Consider photoluminescence in GaAs. I have been told that if excitation of electrons from the valence band happens with the light energy of wavelength of which is higher than the band gap energy, then electron gets excited to the conduction band, but not to the edge of the conduction band. And then by means of interband transitions it "falls" to the edge of the conduction band and then eventually to the valence band emitting photons with the energy equals the band gap. Why photons are not emitted in case of interband transitions? I assume energy goes to phonons, then why transition without creating of phonon is not possible? Thank you.