Session T4: Cold Collisions

Chemical Reactions of Li and CaH at 1 Kelvin

Using cryogenic helium buffer-gas cooling, we have prepared dense samples of atomic lithium and molecular calcium monohydride at temperatures as low as 1 Kelvin. We have measured the Li~+~CaH~$\rightarrow$~LiH~+~Ca chemical reaction, observed in both the accelerated disappearance of CaH and in the appearance of the LiH molecule. For unpolarized reactants, we have measured a reaction rate coefficient of $3.6\times10^{-10}$~cm$^3$~s$^{-1}$, with an uncertainty of a factor of 2. The methods of reaction rate measurement and detection of LiH molecules will be presented.   

High Partial Wave Multichannel Quantum Defect Theory for Cold Collisions

We introduce a formulation of multichannel quantum defect theory which is numerically stable for high partial waves at ultracold energies and use this formulation to study Fano-Feshbach resonances in alkali atom collisions. Fano-Feshbach resonances have been observed for a variety of cold collisions, and many of these have been well described by theoretical models. Such measurement of high partial wave resonances could be used to improve precision of theoretical models of scattering. These resonances can be calculated using a full close-coupling scheme, but this method becomes very time-consuming as the number of partial waves grows and the resolution of magnetic field is increased. Multichannel quantum defect theory allows for a much faster and quantitatively accurate calculation of these resonances for alkali atoms.   

Investigation of a hybrid quantum system of ultracold atoms and trapped ions

Hybrid quantum experiments with single ions immersed in quantum gases are starting to be used as versatile systems for experiments in quantum information science, atomic physics and cold chemistry. We deterministically position radio-frequency trapped $^{174}$Yb$^{+}$ ions inside a Bose Einstein condensate of $^{87}$Rb atoms and achieve independent control on the motional and internal states of both species. We investigate the fundamental atom-ion interactions by characterizing elastic and inelastic collisions and measure their energy-dependent collision rates. In the presence of near resonant light interactions between both species are strongly modified, leading to inelastic scattering rates that are more than three orders of magnitude higher compared to collisions in the ground states. We analyze the process at the single particle level with ion trap mass spectroscopy to identify the underlying interaction channels. The emerging understanding of the state-dependent interactions between the two quantum systems paves the way for applications in quantum information science and cold-matter research.   

The role of electronic excitation in ultracold atom-ion chemistry

The role of electronic excitation in chemical reactions between ultracold Ca atoms and Ba$^+$ ions confined in a hybrid trap is studied. This prototypical system is energetically precluded from reacting in its ground state, allowing a particularly simple interpretation of the data. It is found that while electronic excitation of the ion can critically influence the chemical reaction rate, electronic excitation in the neutral atom is relatively unimportant. It is experimentally demonstrated that with the correct choice of the atom-ion pair it is possible to mitigate the unwanted effects of these chemical reactions in ultracold atom-ion environments, marking a crucial step towards the next generation of hybrid devices.   

Interspecies probe of Feshbach molecule formation and stability

Feshbach resonances are an integral tool in ultracold atomic physics allowing for two-body interaction tuning and molecular dimer formation. The two lowest energy states of the $^6$Li atom exhibit a broad Feshbach resonance at 834 Gauss which can be utilized to link pairs of atoms into dimers. We study the formation and dynamics of shallow Li$_2$ Feshbach dimers in the presence of a second species, $^{174}$Yb, at ultracold temperatures. The collisional stability of the Li-Yb mixture is adequate to allow time-resolved studies of these interactions. We will report on the observed modifications of Li$_2$ formation and stability due to the presence of Yb, as well as the concomitant effect on the Yb gas.   

Recent results on a new method for producing ultracold molecular ions

We present recent results from our experimental effort to produce ultracold, internal ground-state BaCl$^{+}$ ions using a Ca MOT. The method utilizes sympathetic cooling due to the strong collisions between co-trapped molecular ions and laser-cooled neutral atoms which should efficiently cool both the internal and external molecular ion degrees of freedom. Samples of such ultracold molecular ions find applications in ultracold chemistry, precision measurement and quantum computation.   

Non-reactive collisions of sodium and silver atoms with nitrogen molecules

We present a quantal study of elastic and rotationally inelastic collisions of Na and Ag with N$_2$ for energies between 0.1 and 5000 cm$^{-1}$. We obtain the two-dimensional potential energy surface of the ground state of the NaN$_2$ and AgN$_2$ complexes using CCSD(T) methods with the nitrogen molecule frozen at its equilibrium geometry. Using these potentials, we compute the rotationally elastic and inelastic scattering, differential, and momentum transfer cross sections for several initial rotational levels using the close-coupling approach. We also investigate the temperature dependence of the rates corresponding to these collisions. We discuss the importance of sodium-nitrogen collisions in the study of laser guide stars as well as the possibility of substituting Ag for Na in experiments by comparing the cross sections and rates for both systems.   

Zeeman Relaxation in Cold Aluminum--Helium and Antimony--Helium Collisions

We present the combined experimental and theoretical study of aluminum--helium and antimony--helium collisions at 800mK. Zeeman relaxation in atom--helium collisions can serve as a probe of the atom--helium interaction potentials. The relaxation mechanisms are different for the two species. In the case of aluminum, due to a spherical electron distribution, Zeeman relaxation is expected to be slow in the pure $\,^2 P_{1/2}$ ground state. However, during a collision the anisotropic $\,^2 P_{3/2}$ excited state is mixed with the ground state, causing rapid relaxation. Our results further confirm the theoretical model previously developed for indium and gallium. In the case of antimony, despite being nominally a spherical S--state ($\,^4 S_{3/2}$), spin--orbit coupling mixes states with nonzero angular moment into the ground state, and hence introduces electronic anisotropy into its interaction with helium. This work extends our understanding of cold collisions in pnictogens.   

Charge transfer processes in ultracold atom-ion collisions

We investigate charge transfer processes occurring in ultracold collisions of atoms and ions, and explore the effect of external magnetic fields. Our calculations include hyperfine interactions between the ion and the neutral atoms. We discuss how these interactions affect scattering processes, and may lead to detectable resonances. These resonances can be used to control charge transfer which may have applications to quantum information processing. We present results for collisions between various alkaline-earth atom-ion systems.   

Single trapped ions sympathetically cooled by ultracold atoms

We investigate the interaction of single trapped ions ($^{138}$Ba$^{+}$ or $^{87}$Rb$^{+}$) with an ultracold cloud of optically confined $^{87}$Rb atoms. In these experiments, the ion is held in a linear Paul trap and is immersed in the center of the atomic cloud. The atom-ion interaction gives rise to a strong and long-range $\frac{1}{r^4}$ polarization potential yielding novel and complex interaction dynamics. Charge transfer processes and elastic scattering have been observed at millikelvin collision energies [1,2], the energy scale being set by the trap-driven excess micromotion of the ion. Using improved field compensation techniques, we reduce the energy of the excess micromotion to the Ba$^{+}$ sub-Doppler regime ($\approx k_B 300\,\mu$K) and examine the influence of ion micromotion energy over a wide range. In performing these experiments on $^{87}$Rb$^{+}$ ions we show the applicability of this buffer-gas cooling method to ionic species not amenable to laser cooling. By decreasing the ion energy even further we are aiming at novel experiments, such as the production of ultracold, charged molecules in a well-defined quantum state.\\[4pt] [1] S. Schmid et al, Phys. Rev. Lett. \textbf{105}, 133202 (2010)\\[0pt] [2] C. Zipkes et al, Phys. Rev. Lett. \textbf{105}, 133201 (2010)