Indirect optical transition

An optical interband transition that requires another momentum source, commonly a phonon, to connect initial and final electronic wavevectors. Near an indirect band gap, phonon emission gives a threshold photon energy $E_g+\hslash \Omega$, while phonon absorption gives $E_g-\hslash \Omega$ and requires an occupied phonon mode. The temperature dependence of the absorption therefore contains phonon-population information in addition to electronic occupations.