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The influence of an external magnetic field on the normal incidence reflectivity of CuCl single crystals has been measured in the Voigt and Faraday configurations. The results are discussed in terms of the dielectric function calculated from the reflectivity spectrum by a Kramers-Kronig transformation. In the Voigt configuration two new exciton bands can be observed, which are attributed to a triplet and a longitudinal exciton. Their energetic positions are used to determine the parameters of electron-hole exchange interaction in the 1s excitons, and the spin-orbit splitting parameter of the upper valence band. The excitonic g-values are determined from the Faraday configuration measurements and can be interpreted on the basis of the tight binding model.

Low temperature crystallographic phase transitions in DyVO4 and TbVO are attributed to a co-operative Jahn-Teller effect. A preliminary report of the theory is given, based on a harmonic oscillator Hamiltonian for the crystal and an electron lattice coupling which is small compared to typical phonon zero point energies. An Ising Hamiltonian is shown to describe the co-operative ordering.

A Hamiltonian for the co-operative Jahn-Teller transition in DyVO4 is derived with both optic mode and strain couplings. A molecular field theory for this Hamiltonian predicts a second order phase transition and a dependence of the ground state splitting on stress. Contributions of phonon modes and strain can be distinguished by a comparison of the elastic constants and the parameters derived from the stress dependence of the ground state splitting. A correction to these molecular field results is derived by accounting for fluctuations. The effects of a magnetic field are investigated in the molecular field approximation; it is found that if the field is applied in the basal plane simultaneously with the stress at low temperatures, different distortions can be stabilized. The form of the first order phase boundary for these distortions is derived and an analogy is drawn between this ‘pseudospin flop’ transition and antiferromagnetic spin flop.

The magnetic circular dichroism of the K-band in the alkali halides RbCl, RbBr, RbI, KCl, KBr, and KI has been measured at temperatures between 4.2 and 77 °K. The magnetic field up to 50 kG was produced by a superconducting solenoid. We interpret the experimental results using the method of moments. The spin-orbit splitting and the orbital g-factors of the excited states due to the K-band are determined. Furthermore we obtain matrix elements describing the mixture of the excited states giving rise to F-and K-bands. In accordance with calculations of Smith [15] the measurements confirm the Mott-Gurney model, which attributes the K-band mainly to 1s-3p transitions. Der magnetische zirkulare Dichroismus der K-Bande wurde in den Alkalihalogeniden RbCl, RbBr, RbJ, KCl, KBr und KJ bei Temperaturen von 4.2 bis 77 °K gemessen. Als Magnet diente eine Supraleitungsspule, mit der Magnetfelder bis zu 50 kG erzeugt werden konnen. Die Auswertung der Messungen geschieht nach der Methode der Momente. Man erhalt Spin-Bahn-Aufspaltungen und orbitale g-Faktoren der angeregten Zustande des K-Uberganges. Auserdem werden Matrixelemente bestimmt, die die Mischung der angeregten Zustande des K- und F-Uberganges beschreiben. In Ubereinstimmung mit Rechnungen von Smith [15] bestatigen die Messungen das Mott-Gurney-Modell, das die K-Bande im wesentlichen 1s-3p-Ubergangen zuordnet.

Optical absorption measurements on TbVO 4 indicate that at 33°K there is a second order crystallographic phase change. Although no magnetic ordering occurs, the distortion direction can be controlled by a magnetic field. This behaviour is compared to DyVO 4 , in which a similar distortion occurs along different crystallographic directions.

Transverse magnetoreflection measurements give evidence for a longitudinal and a triplet state near the first exciton band in CuCl.

Optical absorption spectroscopy is used to investigate the low lying energy levels of DyVO 4 under uniaxial stress and magnetic fields at temperatures above and below the co-operative Jahn-Teller transition. The results agree with the molecular field theory of the previous paper using the parameter σ 0 = 64 ± 4 kg/mm 2 (which represents the stress of e aa − e bb type required to split the ground state energy levels by 27 cm −1 at high temperatures). The behaviour of dilute Dy 3+ in YVO 4 is studied and predicted from the theory. Deviations from the molecular field theory are found and discussed. Comparison of the results with known elastic constants of DyVO 4 indicates that strain coupling dominates the optic mode coupling.

Circular dichroism measurements were used to determine the spin orbit splitting Δ and the orbital g -factor g orb of the excited state belonging to the K absorption in RbCl.