QUANTUM KINETIC ELECTRON-PHONON INTERACTION IN GAAS - ENERGY NONCONSERVING SCATTERING EVENTS AND MEMORY EFFECTS

Highly energetic electrons in GaAs emitting LO phonons are studied via femtosecond transmission spectroscopy. It is demonstrated for the fir st time that energy is not conserved on a time scale as short as a LO oscillation period: A replica of the initial distribution starts spect rally broadened before it sharpens into the excitation shape. This sce nario is repeated for every step in the phonon cascade. Simulations co mbining an exact solution of quantum kinetics within a simplified mode l and a calculation of the polarization including Coulomb effects quan titatively agree with the data.