahn-Teller Effect in Localized Systems, Elsevier, Amsterdam, 1984, pp. 251-346. 

The structural (martensitic) transition in Ni2MnGa-based Heusler alloys was described as driven by a band Jahn-Teller effect [ff, 12], In the framework of this model, the results of theoretical [12] and experimental [If] studies of the redistribution of magnetization upon martensitic transformation in Ni2MnGa alloys have been discussed. 

Another factor contributing to the asymmetry and breadth of absorption bands in crystal field spectra of transition metal ions is the dynamic Jahn-Teller effect, particularly for dissolved hexahydrated ions such as [Fe(H20)6]2+ and [Ti(H20)6]3+, which are not subjected to static distortions of a crystal structure. The degeneracies of the excited 5Eg and 2Eg crystal field states of Fe2+ and Ti3+, respectively, are resolved into two levels during the lifetime of the electronic transition. This is too short to induce static distortion of the ligand environment even when the cations occupy regular octahedral sites as in the periclase structure. A dual electronic transition to the resolved energy levels of the Eg excited states causes asymmetry and contributes to the broadened absorption bands in spectra of most Ti(m) and Fe(II) compounds and minerals (cf. figs 3.1,3.2 and 5.2). 

Shankland, T. J. (1968) Pressure shift of absorption bands in MgO Fe2+ the dynamic Jahn-Teller effect. J. Phys. Chem. Solids, 29,1907-9. 

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