Optical absorption by Cu-Mn pairs in KZnF3

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Abstract
A crystal of KZnF3 containing added Mn2+ and Cu2+ has been shown to have three absorption regions associated with nearest neighbour pairs of Cu2+ and Mn2+. Two correspond to electronic excitation energy on the Mn2+ and the third is an electron transfer process. All three occur only for the pair in which the tetragonal axis of the Jahn-Teller distortion around the Cu2+ is directed towards the Mn2+. Temperature dependence measurements of the absorption intensity can be accounted for by using an exchange energy of 130+or-10 cm-1 for this pair in the ground state. The two absorption regions corresponding to Mn2+ excitation comprise mainly the Delta S=0 transitions (4A1g at 25052 cm-1 and 4Egb at 29980 cm-1) although three other lines are assigned to Delta S=-1 transitions (4A1g at 25244.7 cm-1 and 25254.3 cm-1 (split by spin-orbit interaction) and 4Egb at 30460 cm-1). These latter assignments demand exchange energies of -100+or-5 cm-1 and -240+or-30 cm-1 respectively, for the two excited states and are of opposite sign to the ground state value.