Charge-Transfer Spectrum of the Tetrachlorocuprate Ion

The polarized ultraviolet absorption spectra of the tetrahedral CuCl4= ion oriented in single crystals of Cs2CuCl4 and Cs2ZnCl4 have been observed. The spectral bands are identified by means of an analysis which takes into account the observed variations with lattice of the band intensities and positions and which allows for the effects upon the spectra of terms of rhombic symmetry known to be present in the ionic Hamiltonian. The resulting band assignments differ from those proposed by Ferguson. Salient differences between the observed Cs2CuCl4 spectra and those reported by Ferguson are noted and tentatively explained.