OPtical absorption spectra of KNiF_3 and NiO stoichiometric single-crystals were measured at 5 K in the range from 4000 to 50000 cm~-1. The observed bands are interpreted based on the energy levels diagram for Ni~2+ (3d~8) ion in a cubic crystal field. The crystal-field parameter Dq is equal to 766 cm~-1 in KNif_3 and 890 cm~-1 in NiO. Particular attention is paid to the band due to the magnetic-dipole ~3A_2g(F)->~3T_2g(F) transition, located at 7700 cm~-1 in KNiF_3 and at 8900 cm~-1 in NiO. We show that the energy difference between the two peakds, located at the low energy side of the band, is related to the zone-center (k=0) one-magnon energy, so that the low-energy peak is attributed to the pure exciton transition, whereas the high-energy peak to the exciton-magnon excitation. The estiamted one-magnon energies are 25+-5 cm~-1 in KNiF_3 and 39+-3 cm~-1 in NiO.
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