Observation of singlet-scattering channels in long-range Rydberg dimers

J. Deiglmayr, H. Sassmannshausen and F. Merkt

ETH Zurich, Laboratory of Physical Chemistry, Switzerland

Long-range Rydberg dimers are molecules in which one of the atoms is in the ground state and the other one is in a highly excited electronic state, a Rydberg state. The binding of these molecules is caused by the elastic scattering of the Rydberg electron off the ground-state atom, which is located within the electron orbit. This type of molecular bond was initially proposed by Greene et al. [1] and has been observed experimentally in the photoassociation of ultracold rubidium and cesium atoms close to transitions to s, p and d Rydberg states [2]. We report on the formation of long-range Rydberg molecules correlated to np Rydberg states of cesium with principal quantum number n ranging from 26 to 34 [3]. The observed binding energies reach up to 400 MHz  and the internuclear separations exceed 45 nm at n=26. The photoassociation spectra reveal two types of molecular states recently predicted by Anderson et al. [4]: Bound states for which the binding results only from triplet s-wave scattering, and more weakly bound states with contributions from both singlet and triplet s-wave scattering. The experimental observations are well described by a model including s-wave scattering, the hyperfine interaction of the ground-state atom and the fine structure of the Rydberg atom. By adjusting only the values of the zero-energy singlet and triplet s-wave scattering lengths  we are able to reproduce the experimental binding energies within the experimental uncertainties. This yields the first experimental determination of the zero-energy singlet s-wave scattering length, which is in reasonable agreement with an extrapolation from higher-energy electron-Cs scattering data.

[1] C.H. Greene, A.S. Dickinson and H.R. Sadeghpour 2000, Phys. Rev. Lett. 85, 2458
[2] L.G. Marcassa and J.P. Shaffer 2014, Adv. At.Mol. Opt. Phys. 63, 47
[3] H. Sassmannshausen, F. Merkt and J. Deiglmayr 2015, Phys. Rev. Lett. 114, 133201
[4] D.A. Anderson, S.A. Miller and G. Raithel 2014, Phys. Rev. Lett. 112, 163201