ABSTRACT
This chapter discusses electron transitions related to the absorption or emission of photons, and electron transitions with participation of the excited states. Processes of optical interband transitions in solids are rather diverse. They involve interband transitions near the edge of the absorption band in polar semiconductors for which polaron and exciton effects are important. Dexter has considered a quadratic Stark effect in the exciton placed in an homogeneous electric field. Irradiation of some solids by high-energy electrons or gamma-quanta produces electrons due to ionization of the molecules of the medium. Some of the electrons are trapped by the defects of the crystal lattice or by other traps for rather long times. Light absorption by impurity centers in crystals usually leads to considerable distortion of the crystal lattice near the impurity center along with a change of the electron state, and represents therefore a typical electron-nuclear transition.
