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We report the first observation of quantum beats in the resonance fluorescence of a bound exciton in a semiconductor, the ionized-donor exciton complex in CdS. Measurements of the polarization behavior and magnetic-field dependence of the beats permitted a detailed investigation of strain eff'ects and magneto-optical parameters of the states. From the damping of the beats, coherence times of the order of several hundreds of picoseconds are deduced. For the neutral-donor and -acceptor bound exciton states, very rapid dephasing with coherence times of the order of 20 ps is found.PACS numbers: 42.50. Md, 71.35.+Z, 78.20.Ls, Phase relaxation is of fundamental importance for the understanding of the dynamical behavior of electronic states. In solids, phase relaxation times in the picosecond and subpicosecond regime require high time resolution, and therefore only nonlinear optical techniques, like photon echo [1,2], transient absorption correlation [3], or four-wave mixing [4], hitherto allowed exploration of the dephasing of continuum electron-hole states and excitons in various semiconductor systems. However, an alternative, linear method to investigate dephasing of quantum states in solids is quantum-beat spectroscopy. This technique has widely been used in studies of atoms and molecules [5], but most recently, by studying the resonance fluorescence of free exciton states in AgBr [6], it was demonstrated to be applicable for solids too. In this technique, a set of nearly degenerate electronic states, e.g., split by a magnetic field, is excited coherently by a short laser pulse with a spectral width larger than the energy splitting of the levels. The beats show up as oscillations in the time-dependent resonance fluorescence from these states. From the damping of the beats, the coherence time Tcoh (also called dephasing time T2) of the states can be deduced, which is defined through l/Tcoh~2/ri + l/r2, where T\ and T2 denote the energy relaxation and pure dephasing times. The beating frequencies are directly related to the energy splitting of the states, which can be varied by the magnetic field, allowing a determination of important magneto-optical parameters like electron and hole g factors. A particular advantage of quantum beats in spontaneous fluorescence is that they are not aff'ected at all by an inhomogeneous broadening of the states. Therefore, small splittings can be measured that are masked in the spectrum and not accessible to conventional spectroscopy. From a fundamental point of view, beats in spontaneous fluorescence originating from the superposition of wave functions have to be distinguished from beating phenomena observed in investigations of exciton states using nonlinear four-wave mixing [7,8], as these are due to interference of coherent polarizations oscillating at different frequencies.In order to be able to use the full potential of quantum-beat spectroscopy, the coherence time of the states investigated should be long, corresponding to a small homogeneous linewidth Fhom'^^^/rcoh-In semiconduc...

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Very low energy electrons in a solid should behave like Bloch electrons and will interact with perturbations of the atomic lattice, that is, with phonons. So we use the acoustic phonon scattering for replacing the elastic binary encounter approximation of the Mott scattering for electrons with low energies E < 100 eV. For ballistic electrons (1 eV < E < Eg) and higher energies up to 1 keV we determined the acoustic phonon scattering and the impact ionization rate by means of the “backscattering-versus-range” proof and respective η(E0) − R(E0) diagrams. Electron trajectories demonstrate the relatively short range of primary electrons (PE) with energies E > 50 eV due to strong impact ionization losses (cascading) and the much greater range of secondary electrons (SE) with E < 50 eV, finally as a consequence of less effective phonon losses. The field-dependent transport parameters allow us to model the self-consistent charge transport and charging-up of insulating SiO2 layers during electron bombardment maintained by the current components of primary electrons jPE, secondary electrons jSE, and associated ballistic holes jBH, as well as by Fowler–Nordheim field injection jFN from the substrate. The resulting distributions of currents j(x,t), charges ρ(x,t), electric fields F(x,t), and the potential V(x,t) across the dielectric layer explain the phenomena of field-enhanced and field-blocked secondary electron emission with rates δ [gel ] 1.