Optical absorption by impurities in semiconductors

Parts of the linear and quadratic coupling of the electron–phonon system are included in a discussion of optical absorption and emission by an electron that is bound to an impurity in a semiconductor and is in interaction with the lattice vibrations. An adiabatic approach is not used. Comparison is made between an approach based on the Bethe–Sommerfeld expression in the representation in which the Hamiltonian is diagonal and one based on a correlation function approach.