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The initial cooling of hot carriers and the subsequent exciton formation in GaSe are studied by time-resolved photoluminescence (PL) using femtosecond up-conversion techniques. From the time-resolved PL spectra of this layered III-VI semiconductor two different energy relaxation channels are derived. After an initial subpicosecond cooling due to Fr\"ohlich-type interaction of carriers with longitudinal optical E^' (2^2) phonons a slower regime follows, which is dominated by deformation potential interaction with the nonpolar optical A₁^' (1^2) phonons. The coupling constant for nonpolar optical phonon scattering is derived. The subsequent formation of excitons is studied at different carrier densities and detection energies. A cross section for the free-exciton formation is determined based on a rate equation model.
Nüsse et al. (Fri,) studied this question.