ABSTRACT
This chapter discusses electron transitions related to the absorption or emission of photons, and electron transitions with participation of the excited states. Due to light absorption a polaron of small radius may leave the site of the crystal lattice where it is localized in the unexcited electron state and pass to another small polaron site also in the unexcited electron state. 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. Models of exiton absorption with due account of the interaction with phonons or impurities creating local electric fields are used in most work for the explanation of the Urbach rule. Dow and Redfield have assumed that the exponential dependence of the edge of the absorption band is due to the Franz–Kjeldysh effect averaged over the chaotic electric fields.
