High resolution absorption and magnetic circular dichroism of spin-forbidden ligand field transitions in Cs3CoBr5

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Abstract
The axial absorption spectrum of Cs3CoBr5 has been recorded from 3100 to 6500 Å at 4·2 K with a spectral bandpass of 0·1 Å and the magnetic circular dichroism spectrum at 4·2 K and 47·5 kG over the same wavelength range with a spectral bandpass of 1 Å. The electronic origins of the spinorbit and tetragonal field components of most of the doublet ligand field states are definitively located and assigned. In a number of instances it has also proved possible to determine whether the doublet gains its intensity from the 3/2 U′ or 5/2 U′ spin-orbit component of 4 T 1, and arguments are also presented to show that the higher energy doublets derive the greater part of their intensity from the lowest 4 T 1 charge transfer state and not the 4 T 1(P) ligand field state. The vibronic sidebands accompanying the doublet transitions are also assigned to internal modes of CoBr4 2- and to lattice modes. In a number of the transitions non-totally symmetric modes are excited with greater intensity than totally symmetric.