= 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 $E_g+\hbar\Omega$, while phonon absorption gives $E_g-\hbar\Omega$ and requires an occupied phonon mode. The temperature dependence of the absorption therefore contains phonon-population information in addition to electronic occupations.
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