Franck-Condon modes in Sr2MnWO6 double perovskite

Molecules and color centers in crystals are known to exhibit a form of electron-phonon coupling, which is understandable by the Franck-Condon mechanism. In a Raman spectrum, this process is seen as a series of equidistant peaks whose relative intensities depend on the induced atomic vibration mode. The process has been predicted to be observable in crystals, but the difficulty in observing them has been related to the fact that the excited electron states tend to be delocalized. Correspondingly, it has been assumed that a self-trapping (localization) of the electron is necessary. Our polarization resonance Raman scattering data revealed two different series of Franck-Condon modes in double perovskite Sr2MnWO6 single crystals. One series corresponded to the Mn d orbital electron transition to the conduction band, whereas the second series corresponded to e(g) to e(g) transition (localized electron). The lattice distortion was estimated from the observed Franck-Condon peak positions and intensities.