  We report on the observation of two vibrational series of pure trilobite
rubidium Rydberg molecules. They are created via three-photon photoassociation
and lie energetically more than 15 GHz below the atomic 22$F$ state of
rubidium. In agreement with theoretical calculations, we find an almost perfect
harmonic oscillator behavior of six vibrational states. We show that these
states can be used to measure electron-atom scattering lengths for low energies
in order to benchmark current theoretical calculations. The molecules have
extreme properties: their dipole moments are in the range of kilo-Debye and the
electronic wave function is made up of high angular momentum states with only
little admixture from the nearby 22$F$ state. This high-$l$ character of the
trilobite molecules leads to an enlarged lifetime as compared to the 22$F$
atomic state. The observation of an equidistant series of vibrational states
opens an avenue to observe coherent molecular wave-packet dynamics.
