Low temperature luminescence spectra of the d10s2 complexes Cs2MX6 (M = Se, Te and X = Cl, Br). The Jahn—Teller effect in the Γ−4(3T1u) excited state

Abstract The emission spectra of the title compounds in microcrystalline form have been measured at 10 K. The extensive vibrational progression in the e g mode is indicative of a tetragonal Jahn—Teller distortion in the Γ − 4 ( 3 T 1u ) excited state. The vibronic coupling of a threefold electronic state with a doubly degenerate e g mode (T—e coupling), linear in the nuclear coordinates, has been reinvestigated considering spin—orbit coupling up to second order perturbation on energy levels which result from an a 1 1g t 1 1u electron configuration. For an estimation of Jahn—Teller coupling constants, the intensity distributions in the progressions were compared with the theoretical line shape functions which were obtained from a model which also permits the determination of potential energy minima and vibrational fundamentals of the excited state. The unusually large increase in the e g vibrational frequency compared to the ground state is due to Jahn-Teller forces which distort the potential surface, yielding steeper excited state energy curves.