Person:

Dalgarno, Alexander

An efficient method is presented for rigorous quantum calculations of atom-molecule and molecule-molecule collisions in a magnetic field. The method is based on the expansion of the wavefunction of the collision complex in basis functions with well-defined total angular momentum in the body-fixed coordinate frame. We outline the general theory of the method for collisions of diatomic molecules in the (^{2}\Sigma) and (^{3}\Sigma) electronic states with structureless atoms and with unlike (^{2}\Sigma) and (^{3}\Sigma) molecules. The cross sections for elastic scattering and Zeeman relaxation in low-temperature collisions of CaH((^{2}\Sigma^{+})) and NH((^{3}\Sigma^{-})) molecules with (^{3})He atoms converge quickly with respect to the number of total angular momentum states included in the basis set, leading to a dramatic >10-fold enhancement in computational efficiency compared to the previously used methods [A. Volpi and J. L. Bohn, Phys. Rev. A 65, 052712 (2002); R. V. Krems and A. Dalgarno, J. Chem. Phys. 120, 2296 (2004)]. Our approach is thus well suited for theoretical studies of strongly anisotropic molecular collisions in the presence of external electromagnetic fields.

A rigorous quantum theory of atomic collisions in the presence of radio frequency (rf) magnetic fields is developed and applied to elucidate the effects of combined dc and rf magnetic fields on ultracold collisions of Rb atoms. We show that rf fields can be used to induce Feshbach resonances, which can be tuned by varying the amplitude and frequency of the rf field. The rf-induced Feshbach resonances occur also in collisions of atoms in low-field-seeking states at moderate rf field strengths easily available in atom chip experiments, which opens up the world of tunable interactions to magnetically trappable atomic quantum gases.

We present a theoretical study of molecular-trap loss induced by collisions with slow atomic beams based on an explicit analysis of collision kinematics in the laboratory frame and a rigorous quantum description of atom-molecule scattering in external fields. The theory is applied to elucidate the effects of nonuniform magnetic and optical trapping fields on low-temperature collisions of OH ((J=\frac{3}{2},M_J=\frac{3}{2},f)) molecules with (^{4})He atoms. Our calculations quantify the extent to which both elastic and inelastic cross sections are suppressed by external trapping fields, clarify the role of small-angle scattering in trap loss, and may benefit future experiments on collisional cooling of molecules in electromagnetic traps. The calculated cross sections for trap loss in (^{4})He + OH collisions are consistent with recent experimental observations at low beam energies [ B. C. Sawyer (et al.) Phys. Rev. Lett. 101 203203 (2008)], demonstrating the importance of including the effects of nonuniform trapping fields in theoretical simulations of cold collision experiments with trapped molecules and slow atomic beams.

We present an experimental and theoretical study of atom-molecule collisions in a mixture of cold, trapped N atoms and NH molecules at a temperature of ∼600  mK. We measure a small N + NH trap loss rate coefficient of (\kappa_{loss^{(N+NH)}}) = 9(5)(3)×10(^{-13})  cm(^3) s(^{-1}). Accurate quantum scattering calculations based on ab initio interaction potentials are in agreement with experiment and indicate the magnetic dipole interaction to be the dominant loss mechanism. Our theory further indicates the ratio of N + NH elastic-to-inelastic collisions remains large (>100) into the mK regime.

We report a combined experimental and theoretical study of collision-induced dipolar relaxation in a cold spin-polarized gas of atomic nitrogen (N). We use buffer gas cooling to create trapped samples of (^{14})N and (^{15})N atoms with densities (5(\pm)2) × (10^{12}) (cm^{-3}) and measure their magnetic relaxation rates at milli-Kelvin temperatures. These measurements, together with rigorous quantum scattering calculations based on accurate (ab) (initio) interaction potentials for the (^{7}\Sigma^{+}{u}) electronic state of (N{2}) demonstrate that dipolar relaxation in N+N collisions occurs at a slow rate of ~(10^{-13}) (cm^{3})/s over a wide range of temperatures (1 mK to 1 K) and magnetic fields (10 mT to 2 T). The calculated dipolar relaxation rates are insensitive to small variations of the interaction potential and to the magnitude of the spin-exchange interaction, enabling the accurate calibration of the measured N atom density. We find consistency between the calculated and experimentally determined rates. Our results suggest that N atoms are promising candidates for future experiments on sympathetic cooling of molecules.

We show that weakly bound He-containing van der Waals molecules can be produced and magnetically trapped in buffer-gas cooling experiments, and provide a general model for the formation and dynamics of these molecules. Our analysis shows that, at typical experimental parameters, thermodynamics favors the formation of van der Waals complexes composed of a helium atom bound to most open-shell atoms and molecules, and that complex formation occurs quickly enough to ensure chemical equilibrium. For molecular pairs composed of a He atom and an S-state atom, the molecular spin is stable during formation, dissociation, and collisions, and thus these molecules can be magnetically trapped. Collisional spin relaxation is too slow to affect trap lifetimes. However, 3He-containing complexes can change spin due to adiabatic crossings between trapped and untrapped Zeeman states, mediated by the anisotropic hyperfine interaction, causing trap loss. We provide a detailed model for Ag3He molecules, using ab initio calculation of Ag–He interaction potentials and spin interactions, quantum scattering theory, and direct Monte Carlo simulations to describe formation and spin relaxation in this system. The calculated rate of spin-change agrees quantitatively with experimental observations, providing indirect evidence for molecular formation in buffer-gas-cooled magnetic traps. Finally, we discuss the possibilities for spectroscopic detection of these complexes, including a calculation of expected spectra for Ag3He, and report on our spectroscopic search for Ag3He, which produced a null result.