Simultaneous Electronic Excitation of Exchange-Coupled Pairs of Manganese and Nickel Ions

An investigation of the ultraviolet absorption spectra of KZn_1-x-yMnₓNiyF₃ and KMnₓNi_1-xF₃ crystals is reported. Two regions of absorption have been found, one near 40 000 cm^-1 and the other near 45 000 cm^-1, corresponding to the simultaneous electronic excitation of the Mn²⁺ to the ⁴A1g, ⁴Egᵃ states and the Ni²⁺ to its ¹Eg state in the first case, and of Mn²⁺ to its ⁴Egᵇ state and Ni²⁺ to its ¹Eg state in the second. It is shown from a study of the temperature dependence of the intensity that the mechanism for the absorption of light involves exchange-coupled pairs with the selection rule ΔS=0. The oscillator strengths of the lines and the temperature-dependent shifts of the absorption peaks in the spectra of the concentrated crystals can be accounted for using the theoretical model proposed earlier.