Rydberg atoms are atoms with one electron excited to a state with a high principal quantum number n. The size of the Rydberg atom scales as n2, making them extremely large atoms that are nearly as big as bacteria. Our experimental apparatus combines the ability of producing thermal gases or Bose-Einstein-condensates of ultracold Rubidium atoms with high resolution optical access for the controlled creation of individual Rydberg atoms. This allows us to study the static and dynamic properties of a single Rydberg impurity within the 87Rb background gas.The interaction between the Rydberg electron and neighboring atoms in the ground state can be treated as a scattering problem, since the Rydberg electron is only loosely bound to the ionic core. This scattering process can lead to attractive potential energy curves that can sustain bound molecular states, the ultralong-range Rydberg molecules. In this talk, I will present the influence of the relative spin on these Rydberg molecules, which leads to molecular states bound by mixed singlet-triplet electron-neutral atom scattering [1, 2]. In the second part of the talk spectroscopy of a single Rydberg atom excited within a Bose-Einstein condensate will be presented . We observe a frequency shift propotional to the density, as discovered by Amaldi and Segre in 1934 , as well as a line shape which changes with the principal
quantum number n. The observed line broadening thereby depends precisely on the potential energy curve arising from the interaction between the Rydberg electron and the neutral atom perturbers. In particular, we show the relevance of the p-wave shape resonance in the triplet e-Rb(5S) scattering,which signicantly modies the interaction potential.
 D. A. Anderson et al., Phys. Rev. A 90, 062518 (2014)
 F. Bottcher et al., Phys. Rev. A 93, 032512, (2016)
 M. Schlagmuller et al., Phys. Rev. Lett. 116, 053001 (2016)
 E. Amaldi and E. Segre, Nature 133, 141 (1934)