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Time-dependent spontaneous Raman spectra of optically pumped GaAs have been measured with subpicosecond time resolution. When the optically injected carrier density is less than 10^17 cm^-3, we temporally resolve the growth of the optically induced nonequilibrium LO-phonon population and deduce an average electron-phonon scattering time of about 165 fs. For higher injected carrier densities, substantial time-dependent changes are observed in the spectra which result from screening of the LO phonons by the free carriers and the relaxation of the free carriers to the band edges.
Kash et al. (Mon,) studied this question.