Direct excitation of butterfly states in Rydberg molecules

Since their first theoretical prediction Rydberg molecules have become an increasing field of research. These exotic states originate from the binding of a ground state atom in the electronic wave function of a highly-excited Rydberg atom mediated by a Fermi contact type interaction. A special class of long-range molecular states, the butterfly states, were first proposed by Greene et al.\footnote{Chris H. Greene, A. S. Dickinson, H. R. Sadeghpour, \textbf{PRL} 85, 2458}. These states arise from a shape resonance in the p-wave scattering channel of a ground state atom and a Rydberg electron and are characterized by an electron wavefunction whose density distribution resembles the shape of a butterfly. We report on the direct observation of deeply bound butterfly states of Rydberg molecules of $^{87}$Rb. The butterfly states are studied by high resolution spectroscopy of UV-excited Rydberg molecules. We find states bound up to $-50\:$GHz from the 25P$_{1/2}, F=1$ state, corresponding to binding lengths of $50\:a_0$ to $500\:a_0$ and with permanent electric dipole moments of up to $500\:$Debye. This distinguishes the observed butterfly states from the previously observed long range Rydberg molecules in rubidium.