The fundamental lattice vibrations of the trigonal CdC12 structure (D3d5 ) have been analyzed group theoretically, and symmetry coordinates have been constructed. The primitive cell has nine normal modes of vibration. All six optical modes are either infrared or Raman active, so a complete vibrational analysis is possible. Results of far-infrared measurements on a Fourier-transform spectrometer together with Raman spectra recorded by argon laser excitation are presented for both CdC12 and CdBr2. Symmetry assignments of the observed frequencies are made on the basis of the shift in frequency in going from chloride to bromide and the polarization behaviour. Peaks in the second-order axial infrared absorption spectra are assigned to allowed fundamental combinations. The lattice frequencies of CdC12 are consistent with those of the isomorphic crystals of CoC12 and MnC1 2, which have also been investigated by infrared absorption. An unsuccessful attempt was made to measure the electronic Raman spectra of Fe2+ and Co2+ ions in CdC12 and CdBr2.

The Raman spectra were measured on a Raman system comprising a 5 watt argon laser and a double monochromator, with photoelectric detection. The construction of the Raman spectrometer, and the associated signal processing electronics, is described in detail. Special emphasis has been placed on the use of on-line computer techniques in processing the Raman signal. A signal averaging system has been developed that is superior to both photon counting and lock-in amplification when measuring very weak signals.