Electronic absorption spectra of Ni2+in NiCl2and NiBr2. (Phonon and magnon sidebands)

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Abstract
The optical absorption spectra of the antiferromagnetic insulator NiCl2 and NiBr2 single crystals have been measured between 5*103 and 33*103 cm-1, and from 300 to 4.2 K. The electronic spectrum of Ni2+ in these materials has been analysed using the spin-orbit matrices for a (3d)8 system and the parameters of the crystal field have been derived. It is found that Dq=692 cm-1, lambda =:140 cm-1 and F4=91 cm-1 for NiCl2, and Dq=680 cm-1, lambda =-390 cm-1 and F4=85 cm-1 for NiBr2. A detailed discussion of all the bands of the spectra and comments on the interesting structures of vibronic and magnonic origin of the 3T1g(a) band are given. The absorption peaks located around 16.6*103 cm-1 in NiBr2 are not easily interpreted but their puzzling origin is discussed.