Absorption- and uniaxial stress experiments in the a band of Pb2+ centres in KBr,RbBr, KI and RbI

The absorption lineshape and the stress induced dichroism of the A band of Pb²⁺ centres in KBr, RbBr, KI and RbI have been measured as a function of temperature from 4 K to room temperature. The moments of the absorption bands and of the dichroic spectra have been determined, and the effective coupling constants of the linear electron-lattice interactions of the ¦A〉 state as well as the frequencies of the interaction modes have been evaluated. Above 200 K the temperature dependence of the moments gives strong evidence that nonlinear electron-lattice interactions are important, particularly in the case of KBr:Pb²⁺.