  We present spectroscopy of a single Rydberg atom excited within a
Bose-Einstein condensate. We not only observe the density shift as discovered
by Amaldi and Segre in 1934, but a line shape which changes with the principal
quantum number n. The line broadening depends precisely on the interaction
potential energy curves of the Rydberg electron with the neutral atom
perturbers. In particular, we show the relevance of the triplet p-wave shape
resonance in the Rydberg electron-Rb(5S) scattering, which significantly
modifies the interaction potential. With a peak density of 5.5x10^14 cm^-3, and
therefore an inter-particle spacing of 1300 a0 within a Bose-Einstein
condensate, the potential energy curves can be probed at these Rydberg ion -
neutral atom separations. We present a simple microscopic model for the
spectroscopic line shape by treating the atoms overlapped with the Rydberg
orbit as zero-velocity, uncorrelated, point-like particles, with binding
energies associated with their ion-neutral separation, and good agreement is
found.
