Experimental Characterization of Singlet Scattering Channels in Long-Range Rydberg Molecules

Abstract

We observe the formation of long-range Cs2 Rydberg molecules consisting of a Rydberg and a ground-state atom by photoassociation spectroscopy in an ultracold Cs gas near 6s 1/2 (F =3,4)→np 3/2 resonances (n=26-34). The spectra reveal two types of molecular states recently predicted by D. A. Anderson, S. A. Miller, and G. Raithel [Phys. Rev. A 90, 062518 (2014)]: states bound purely by triplet s-wave scattering with binding energies ranging from 400 MHz at n=26 to 80 MHz at n=34, and states bound by mixed singlet-triplet s-wave scattering with smaller and F -dependent binding energies. The experimental observations are accounted for by an effective Hamiltonian including s-wave scattering pseudopotentials, the hyperfine interaction of the ground-state atom, and the spin-orbit interaction of the Rydberg atom. The analysis enabled the characterization of the role of singlet scattering in the formation of long-range Rydberg molecules and the determination of an effective singlet s-wave scattering length for low-energy electron-Cs collisions.Atoms in Rydberg states of high principal quantum number n are weakly bound systems and are extremely sensitive to their environment. In 1934, Amaldi and Segrè [1] observed the pressure-dependent shift and broadening of the Rydberg series of alkali atoms in a gas cell, an effect which was explained by Fermi [2] as originating from the elastic scattering between slow Rydberg electrons and ground-state atoms within the Rydberg orbit. He modeled the pressure shift using a pseudopotentialwhere a is the scattering length and |Ψ(R)| 2 the probability density of the Rydberg electron at the position R of the neutral perturber. Measurements of pressure shifts in Rydberg states thus provide information on the cross sections of elastic collisions between slow electrons and atoms and molecules [2,3]. Equation (1) implies the existence of oscillating interaction potentials between Rydberg and ground-state atoms [4,5]. A manifestation of such potentials are long-range diatomic molecules in which a ground-state atom having a negative s-wave scattering length is attached to a Rydberg atom at a distance corresponding to an antinode of Ψ(R), as was first pointed out by Greene et al. [6]. Such molecules were first observed experimentally by Bendkowsky et al. [7] following excitation of Rb atoms close to ns 1/2 Rydberg states with n=35-37. Later investigations led to the detection of long-range Rb 2 molecules correlated to np 1/2,3/2 (n=7-12) [8], nd 3/2,5/2 (n=34-40) [9], and nd 3/2,5/2 (n=40-49) [10] dissociation asymptotes and long-range Cs 2 molecules correlated to ns 1/2 (n=31-34) [11] and ns 1/2 (n=37,39,40) [12] asymptotes. The analysis of the experimental data confirmed the overall validity of Eq. (1), revealed contributions from triplet pwave scattering channels, and enabled the determination of triplet s-and p-wave scattering lengths that confirmed theoretical predictions [13].Singlet s-wave scattering lengths are expected to be either positive or much smaller than triplet scattering length...

Ultra-long-range Rydberg molecules in a divalent atomic system

“…The original observations were of dimers involving spherically symmetric S Rydberg states [2], but now measurements have been extended to anisotropic P [5,6] and D [7,8] Rydberg states and to molecules comprising one Rydberg atom and as many as four ground state atoms [9]. Excitation of Rb(nD) [9] and Cs(nP ) [6] Rydberg molecules has revealed considerable spectral complexity due to the Rydberg electron spin-orbit and ground-state hyperfine interactions, which results in mixing of singlet and triplet electronic symmetries. For bosonic alkaline-earth metal atoms, such as 84 Sr studied here, both these interactions are absent.…”

“…Gaps in laser coverage prevented measurement of spectra for n = 31 and n = 32. The binding energies of deeply bound states follow the approximate 1/(n − δ) 6 scaling initially seen in Rb [9], which reflects the scaling of the Rydberg-electron probability density |ψ ns (R)| 2 at the location of the groundstate Sr atom. Deviations from the scaling are evident for weakly bound states and are clearly seen in high-quality spectra for n = 30.…”

“…1) are relatively simple because of the closed-shell 1 S 0 electronic-ground state and lack of nuclear spin for 84 Sr. Atom loss is an indirect method of detecting excitation compared to the traditional technique of pulsed-field ionization and chargedparticle detection [2,[4][5][6][7]9], but it yields a high signalto-noise ratio. A comprehensive understanding of the decay channels of ultralong-range molecules is still lacking, but a large fraction of excitations should lead to measurable ground-state atom loss.…”

Lifetimes of ultralong-range strontium Rydberg molecules in a dense Bose-Einstein condensate

“…Initial experiments focused on creation of dimer molecules comprising a ground-state rubidium atom bound to a spherically-symmetric Rb(ns) Rydberg atom [2]. Measurements have since been extended to include other Rydberg species, anisotropic P and D Rydberg states and, using Cs(ns) Rydberg atoms, to the creation of so-called trilobite states with very large permanent electric dipole moments [3][4][5][6][7][8][9][10][11][12]. Detailed spectroscopic studies have revealed the formation of molecules comprising one Rydberg atom and up to four bound ground-state atoms [13].…”

Electronic structure of ultralong-range Rydberg penta-atomic molecules with two polar diatomic molecules

“…These ultralong-range Rydberg molecules were first experimentally observed for Rb atoms in a swave Rydberg state [14], and current experiments focus on exploring these Rydberg molecules formed by higher angular momentum Rydberg states [15, 17? -20], and by other atomic species such as Cs [18,19] and Sr [21,22]. In a mixture of ultracold atoms and ultracold molecules, Rydberg atoms could be created by standard two-photon excitation schemes.…”