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Observation of ultralong-range Rydberg molecules


Rydberg atoms have an electron in a state with a very high principal quantum number, and as a result can exhibit unusually long-range interactions. One example is the bonding of two such atoms by multipole forces to form Rydberg–Rydberg molecules with very large internuclear distances1,2,3. Notably, bonding interactions can also arise from the low-energy scattering of a Rydberg electron with negative scattering length from a ground-state atom4,5. In this case, the scattering-induced attractive interaction binds the ground-state atom to the Rydberg atom at a well-localized position within the Rydberg electron wavefunction and thereby yields giant molecules that can have internuclear separations of several thousand Bohr radii6,7,8. Here we report the spectroscopic characterization of such exotic molecular states formed by rubidium Rydberg atoms that are in the spherically symmetric s state and have principal quantum numbers, n, between 34 and 40. We find that the spectra of the vibrational ground state and of the first excited state of the Rydberg molecule, the rubidium dimer Rb(5s)–Rb(ns), agree well with simple model predictions. The data allow us to extract the s-wave scattering length for scattering between the Rydberg electron and the ground-state atom, Rb(5s), in the low-energy regime (kinetic energy, <100 meV), and to determine the lifetimes and the polarizabilities of the Rydberg molecules. Given our successful characterization of s-wave bound Rydberg states, we anticipate that p-wave bound states9, trimer states10 and bound states involving a Rydberg electron with large angular momentum—so-called trilobite molecules5—will also be realized and directly probed in the near future.

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Figure 1: Electron probability density and molecular potential for the 35 s state.
Figure 2: Spectra of the Rydberg states 35 s , 36 s and 37 s.
Figure 3: Measured and calculated binding energies, EB.
Figure 4: Stark map of the atomic 35 s state and the molecular 3Σ(5 s –35 s)(v = 0) state.


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We would like to thank C. Greene and J. M. Rost for discussions and P. Kollmann for his contribution in the early stage of the experiment. This work is supported by the Deutsche Forschungsgemeinschaft as part of the SFB/TRR21 and under contract PF 381/4-1, and by the Landesstiftung Baden-Württemberg. B.B. acknowledges support from the Carl Zeiss foundation and J.P.S. thanks the Alexander von Humboldt foundation for financial support.

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Correspondence to Vera Bendkowsky or Tilman Pfau.

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Bendkowsky, V., Butscher, B., Nipper, J. et al. Observation of ultralong-range Rydberg molecules. Nature 458, 1005–1008 (2009).

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