SPECTROSCOPIC EVIDENCE FOR THE COEXISTENCE OF TETRAGONAL AND TRIGONALMINIMA WITHIN THE EXITED STATE ADIABATIC POTENTIAL-ENERGY SURFACES OFHEXACHLOROTELLURATE AND HEXACHLORO-SELENATE COMPLEXES

Coexistence of Jahn-Teller minima resulting from the coupling to diffe rent accepting modes within the adiabatic potential energy surface (AP ES) is not possible within the framework of linear vibronic coupling t heory. For the lowest exited triplet state T-3(1u) of inorganic comple xes with s(2) electronic ground-state configuration, such a coexistenc e, due to quadratic coupling effects, is discussed. As a direct experi mental evidence two vibronic progressions with different accepting mod es in the emission spectra resulting from a single electronic state ar e observed in the emission spectra of the title compounds. The observa tion of vibronic finestructure in the emission spectra of [TeCl6](2-) is reported for the first time. (C) 1998 American Institute of Physics . [S0021-9606(98)00143-3].