Raman scattering by two LO-phonons near Γ in GaAs

Near resonance the second order phonon spectra of semiconductors exhibit peaks which correspond to 2 LO phonons with wave-vector $\text{q}\text{→}\text{?o}$ (2 LO(Γ)). The frequencies of these 2 LO(Γ)-peaks are studied for photon energies varying across the E_o + Δ_o gap of GaAs. These peaks are shown to be slightly shifted to lower frequencies compared with the exact 2 LO(Γ) frequency. The peak positions when plotted as a function of the incident photon energy display an oscillatory behaviour which corresponds to the fact that LO-phonons of different wave vectors are created in the Raman process as the photon energy is tuned across the E_o+Δ_o resonance.