MAGNETIC-FIELD EFFECT ON THE VIBRONIC STRUCTURE OF A T-4(2) LEVEL OF A D(5) ION COUPLED TO EPSILON-VIBRATIONAL MODES

A high resolution study of the magnetic field effect on the fundamenta l vibronic lines (FVL) of the T-4(2) level at lower energy of Mn2+ in pure cubic ZnS has been made in order to analyze the vibronic structur e of this level. First, very detailed excitation spectra sigma(+) and sigma(-) of the two observed FVLs corresponding to transitions (6)A(1) --> T-4(2) have been obtained by using polarization-modulated laser s pectroscopy performed at 1.9 K, for B parallel to [001] and B parallel to [111]. Second, the theoretical energies and relative dipole streng ths (RDS) of the transitions between the Zeeman levels 6A1 (M-S = -5/2 to M-S = +5/2) of the fundamental level and the Zeeman levels of the Kramers doublets Gamma(6)(T-4(2)), Gamma(7)(T-4(2)), and spin quartets Gamma(8)'(3/2) (T-4(2)), Gamma(8)'(5/2) (T-4(2)), have been analyzed in terms of the Zeeman Hamiltonian mu(B)g(e)S.H and in terms of three coefficients which account for the splitting and RDS of the fundamenta l vibronic lines in zero magnetic field. Finally, by comparing the the oretical and experimental spectra, it is shown that the model of a str ong coupling to E-vibrational modes must be rejected and that, for the proposed model, the two observed lines are associated to transitions \(6)A(1)] --> \Gamma(8)'(5/2)(T-4(2))] and \(6)A(1)] --> \Gamma(6)(T-4 (2))], the transition \(6)A(1)] --> \Gamma(8)'(3/2) (T-4(2))] being st rongly attenuated by selective intensity transfer due to a medium coup ling to E-vibrational modes, and the transition \(6)A(1)] --> \Gamma(7 )(T-4(2))] being forbidden by symmetry.