Interpretation of the temperature dependence of Ag+ dipole strengths in alkali halides

The absorption spectrum of Ag+ doped RbCl is measured at various low temperatures in the spectral range from 6.4 to 5.0 eV. The temperature dependence of the Ag+ dipole strength in RbCl is different from that found in NaCl, KCl, and KBr. The different temperature behaviours are interpreted as being due to the different lattice potentials seen by the Ag+ in the various host crystals. The static potentials for Ag+ in the crystals are calculated by a semiphenomenological method. For NaCl, KCl, and KBr a potential minimum is found at the symmetry centre. For RbCl, however, a potential barrier exists at the symmetry centre and potential minima occur at 0.25 Å away from the centre in the [111] direction.