Multiphonon optical bands of Cr3+ ion in sulphide spinels:: a pseudo Jahn-Teller model

In this article we report the adiabatic and dynamic solutions of the pseudo Jahn-Teller (JT) problem in the orbital triplet T-4(2) (t(2)(2)e) of Cr3+ ion resulting from the combined effect of tetragonal (E) vibrations and trigonal crystal field in sulphide spinels lattice. We discuss the general properties of the adiabatic surfaces that give a clear qualitative insight on the main features of the optical bands. The shapes of the absorption bands are calculated on basis of the quantum-mechanical solution of the dynamic pseudo JT problem taking into account the effective electric-dipole ;moment involving odd-parity crystal field potential. Pseudo JT ((4)A(1)+ E-4)circle times e interaction is shown to produce a complicated structure of the multiphonon optical bands and leads to a specific polarization effect. Calculated shape function for the U-band absorption proves to be in a good agreement with the excitation spectrum of alpha - ZnAl2S4 tiospinel doped with chromium ions, the experimentally detected peculiarities of the structure of this band are shown to be an evidence of strong pseudo JT interaction in the excited state.