ROLE OF EXCITONS IN DOUBLE RAMAN RESONANCES IN GAAS QUANTUM-WELLS

Raman scattering by longitudinal-optical phonons has been measured in GaAs-AlAs multiple quantum wells at high magnetic fields. Doubly reson ant scattering processes are observed at photon energies corresponding to magneto-excitons with different principal quantum numbers for the incoming and outgoing channels. The existence of these initially forbi dden scattering processes, their resonance energies, and their relativ e intensities are correctly reproduced by our theoretical description. The model takes into account the excitonic nature of the intermediate states, as well as scattering processes involving a nonzero in-plane phonon wave vector, which is required to allow inter-landau level scat tering of the exciton.