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Observation of single-optical-phonon emission.

We report an observation of a single-optical-phonon emission by monoenergetic hot electrons traversing thin n + type GaAs and thin undoped AlGaAs layers in times much shorter than the classical phonon period. This was done by injecting ballistic electrons into the thin layers with energy around the threshold for optical phonon emission and monitoring their exit energy. We estimate a scattering time of — 200 fsec for electrons with energy of about 85 meV in « + type GaAs, and —550 fsec for 40-meV electrons in undoped AlGaAs.