[en] In this thesis the study influence of residual gases on the lifetime, the temperature dependence of the quantum yield, and the influence of activation with potassium on both effects for GaAs-photocathodes is described. (HSI)

[en] Electron impact ionization cross sections σ_q,q+n for gallium ions have been measured for the reaction e + Ga^q+ → Ga^(q+n)+ + (n + 1)e for single (n = 1, q = 1, 3, 8) and triple (n = 3, q = 3, 4) ionization. The measurements have been performed using the crossed-beams technique in an energy range from the respective ionization threshold up to 1 keV, partly up to 6 keV. The cross sections for single ionization of ions in the charge state q = 1 and 3 show contributions below the ground state threshold caused by ionization of ions in excited, long-lived states in the parent ion beam. The cross sections are compared with the semiempirical Lotz formula. The cross section for Ga⁴⁺ triple ionization shows contributions from inner-shell processes. Semiempirical formulae are in good agreement with the experimental data only at energies below the cross section maximum. (orig.)

[en] Absolute cross sections σ_q,q+n for the multiple ionization of C^q+, N^q+ and O^q+ ions were measured for the reactions: e + A^q+ → A^(q+n)+ + (n + 1)e. The measurements were performed for electron energies up to 6.5 keV using the animated crossed-beams method at the Giessen electron-ion crossed-beams set-up. K-shell ionization with subsequent autoionization or even double-autoionization strongly contributes to the cross sections. (orig.)

[en] Absolute cross-sections for electron impact induced fragmentation, fragmentation-single-ionisation and fragmentation-double-ionisation were measured for the fullerene ions C₆₀⁺ and C₅₈⁺. For the fullerene ion C₆₀²+, fragmentation-single-ionisation was measured with a loss of n C₂ molecules (n=1, 2, 3). The cross-sections were determined from the respective threshold up to an electron energy of 1 keV using the animated crossed-beams technique

[en] Short communication

[en] Experimental measurements of the ionization cross section for O^q+, where q=1-4, performed with a crossed-beam apparatus are presented and compared with theoretical calculations. For O⁺, the experimental measurements are in very good agreement with configuration-average time-dependent close-coupling calculations. For the remaining oxygen ions, the experimental measurements are in good agreement with time-independent distorted-wave calculations. As expected, the accuracy of the perturbative distorted-wave calculations improves with increasing ion charge

[en] Absolute total cross sections for electron-impact single ionization of Al^q+(q=3-7) ions are investigated experimentally as well as theoretically. Employing the crossed-beams technique, the cross sections have been measured from below the respective ground state threshold up to 1 keV and partly up to 5.5 keV. The theoretical calculations were performed using the configuration-averaged {distorted-wave} method, including both direct ionization and excitation-autoionization contributions from the 2p-shell. The measurements are in good agreement with the calculations considering a small content of metastable ions in the incident ion beam. Because of the strong dominance by direct ionization processes the semiempirical Lotz formula can describe the measured data as well. (author)

[en] Polarized electrons have been accelerated in the electron stretcher accelerator ELSA for the first time. Up to 2.1 GeV the polarization of the electron beam supplied by the 120 keV polarized electron source has been measured with a Moeller polarimeter. Preliminary results of polarization measurements at high energies and the performance of the source are presented