We analyze the possibility of observing parity-forbidden transitions of shallow-donor electrons by means of a two-photon absorption. We consider two energy levels of an impurity, coupled by a two-photon transition and interacting with the crystal. To describe the dynamics of the system, we introduce the master equation for the reduced density matrix. In this equation, the two-photon interaction is computed with the help of the Coulomb-Green's function, while the interaction with the crystal is phenomenologically taken into account by appropriate damping constants. The numerical calculations for GaAs and CdTe show that the observation of two-photon absorption by shallow donors in semiconductors seems to lie within the present experimental possibilities of the photothermal ionization technique.

• Received 15 February 1977

DOI:https://doi.org/10.1103/PhysRevB.18.2755