Low-temperature optical absorption of nickel fluosilicate crystals

Abstract

The vibronic absorption spectrum of the Ni⁺⁺ ion in an electric field of trigonal (nearly cubic) symmetry at temperatures down to 4°K is reported and analysed in terms of ligand field and vibronic theory. The positions, widths, shapes and strengths of the bands are satisfactorily explained, together with their associated fine structures. In particular, the complex structure of the red band is well explained in detail as a consequence of the mutual perturbation of the ¹E and ³T₄ levels, which strongly modifies the vibronic coupling and results in two fairly narrow bands, each showing resolved vibrational structure, superposed on a broad band. A Trees correction term, αL(L+1), helps the agreement between observed and calculated electronic levels. The following parameters are found to give good agreement: Δ = 9100 cm⁻¹, B=955 cm⁻¹, C=3750 cm⁻¹, α=80 cm⁻¹, ζ = 600 cm⁻¹.