Observation of Cesium (nD_5/2+6S_1/2) Ultralong-Range Rydberg-Ground Molecules

Ultralong-range Cs_2 Rydberg-ground molecules (nD_5/2 + 6S_1/2F) (33 \leq n \leq 39, F=3 or 4) are investigated by a two-photon photo-association spectroscopy of an ultracold Cs gas. Two vibrational ground molecular spectra of triplet ^3\!\varSigma and hyperfine mixed singlet-triplet ^1,3\!\varSigma molecular states and their corresponding binding energies are attained. The experimental observations are simulated by an effective Hamiltonian including low energy electron scattering pseudopotentials, the spin-orbit interaction of the Rydberg atom, and the hyperfine interaction of the ground-state atom. The zero-energy singlet and triplet s-wave scattering lengths are extracted by comparing the experimental observations and calculations. Dependences of the measured binding energies on the effective principal quantum number, n_\rm eff = n - \delta_D (\delta_D is the quantum defect of Rydberg D state), yield the scaling of n_\rm eff^-5.60 \pm 0.16(^3\!\varSigma,F=3), n_\rm eff^-5.62 \pm 0.16(^3\!\varSigma,F=4) for deep triplet potential and n_\rm eff^-5.65 \pm 0.38(^1,3\!\varSigma, F=3), n_\rm eff^-6.19 \pm 0.14(^1,3\!\varSigma,F=4) for shallow mixed singlet-triplet potential well. The simulations of low-energy Rydberg electron scattering show agreement well with the experimental measurements.