Jahn-Teller effect in the emission and absorption spectra of ZnS:Cr2+ and ZnSe:Cr2+ -: art. no. 075206

The Jahn-Teller effect is invoked to explain the fine structure (isolated zero-phonon lines) observed in both the infrared emission and absorption spectra of substitutional Cr2+ impurities in ZnSe and ZnS. The ground D-5(2) term of Cr2+ is split by crystal field into a T-5(2) ground multiplet and an excited E-5 multiplet. We look at transitions among levels belonging to these two multiplets, which happen to be in the near infrared region. Spin-orbit and spin-spin interactions are taken into account. The Jahn-Teller coupling is introduced as a linear coupling considering both epsilon and tau(2) phonons. The Lanczos-recursion procedure with a proper choice of the initial state is used to calculate the vibronic functions and energies. It is found that epsilon modes only lead to intensities that do not agree well with those of the zero-phonon doublet observed both in emission and absorption in the cases of ZnS and ZnSe, while tau(2) modes give a good explanation of transition energies and transitions strengths in the same cases. A discussion of the relatively high strength of the vibronic coupling for Cr in comparison with other impurities in the same compounds is also included.