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...

Transitions between high Rydberg states of Cs atoms have been studied by high-resolution millimeter-wave spectroscopy of an ultracold sample. The spectroscopic measurements were performed after loading the atoms into a magneto-optical trap. Switching off all trapping fields and compensating the stray electric and magnetic fields to below 1 mV/cm and 2 mG, respectively, prior to the spectroscopic measurement enabled the recording of millimeter-wave spectra of Rydberg states with principal quantum number beyond n = 100 under conditions where the inhomogeneous broadening by stray fields is minimal and no dephasing of the Rydberg-atom sample can be detected over measurement times up to 60 µs. The Fourier-transform-limited linewidths of better than 20 kHz enabled the observation of the hyperfine structure of nS and nP Rydberg states of Cs beyond n = 90. The analysis of the lineshapes of transitions to Rydberg states with n ≈ 230 indicated that field inhomogeneities across the atomic sample represent the dominant cause of spectral broadening at high n values. The analysis also revealed that the initial polarization of the atomic sample (F = 4, M F = 4) is preserved for several tens of microseconds, the depolarization being caused by slow precession along the magnetic stray fields.

We observe the direct excitation of pairs of Cs atoms from the ground state to molecular states correlating asymptotically to nsn'f asymptotes. The molecular resonances are interpreted as originating from the dipole-quadrupole interaction between the nsn'f pair states and close-by npnp asymptotes (22≤n≤32). This interpretation is supported by Stark spectroscopy of the pair states and a detailed modeling of the interaction potentials. The dipole-quadrupole interaction mixes electronic states of opposite parity and, thus, requires a coupling between electronic and nuclear motion to conserve the total parity of the system. This non-Born-Oppenheimer coupling is facilitated by the near-degeneracy of even- and odd-L partial waves in the atom-atom scattering which have opposite parity.

Hydrogen atoms in Rydberg states with principal quantum numbers between 23 and 70 have been accelerated, decelerated, and electrostatically trapped using a surface-electrode Rydberg-Stark decelerator. By applying a set of oscillating electrical potentials to a two-dimensional array of electrodes on a printed circuit board (PCB), a continuously moving, three-dimensional electric trap with a predefined velocity and acceleration is generated. From an initial longitudinal velocity of 760  m/s, final velocities of the Rydberg atoms ranging from 1200  m/s to zero velocity in the laboratory-fixed frame of reference were achieved. Accelerated or decelerated atoms were detected directly by pulsed electric-field ionization. Atoms trapped at zero mean velocity above the PCB were reaccelerated off the PCB before field ionization.

Long-range metastable molecules consisting of two cesium atoms in high Rydberg states have been observed in an ultracold gas. A sequential three-photon two-color photoassociation scheme was employed to form these molecules in states, which correlate to np(n + 1)s dissociation asymptotes. Spectral signatures of bound molecular states are clearly resolved at the positions of avoided crossings between long-range van der Waals potential curves. The experimental results are in agreement with simulations based on a detailed model of the long-range multipole-multipole interactions of Rydbergatom pair states. We show that a full model is required to accurately predict the occurrence of bound Rydberg macrodimers. The macrodimers are distinguished from repulsive molecular states by their behavior with respect to spontaneous ionization and possible decay channels are discussed.A new regime of ultracold chemistry has been established by the possibility to routinely produce ultracold atomic samples with translational temperatures below 1 mK using laser cooling [1,2] and trapping [3,4]. The characteristic feature of this regime is that reactions are not driven by thermodynamical quantities, i.e. temperature or pressure, but by the precise manipulation of the internal quantum states of the constituents and the interactions between them. Landmark results in the emerging field of ultracold chemistry include the formation of ultracold molecules in the absolute ground state using photoassociation [5][6][7] or magneto-association [8,9], the control of chemical reactions by manipulating the quantum statistics of the reactants [10,11], or the photodissociation of molecules with full control over reactant and product channels [12]. The enabling factor in all these studies is the control of long-range interaction potentials, which dominate the dynamics at low collision energies.An extreme case of long-range interactions are van der Waals forces between two atoms in Rydberg states, scaling in the case of an off-resonant dipole-dipole interaction with the internuclear separation R as R −6 and with the principal quantum number n as n 11 [13]. Metastable molecular states with internuclear separations exceeding 1 µm, so called macrodimers, are predicted to result from these interactions [14][15][16]. The molecular potential minima supporting these bound states arise from avoided crossings between long-range potential-energy curves correlating to different dissociation asymptotes of the doubly-excited dimers (see Fig. 1). Astonishingly, such macrodimers are predicted to have lifetimes limited by radiative decay of the constituent atomic Rydberg states [17], even though they are energetically located in the Cs + Cs + + e − ionization continuum and the density of electronic states is extremely high, two conditions one would expect to lead to fast autoionization [18]. Experimentally, these molecular states have remained elusive. Molecular resonances were observed in the Rydberg-excitation spectrum of ultracold gases and were identified as arisi...