Light emission from impurity states in solids: conventional and coherent phonon descriptions

The phonon-assisted luminescence in AgBr:I is discussed in the framework of a conventional configuration coordinate model in which both optical and acoustic phonons are coupled to the excited electronic states. The authors derive the expression for the luminescence intensity using the coherent phonon description for the displaced oscillator (excited impurity) in a consequent way. As compared to the conventional derivation they gain in simplicity and transparency of the calculation. With the formula derived, the experimental results concerning the spectral features of the luminescence as well as their dependence on temperature are well reproduced. In particular the importance of the acoustical phonon contribution in determining the correct intensity and its temperature dependence is stressed. Furthermore the question of the interpretation of the experiments in terms of the introduced coherent phonon states is discussed.