Abstract
In order to describe the strong temperature dependence of the infrared absorption of Cu+ in NaCI a model is investigated in which the Cu+ is coupled to its nearest neighbours by harmonic and cubic anharmonic forces. The problem is solved by means of retarded Green's functions solving a Dyson equation with a mass operator of second order in the cubic anharmonicity. The pure crystal is described by a Debye model because the impurity‐induced absorption takes place in the very far infrared (ωR ≈ 1/10 Debye frequency). By iterating the mass operator a good agreement between theory and experiment is reached.