Abstract
The authors study the optical absorption spectrum of one-dimensional (1D) and three-dimensional (3D) insulators with impurities, using the Frenkel exciton model and the Green function formalism. For a single substitutional impurity an exact solution is presented within the proposed model, and it is shown that the absorption intensity of local modes may be drastically modified (enhanced or diminished) due to its proximity in energy to the host crystal exciton band. This effect is stronger in 3D than in 1D systems. Finite impurity concentrations c are treated within the average t-matrix approximation (ATA), which reproduces the exact single-impurity results for c to 0. The authors discuss the application of their results to the determination of microscopic parameters in disordered systems from the knowledge of position and relative intensity of impurity and host peaks.
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