26 research outputs found
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Isotope Effects in Complex Scattering Lengths for He Collisions With Molecular Hydrogen
We examine the effect of theoretically varying the collision-system reduced mass in collisions of He with vibrationally excited molecular hydrogen and observe zero-energy resonances for select atomic “hydrogen” masses less than 1 u or a “helium” mass of 1.95 u. Complex scattering lengths, state-to-state vibrational quenching cross sections, and a low-energy elastic scattering resonance are all studied as a function of collision-system reduced mass. Experimental observations of these phenomena in the cold and ultracold regimes for collisions of He and He with H, HD, HT, and DT should be feasible in the near future.Astronom
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Formation and dynamics of van der Waals molecules in buffer-gas traps
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.Astronom
Formation and dynamics of van der Waals molecules in buffer-gas traps
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 relaxations are too slow to affect trap lifetimes.
However, helium-3-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.Comment: 20 pages, 13 figure
Rate of formation of hydrogen molecules by three body recombination during primordial star formation
ABSTRACT Astrophysical models of primordial star formation require rate constants for three-body recombination as input. The current status of these rates for H 2 due to collisions with H is far from satisfactory, with published rate constants showing orders of magnitude disagreement at the temperatures relevant for H 2 formation in primordial gas. This Letter presents an independent calculation of this recombination rate constant as a function of temperature. An analytic expression is provided for the rate constant which should be more reliable than ones currently being used in astrophysical models