The low temperature (T = 5-40 K) capture of free electrons into hydrogenlike antimony centers in germanium has been studied by a time-resolving experiment using the free electron laser FELBE. The analysis of the pump-probe signal reveals a typical capture time of about 1.7 ns that decreases with pump energy to less than 1 ns whi le the number of ionized donors increases. The dependence on the pump-pulse energy is well described by an acoustic phonon-assisted capture process. In the cases when (i) a significant number of the electrons is in the conduction band (flux densities larger than 5 × 1025 photons/(cm2 s), (ii) the lattice temperature is above ∼20 K, or (iii) a static electric field above ∼2 V/cm is applied to the crystal, the pump-probe technique reveals an additional intraband relaxation process with a characteristic time of ∼100 ps, which is much shorter than that of the capture of free electrons into the antimony ground state. © 2014 American Physical Society.
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