Lattice relaxation and multiphonon transitions

Abstract Abstract The phenomenon of lattice relaxation, namely, that the atomic configuration surrounding a localized centre in solids changes with its change of electronic state, leads to various forms of multiphonon transitions. In optical emission or absorption spectra, such multiphonon transitions can cause shift and broadening of the spectral lines or give rise to distinct multiphonon peaks. Certain basic theoretical concepts relating to such multiphonon optical spectra are explained and a special discussion of the proper interpretation of the Franck-Condon principle in this connection is given. Lattice relaxation can also give rise to nonradiative transitions, which are important for carrier recombinations in semiconductors and quenching of luminescence.