Localized exciton-phonon complex as an intermediate state in light scattering

A $\text{sharp}$ LO-phonon replica of the first order Raman spectrum of (Ga_xIn_1-xP and Ga(As_1-xP_x) has been observed for the first time. The experiment shows strong resonance with varying incoming photon energy which only appears in the presence of alloying and disorder. The results can only be understood if the relevant resonant intermediate state is a localized sharp exciton strongly interacting with the LO-phonon system, i.e. a localized exciton-phonon complex. Under those conditions, a simple model yields a ratio of the two-phonon to one-phonon intensities which is given by straightforward Franck-Condon overlaps.