Photodissociation of Trapped Rb_{2}^{+}: Implications for Simultaneous Trapping of Atoms and Molecular Ions

Spin-Conservation Propensity Rule for Three-Body Recombination of Ultracold Rb Atoms

We explore the physical origin and the general validity of a propensity rule for the conservation of the hyperfine spin state in three-body recombination. This rule was recently discovered for the special case of ^{87}Rb with its nearly equal singlet and triplet scattering lengths. Here, we test the propensity rule for ^{85}Rb for which the scattering properties are very different from ^{87}Rb. The Rb_{2} molecular product distribution is mapped out in a state-to-state fashion using resonance-enhanced multiphoton ionization detection schemes which fully cover all possible molecular spin states. Interestingly, for the experimentally investigated range of binding energies from zero to ∼13  GHz×h we observe that the spin-conservation propensity rule also holds for ^{85}Rb. From these observations and a theoretical analysis we derive an understanding for the conservation of the hyperfine spin state. We identify several criteria to judge whether the propensity rule will also hold for other elements and collision channels.

Observation of Chemical Reactions between a Trapped Ion and Ultracold Feshbach Dimers

We measure chemical reactions between a single trapped ^{174}Yb^{+} ion and ultracold Li_{2} dimers. This produces LiYb^{+} molecular ions that we detect via mass spectrometry. We explain the reaction rates by modeling the dimer density as a function of the magnetic field and obtain excellent agreement when we assume the reaction to follow the Langevin rate. Our results present a novel approach towards the creation of cold molecular ions and point to the exploration of ultracold chemistry in ion molecule collisions. What is more, with a detection sensitivity below molecule densities of 10^{14}  m^{-3}, we provide a new method to detect low-density molecular gases.

Electronic transitions in Rb2+ dimers solvated in helium

We have measured depletion spectra of the heteronuclear (85Rb87Rb+) dimer cation complexed with up to 10 He atoms. Two absorption bands are observed between 920 and 250 nm. The transition into the repulsive 12Σu+state of HeRb2+gives rise to a broad feature at 790 nm (12,650 cm−1); it exhibits a blueshift of 98 cm−1per added He atom. The transition into the bound 12Πustate of HeRb2+reveals vibrational structure with a band head at ≤ 15,522 cm−1, a harmonic constant of 26 cm−1, and a spin–orbit splitting of ≤ 183 cm−1. The band experiences an average redshift of − 38 cm−1per added He atom. Ab initio calculations rationalize the shape of the spectra and spectral shifts with respect to the number of helium atoms attached. For a higher number of solvating helium atoms, symmetric solvation on both ends of the Rb2+ion is predicted.

Photon-mediated charge exchange reactions between 39K atoms and 40Ca+ ions in a hybrid trap

We present experimental evidence of charge exchange between laser-cooled potassium ³⁹K atoms and calcium ⁴⁰Ca⁺ ions in a hybrid atom-ion trap and give quantitative theoretical explanations for the observations. The ³⁹K atoms and ⁴⁰Ca⁺ ions are held in a magneto-optical (MOT) and a linear Paul trap, respectively. Fluorescence detection and high resolution time of flight mass spectra for both species are used to determine the remaining number of ⁴⁰Ca⁺ ions, the increasing number of ³⁹K⁺ ions, and ³⁹K number density as functions of time. Simultaneous trap operation is guaranteed by alternating periods of MOT and ⁴⁰Ca⁺ cooling lights, thus avoiding direct ionization of ³⁹K by the ⁴⁰Ca⁺ cooling light. We show that the K-Ca⁺ charge-exchange rate coefficient increases linearly from zero with ³⁹K number density and the fraction of ⁴⁰Ca⁺ ions in the 4p ²P_1/2 electronically-excited state. Combined with our theoretical analysis, we conclude that these data can only be explained by a process that starts with a potassium atom in its electronic ground state and a calcium ion in its excited 4p ²P_1/2 state producing ground-state ³⁹K⁺ ions and metastable, neutral Ca (3d4p ³P₁) atoms, releasing only 150 cm^-1 equivalent relative kinetic energy. Charge-exchange between either ground- or excited-state ³⁹K and ground-state ⁴⁰Ca⁺ is negligibly small as no energetically-favorable product states are available. Our experimental and theoretical rate coefficients are in agreement given the uncertainty budgets.

Vibrational Quenching of Weakly Bound Cold Molecular Ions Immersed in Their Parent Gas

Hybrid ion–atom systems provide an excellent platform for studies of state-resolved quantum chemistry at low temperatures, where quantum effects may be prevalent. Here we study theoretically the process of vibrational relaxation of an initially weakly bound molecular ion due to collisions with the background gas atoms. We show that this inelastic process is governed by the universal long-range part of the interaction potential, which allows for using simplified model potentials applicable to multiple atomic species. The product distribution after the collision can be estimated by making use of the distorted wave Born approximation. We find that the inelastic collisions lead predominantly to small changes in the binding energy of the molecular ion.

Fast In Situ Observation of Atomic Feshbach Resonances by Photoassociative Ionization

We propose and experimentally investigate a scheme for observing Feshbach resonances in atomic quantum gases in situ and with a high temporal resolution of several tens of nanoseconds. The method is based on the detection of molecular ions, which are optically generated from atom pairs at small interatomic distances. As a test system we use a standard rubidium gas (^{87}Rb) with well known magnetically tunable Feshbach resonances. The fast speed and the high sensitivity of our detection scheme allows us to observe a complete Feshbach resonance within one millisecond and without destroying the gas.