Multiphonon Raman scattering in semiconductor nanocrystals: Importance of nonadiabatic transitions

Multiphonon Raman scattering in semiconductor nanocrystals is treated taking into account both adiabatic and nonadiabatic phonon-assisted optical transitions. Because phonons of various symmetries are involved in scattering processes, there is a considerable enhancement of intensities of multiphonon peaks in nanocrystal Raman spectra. Cases of strong and weak band mixing are considered in detail. In the first case, fundamental scattering takes place via internal electron-hole states and is participated in by s and d phonons, while in the second case, when the intensity of the one-phonon Raman peak is strongly influenced by the interaction of an electron and of a hole with interface imperfections (e.g., with trapped charge), p phonons are most active. Calculations of Raman scattering spectra for CdSe and PbS nanocrystals give a good quantitative agreement with recent experimental results.