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International Symposium on Molecular Spectroscopy (ISMS)

MOLECULE FORMATION AND STATE-CHANGING COLLISIONS OF SINGLE RYDBERG ATOMS IN A BEC

K. S. Kleinbach, Chris H. Greene, Jesús Pérez‐Ríos, Tilman Pfau, Sebastian Hofferberth, Robert Löw, Fabian Böttcher, Karl Westphal, Tara Cubel Liebisch, Michael Schlagmüller

Published Jun 23, 2015• 0 citations
Publisher

Abstract

A single Rydberg excitation in the high-density and low-temperature environment of a Bose-Einstein condensate (BEC) leads to a fascinating testbed of low-energy electron-neutral and ion-neutral scattering. In particular the small interparticle spacing in a BEC makes it possible to study the role of ion-neutral interactions in l-changing collisions on time scales much shorter than the Rydberg lifetime. We take advantage of the mean field density shift, caused by elastic electron-neutral collisions, to probe density dependent shells of the $^{87}$Rb BEC and thereby measure the l-changing collision time versus density and principal quantum number. We report on l-changing collisions due to inelastic scattering of the Rydberg electron with a neutral atom located near the Rydberg ionic core. We measure timescales of both the l-changing collision and the Rb$_{2}$ molecule formation of less than one microsecond for n $<$ 100 at the highest BEC densities. We extract a change in kinetic energy of the Rydberg atoms that matches well with the energy gap to the next-lowest manifold. We measure Rb$_{2}$ signal that decreases with increasing principal quantum number. The mechanism and timescales of the l-changing collision are compared with simulations including the motion of the ionic core and neutral atoms, as well as the Rydberg electron.

DOI
10.15278/isms.2015.tj12

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A single Rydberg excitation in the high-density and low-temperature environment of a Bose-Einstein condensate (BEC) leads to a fascinating testbed of low-energy electron-neutral and ion-neutral scattering. In particular the small interparticle spacing in a BEC makes it possible to study the role of ion-neutral interactions in l-changing collisions on time scales much shorter than the Rydberg lifetime. We take advantage of the mean field density shift, caused by elastic electron-neutral collisions, to probe density dependent shells of the 87^{87}87Rb BEC and thereby measure the l-changing collision time versus density and principal quantum number. We report on l-changing collisions due to inelastic scattering of the Rydberg electron with a neutral atom located near the Rydberg ionic core. We measure timescales of both the l-changing collision and the Rb2_{2}2​ molecule formation of less than one microsecond for n <<< 100 at the highest BEC densities. We extract a change in kinetic energy of the Rydberg atoms that matches well with the energy gap to the next-lowest manifold. We measure Rb2_{2}2​ signal that decreases with increasing principal quantum number. The mechanism and timescales of the l-changing collision are compared with simulations including the motion of the ionic core and neutral atoms, as well as the Rydberg electron.
We present spectroscopy of a single Rydberg atom excited within a Bose-Einstein condensate. We not only observe the density shift as discovered by Amaldi and Segrè in 1934, but a line shape that changes with the principal quantum number n. The line broadening depends precisely on the interaction potential energy curves of the Rydberg electron with the neutral atom perturbers. In particular, we show the relevance of the triplet p-wave shape resonance in the e^{-}-Rb(5S) scattering, which significantly modifies the interaction potential. With a peak density of 5.5×10^{14} cm^{-3}, and therefore an interparticle spacing of 1300 a_{0} within a Bose-Einstein condensate, the potential energy curves can be probed at these Rydberg ion-neutral atom separations. We present a simple microscopic model for the spectroscopic line shape by treating the atoms overlapped with the Rydberg orbit as zero-velocity, uncorrelated, pointlike particles, with binding energies associated with their ion-neutral separation, and good agreement is found.
Within a dense environment (ρ ≈ 10 14 atoms=cm 3 ) at ultracold temperatures (T < 1 μK), a single atom excited to a Rydberg state acts as a reaction center for surrounding neutral atoms.At these temperatures, almost all neutral atoms within the Rydberg orbit are bound to the Rydberg core and interact with the Rydberg atom.We have studied the reaction rate and products for nS 87 Rb Rydberg states, and we mainly observe a state change of the Rydberg electron to a high orbital angular momentum l, with the released energy being converted into kinetic energy of the Rydberg atom.Unexpectedly, the measurements show a threshold behavior at n ≈ 100 for the inelastic collision time leading to increased lifetimes of the Rydberg state independent of the densities investigated.Even at very high densities (ρ ≈ 4.8 × 10 14 cm -3 ), the lifetime of a Rydberg atom exceeds 10 μs at n > 140 compared to 1 μs at n ¼ 90.In addition, a second observed reaction mechanism, namely, Rb þ 2 molecule formation, was studied.Both reaction products are equally probable for n ¼ 40, but the fraction of Rb þ 2 created drops to below 10% for n ≥ 90.
A single Rydberg atom impurity excited in a BEC is a system that can be utilized to measure the quantum mechanical properties of electron - neutral scattering andthe electron probability density of a Rydberg atom. The Rydberg electron – neutral atom scattering process, is a fundamental scattering process, which can be described via Fermi’s pseudopotential as V{vec{r},vec{R} )=2pi textit{a}[k(R)]delta^{(3)}(vec{r}-vec{R}). The scattering length is dependent on the momentum of the Rydberg electron, and therefore is dependent on the separation of the Rydberg electron from the ion core. At the classical outermost turning point of the electron, it has the slowest momentum leading to s-wave dominated scattering potentials 10’s of MHz in depth for n<40 (Greene et al. PRL 85 2458 (2000), Bendkowsky et al. PRL 105 163201 (2010)). In alkali atoms there is a shape resonance for p-wave scattering, which becomes relevant at ion-neutral separations of ~75nm (I.I. Fabrikant J.Phys B 19, 1527 (1985)). This shape resonance potential is several GHz deep, spanning the energy level spacing between n and n-1 principal quantum numbers. At high BEC densities of 5x10^14cm-3 the nearest neighbor spacing is less than 70nm. A Rydberg atom excited within a BEC, is an excitation of the Rydberg atom and all N neutral atoms located within the Rydberg orbit, described as nS+N x 5S. The nS+N x 5S state is density shifted from the Rydberg resonance. Not only does the distribution of atoms within the Rydberg orbit lead to a density shift, but, at these high densities, atoms excited in the nS+N x 5S state near the shape resonance potential cause large perturbations to the density shift, leading to a line broadening. Therefore the spectroscopic line shape of a Rydberg atom in a BEC allows us to probe the theoretically calculated p-wave shape resonance potential. Furthermore, we can observe and measure the dynamics of neutrals excited in the nS+N x 5S state. In the ultracold regime of a BEC, the background neutral atoms within the Rydberg orbit have kinetic energies of a few kHz, and experience large forces due to the GHz-deep shape resonance potentials. An atom dragged into this deep potential leads to an exothermic state-changing collision. We measure the timescale of this state-changing collision and compare to semi-classical calculations of the neutral atoms evolving in the potential of the two-particle nS+ 5S system. We also measure the change in energy from the original nS state to the product state, (n-4)L (L<3). On time scales shorter than the state-changing collisions, which for n<100 is on the order of 10 microseconds, the neutral atoms will evolve and collect in the shallower electron-neutral potentials, which mimic the electron probability density of the Rydberg atom.With n<100, the Rydberg atom has a diameter greater than 2 micrometers. With an imaging system with <1 micrometer resolution, we expect to observe a macroscopic change in the density profile of the BEC indicating an nS versus nD Rydberg state. The BEC would serve as a contrast agent for observing textbook atomic wavefunctions (Karpiuk et al. New Journal of Physics 17, 053046 (2015)).
The lifetimes and decay channels of ultralong-range Rydberg molecules created in a dense Bose-Einstein condensate are examined by monitoring the time evolution of the Rydberg population using field ionization. Studies of molecules with values of the principal quantum number, nnn, in the range n=49n=49n=49 to n=72n=72n=72 that contain tens to hundreds of ground-state atoms within the Rydberg electron orbit show that their presence leads to marked changes in the field ionization characteristics. The Rydberg molecules have lifetimes of $\ensuremath{\sim}1\ensuremath{-}5\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{s}$, their destruction being attributed to two main processes: formation of Sr2+{\mathrm{Sr}}_{2}{}^{+}Sr2​+ ions through associative ionization and dissociation induced through LLL-changing collisions. The observed loss rates are consistent with a reaction model that emphasizes the interaction between the Rydberg core ion and its nearest-neighbor ground-state atom. The measured lifetimes place strict limits on the time scales over which studies involving Rydberg species in cold, dense atomic gases can be undertaken and limit the coherence times for such measurements.
We investigate inelastic collision dynamics of a single cold ion in a Bose-Einstein condensate. We observe rapid ion-atom-atom three-body recombination leading to the formation of weakly bound molecular ions followed by secondary two-body molecule-atom collisions quenching the rovibrational states towards deeper binding energies. In contrast to previous studies exploiting hybrid ion traps, we work in an effectively field-free environment and generate a free low-energy ionic impurity directly from the atomic ensemble via Rydberg excitation and ionization. This allows us to implement an energy-resolved field-dissociation technique to trace the relaxation dynamics of the recombination products. Our observations are in good agreement with numerical simulations based on Langevin capture dynamics and provide a complementary means to study stability and reaction dynamics of ionic impurities in ultracold quantum gases.
The lifetimes and decay channels of ultralong-range Rydberg molecules created in a dense Bose-Einstein condensate (BEC) are examined by monitoring the time evolution of the Rydberg population using field ionization. The Rydberg molecules, which contain tens to hundreds of ground state atoms within the electron orbit, have lifetimes of ∼ 1 to 5 µ s, their destruction being attributed to two main processes: formation of ions through associative ionization, and dissociation induced through L -changing reactions. The observed loss rates are consistent with a reaction model that emphasizes the interaction between the Rydberg core ion and its nearest neighbor ground state atom. The application of this model to earlier measurements of strontium dimer lifetimes at lower densities is discussed.