DYNAMIC JAHN-TELLER INTERACTION WITH GAMMA(5) PHONONS IN THE GROUND-STATE OF CU2-VI SEMICONDUCTORS( IN CUBIC II)

Optical absorption and emission measurements of Cu2+ as a substitution al impurity in cubic ZnS and ZnTe rue analyzed by means of an electron -phonon coupling model. The D-2 term of Cu2+ is split by a crystal fie ld of tetrahedral symmetry into a (2) Gamma(5) orbital triplet and a ( 2) Gamma(3) orbital doublet. Optical transitions have been observed be tween these two multiplets in ZnS:Cu2+ and within the (2) Gamma(5) gro und state in ZnTe:Cu2+. The theoretical model is based on crystal-fiel d theory and includes the spin-orbit interaction and a dynamic Jahn-Te ller interaction between the electronic (2) Gamma(5) states and a tran sverse acoustic phonon of TI symmetry. Starting from the ten spin-orbi t wave functions appropriate to the orbital tripler and doubler manifo lds, the symmetry-adapted vibronic basis is constructed and used to di agonalize the Hamiltonian matrix. Phonon overtones up to n = 14 are in cluded to ensure convergence of the energy eigenvalues. The measured p ositions and relative intensities of the spectral lines are described with good accuracy by the theoretical model, including covalency effec ts.. In ZnS, comparison between theory and experiment yields the follo wing values of the physical parameters: the crystal-field splitting De lta = 5990.6 cm(-1), the spin-orbit coupling constants lambda(1) = -66 7 cm(-1) and lambda(2) = -830 cm(-1), the phonon energy h omega = 73.5 cm(-1), and the Jahn-Teller stabilization energy E-JT = 474.5 cm(-1) The corresponding parameters in ZnTe are Delta = 6000 cm(-1), lambda(1 ) = -888 cm(-1), lambda(2)= -830 cm(-1), h omega= 38.8 cm(-1), and E-J T= 468.5 cm(-1).