Observation of spin singlet butterfly Rydberg molecules in an ultracold atomic Rb gas

Abstract

We report the observation of spin-singlet ultra-long range Rydberg butterfly molecules consisting of a ground-state atom bound to a Rydberg atom by $P$-wave scattering of $^{87}$Rb Rydberg electrons from $^{87}$Rb(5s) atoms. A three-photon excitation scheme enables the photoassociation of these molecules by weakly admixing Rb($18f_{7/2}$) states. The measured binding energies, kilo-Debye permanent electric dipole moments, and lifetimes are in excellent agreement with theory. Two long-lived vibrational levels, red detuned from the Rb($18f_{7/2}$) threshold, are observed. This experiment is a foundational step in the production of ultra-cold anions and heavy Rydberg ion-pair systems.

Figures (4)

• Figure 1: a) Born-Oppenheimer potential energy curves for the $n=18$, $\Omega = 3/2$$^{87}$Rb$_2$ ULRM. The black curve highlights the potential curve with $M_L \approx 0$ whose oscillations are deep enough to support vibrational states. The red box marks the first butterfly well, magnified in the inset with the calculated vibrational states. b) Electron density distribution of the spin-singlet butterfly molecule.
• Figure 2: Ion Rb$^+$ signal (binned) as a function of frequency detuning relative to the Rb(18$f_{7/2}$) threshold. The first two molecular singlet butterfly resonances are magnified in the two insets. The calculated eigenenergies of all three vibrational states are shown as dashed vertical lines. Please note that the x-axes (detuning) of the insets take the corresponding molecular resonances as their energy zeros, not the atomic resonance (like in the main figure).
• Figure 3: Measurement of the dipole moment for the $\nu=0$ butterfly state. a) Spectrum of the molecular resonance for different electric fields fitted with a convolution of a Lorentzian and two step functions of width $2dE$. b) Width $dE$ as a function of the electric field fitted by $d\sqrt{E^2+E_0^2}$ to obtain the dipole moment of $d=851 \pm 104 \, \mathrm{Debye}$.
• Figure 4: Normalized ion counts as a function of the delay between excitation and ionization for the two molecular and the atomic $18f_{7/2}$ state. The high $\ell$-admixture leads to an increased lifetime of the molecule. The shorter lifetime for the $\nu=1$ state is likely due to the avoided crossings with $M_L=\pm 1$ potentials.