Modeling Photoassociative Spectra of Ultracold NaK + K.

A model for photoassociation of ultracold atoms and molecules is presented and applied to the case of 39K and 23Na39K bosonic particles. The model relies on the assumption that photoassociation is dominated by long-range atom-molecule interactions well outside the chemical bond region. The frequency of the photoassociation laser is chosen close to a bound-bound rovibronic transition from the X1Σ+ ground state toward the metastable b3Π lowest excited state of 23Na39K, allowing us to neglect any other excitation, which could hinder the photoassociation detection. The energy level structure of the long-range 39K···23Na39K excited super-dimer is computed in the space-fixed frame by solving coupled-channel equations, involving the coupling between the 23Na39K internal rotation and the mechanical rotation of the super-dimer complex. A quite rich structure is obtained, and the corresponding photoassociation rates are presented. Other possible photoassociation transitions are discussed in the context of the proposed model.

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.

Tailored Single-Atom Collisions at Ultralow Energies.

We employ collisions of individual atomic cesium (Cs) impurities with an ultracold rubidium (Rb) gas to probe atomic interaction with hyperfine- and Zeeman-state sensitivity. Controlling the Rb bath's internal state yields access to novel phenomena observed in interatomic spin exchange. These can be tailored at ultralow energies, owing to the excellent experimental control over all relevant energy scales. First, detecting spin-exchange dynamics in the Cs hyperfine-state manifold, we resolve a series of previously unreported Feshbach resonances at magnetic fields below 300 mG, separated by energies as low as h×15 kHz. The series originates from a coupling to molecular states with binding energies below h×1 kHz and wave function extensions in the micrometer range. Second, at magnetic fields below ≈100 mG, we observe the emergence of a new reaction path for alkali atoms, where in a single, direct collision between two atoms two quanta of angular momentum can be transferred. This path originates from the hyperfine analog of dipolar spin-spin relaxation. Our work yields control of subtle ultralow-energy features of atomic collision dynamics, opening new routes for advanced state-to-state chemistry, for controlling spin exchange in quantum many-body systems for solid-state simulations, or for determination of high-precision molecular potentials.

State-to-state chemistry for three-body recombination in an ultracold rubidium gas.

Experimental investigation of chemical reactions with full quantum state resolution for all reactants and products has been a long-term challenge. Here we prepare an ultracold few-body quantum state of reactants and demonstrate state-to-state chemistry for the recombination of three spin-polarized ultracold rubidium (Rb) atoms to form a weakly bound Rb2 molecule. The measured product distribution covers about 90% of the final products, and we are able to discriminate between product states with a level splitting as small as 20 megahertz multiplied by Planck's constant. Furthermore, we formulate propensity rules for the distribution of products, and we develop a theoretical model that predicts many of our experimental observations. The scheme can readily be adapted to other species and opens a door to detailed investigations of inelastic or reactive processes.

The B(1)Π and D(1)Π states of LiRb.

We studied the molecule LiRb in the gas phase with high resolution by Fourier-transform spectroscopy of laser induced fluorescence. The spectra were assigned to transitions between the ground state X(1)Σ(+) and B(1)Π or D(1)Π states and showed perturbations. For levels with e symmetry the coupling to the nearby state C(1)Σ(+) was included in the analysis by means of coupled channel calculations. The evaluation gives potential energy curves for all three electronic states and the coupling functions for B-C coupling, which are related to the expectation value of the electronic orbital angular momentum operator L(+) or L(-). The same coupling between C and D states is considered, but is not yet as fixed as in the case B-C because of lack of data. The model was extended to include the Λ-doubling by distant electronic states through effective q-parameters, but their interpretation is incomplete because of several possible perturbing states and too few data.

Spectroscopic study of the 2(2)Σ+ and the 4(2)Σ+ excited states of LiCa.

The 2(2)Σ(+) and 4(2)Σ(+) excited states of (7)Li(40)Ca have been studied by high resolution Fourier-transform spectroscopy. The data on the lower state, 2(2)Σ(+), were obtained by analyzing the rotationally resolved spectra of the thermal emission of LiCa in the 2(2)Σ(+) → X(2)Σ(+) band around 9500 cm(-1). These data contained transitions mainly from v' = 0 and 1 for N' up to 92 and allowed us to derive molecular parameters describing the potential curve of the state close to its minimum. The dataset on the second state, 4(2)Σ(+), is much larger and comes from a laser-induced fluorescence experiment. The levels were excited by a single mode dye laser and the 4(2)Σ(+) → X(2)Σ(+) fluorescence was recorded through a Fourier-transform spectrometer. For both states potential energy curves and Dunham coefficients were derived and the spin-rotation structure was evaluated. The results are compared with theoretical and experimental data from the literature.

The X2Σ+ state of LiCa studied by Fourier-transform spectroscopy.

The paper reports on a successful observation of high resolution Fourier transform spectra of LiCa. The fine structure of the ground state was observed and attributed to effective spin-rotation interaction. The experimental observations are described by two models using potential energy curves. One of them takes into account the fine structure splitting by means of effective constants, the other by means of a R dependent function γ(R), built in the radial Schrödinger equation. Ab initio calculations were performed for γ(R) which comes close to the experimental function.

The X 1Σ+ state of LiRb studied by Fourier-transform spectroscopy.

We report on the first experimental observation of the ground electronic state of the LiRb molecule at high resolution. A large body of experimental data was collected which led to an accurate potential energy curve for this state. Transitions to the lowest triplet state were also observed, but these data are rather fragmentary and allow only a first attempt for the description of this state. Both potentials were used for evaluating published Feshbach resonances of this molecule. We compare the results of this study with those of the related LiK and LiCs molecules.