Atomic Bose-Einstein condensates in ultracold dilute gases represent almost ideal quantum gases, which can be investigated experimentally since 1995. In the latest experiments, the interactions among the atoms plays a crucial role for a variety of phenomena. These interactions can be tuned by means of so called magnetically induced Feshbach resonances, i. e. resonances of the elastic scattering cross section. This thesis reports on the observation of more than 40 magnetically induced Feshbach resonances, for the first time in rubidium 87, which is the isotope used in most of today's experiments on Bose-Einstein condensation. Thereby, the magnetic field range between 0,5 and 1261 G as well as different spin mixtures were examined. The loss of atoms owing to resonantly enhanced inelastic collision processes served as a signature of the resonances. The resonance positions were determined with a high absolute accuracy of 30 mG using microwave spectroscopy. Originally, only four Feshbach resonances were predicted for the |f,mf>=|1,1> Zeeman state due to the coupling to (quasi) bound states with rotational quantum number l=0. In cooperation with Professor B. J. Verhaar and E. G. M. van Kempen from the Technical University in Eindhoven all except one of the resonances could be clearly identified including additional coupling mechanisms and other entrance channels in the analysis. Furthermore an improved set of model parameters for the rubidium interatomic potential could be derived on the basis of the experimentally determined resonance positions. The deviations between experimentally determined and theoretically predicted resonance positions lie now within the range of a few parts per thousand. Within the range of the broadest resonance of the entrance channel |1,1>×|1,1> at 1007 G the observed particle loss was investigated in more detail. On resonance, the particle loss follows a resonantly enhanced three body decay on long time scales. The appropriate rate coefficient exceeds the corresponding value away from the resonance by approximately two orders of magnitude. Moreover, the measurements suggest the existence of an additional loss mechanism acting on time scales faster than 1 ms. In this context, results of other groups give rise to the assumption that coherent couplings between atoms, molecules, and dissociated atom pairs play a crucial role. In addition, this thesis reports on the characterisation of a new apparatus, which allows for the production of Bose-Einstein condensates with a high repetition rate and outstanding reproducibility, indispensable conditions for the accomplished experiments with Feshbach resonances in rubidium 87.     «

Atomic Bose-Einstein condensates in ultracold dilute gases represent almost ideal quantum gases, which can be investigated experimentally since 1995. In the latest experiments, the interactions among the atoms plays a crucial role for a variety of phenomena. These interactions can be tuned by means of so called magnetically induced Feshbach resonances, i. e. resonances of the elastic scattering cross section. This thesis reports on the observation of more than 40 magnetically induced Feshbach re...     »