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Physical Review Letters

Close-Coupling Calculations of Spin-Polarized Hydrogen-Deuterium Collisions

Yea-Hwang Uang, William C. Stwalley

Published Aug 25, 1980• 12 citations
Publisher

Abstract

Close-coupling calculations on the atomic collisions at cryogenic energies in a 100-kG magnetic field between spin-polarized hydrogen (H\ensuremath{\uparrow}) and deuterium (D\ensuremath{\uparrow}) have demonstrated the resonant spin-flip process postulated by Stwalley and the importance of minimizing the presence of D\ensuremath{\uparrow} in the experimental preparations of bulk H\ensuremath{\uparrow} currently in progress. Other interesting features are presented and briefly discussed including multiple Ramsauer-Townsend effects and direct electron and nuclear single- and double-spin-flip processes.

DOI
10.1103/physrevlett.45.627

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Physical Review A•Sep 1, 1994•20 citations
Close-coupling calculations on the atomic collisions at cryogenic energies in a 100-kG magnetic field between spin-polarized hydrogen (H\ensuremath{\uparrow}) and deuterium (D\ensuremath{\uparrow}) have demonstrated the resonant spin-flip process postulated by Stwalley and the importance of minimizing the presence of D\ensuremath{\uparrow} in the experimental preparations of bulk H\ensuremath{\uparrow} currently in progress. Other interesting features are presented and briefly discussed including multiple Ramsauer-Townsend effects and direct electron and nuclear single- and double-spin-flip processes.
For molecular systems, spin relaxation is expected to be suppressed compared to the case of atoms, since the paired electrons in a hydrogen or deuterium molecule are chemically stable, and only weakly interact with the spin of the nucleus. Such systems would be largely insensitive to polarization losses due to spin-exchange collisions, to the interaction of the electron spins with external fields (e.g. the RF-field of a bunched charged-particle beam), and/or to the presence of container walls. Here, we discuss the results of a recent experiment where we obtained evidence that nuclear polarization is maintained, when polarized atoms recombine to molecules on a copper surface (in a magnetic field of 23 mT and at a density of about 1012 molecules⋅cm−3).
Collision experiments employing a preparation and/or an analysis of spin polarization provide a powerful tool to study individual interaction mechanisms. For atoms or ions with nonvanishing nuclear moments the hyperfine interaction before and after the collision may affect the spin polarization and any collision-induced orientation and alignment, thereby substantially reducing experimentally observable effects. This disturbance can be avoided by applying an external magnetic field, as is demonstrated here for the inelastic collision process He+{\mathrm{He}}^{+}He++Na(\ensuremath{\uparrow}) \ensuremath{\rightarrow}He+{\mathrm{He}}^{+}He++Na∗{\mathrm{Na}}^{\mathrm{*}}Na∗(3p). Easily attainable magnetic field strengths are sufficient to decouple electronic and nuclear spins whereby the figure of merit for both, orientation and alignment, is increased by more than one order of magnitude.
We have studied spin-polarized deuterium, ${\mathrm{D}}_{\ensuremath{\downarrow}}$, in the temperature range 0.3 to 0.7 K in a field of 40 kG using electron-spin resonance at 114 GHz. The intrinsic rates for recombination of two D atoms to form D2{\mathrm{D}}_{2}D2​ on the surface of $l\ensuremath{-}^{4}\mathrm{He}$ have been measured for both the ortho and para channels. The two rates are approximately equal and when extrapolated to zero field are 1600 times greater than the rates for H2{\mathrm{H}}_{2}H2​ formation. Doubly spin-polarized deuterium, ${\mathrm{D}}_{\ensuremath{\downarrow},↡}$, has been observed for the first time, with nuclear-spin purities > 0.98 and sample lifetimes as long as 1 h.
Spin-aligned hydrogen (H \ensuremath{\uparrow}) and deuterium (D \ensuremath{\uparrow}) are generally predicted to be stable for $\frac{\mathcal{H}}{T}\ensuremath{\gtrsim}{10}^{6}$ G/K. Magnetic-field-dependent resonances (inverse to field-induced predissociations of high vibrational-rotational levels of HD and D2{\mathrm{D}}_{2}D2​) provide exceptions to this general rule. Their existence suggests 50 kG (H \ensuremath{\uparrow} + D \ensuremath{\uparrow}) and 19 kG (D \ensuremath{\uparrow} + D \ensuremath{\uparrow}) should be avoided for stable H \ensuremath{\uparrow} and D \ensuremath{\uparrow}. Induced predissociation observations should yield ultraprecise resonance information and dissociation limits.
In this thesis we describe the results of two sets of experiments on spin-polarized atomic hydrogen at temperatures below 1 K using a 115 GHz heterodyne ESR spectrometer. First, we have made measurements of the rates of the two-body recombination processes in spin-polarized atomic hydrogen gas and spin-polarized atomic deuterium gas in a 41 kG field. In the second set of experiments, we examined the ESR absorption line associated with the spin-polarized atomic hydrogen atoms adsorbed on surfaces of liquid helium. In hydrogen, a comparision of the measured recombination rates in 41 kG with the corresponding rates measured in different magnetic fields showed that an additional recombination process is activate for the conditions of these experiments. We demonstrate that this is due to a resonant recombination process via the (v,J) = (14,4) level of molecular hydrogen. In spin-polarized deuterium, we found that the recombination rate are much larger than the analogous rates in hydrogen, and therefore that atomic deuterium gas is much less stable than the hydrogen system under comparable conditions. We report the first observation and study of doubly spin-polarized deuterium, a gas in which both the deuteron and electron spins are aligned. At temperatures near 0.1 K, the absorption line of doubly spin-polarized H atoms changes drastically as a peak appears on both sides of the peak associated with the bulk atoms. We have shown that these new peaks are due to the atoms which are adsorbed onto the surface of the liquid helium film that coats the walls of the microwave cavity. The geometry dependent non-zero average magnetic dipolar field due to aligned spins on a planar surface results in a shift of the position of the ESR absorption lines. The lineshapes of the side peaks are very unusual and are best described as ramp-like. Even after extensive attempts to explain this lineshape, we still do not know what mechanism is responsible.
The electron polarization of hydrogen (H) and deuterium (D) atoms in a spin-exchange optical-pumping apparatus has been measured as a function of H (D) density and magnetic-field strength. The atomic density was varied between 4\ifmmode\times\else\texttimes\fi{}1013{10}^{13}1013 and 2.6\ifmmode\times\else\texttimes\fi{}1014{10}^{14}1014 atoms/cm3{\mathrm{cm}}^{3}cm3 and the magnetic-field strength from 120 to 400 mT. The results suggest that the atoms approach spin-temperature equilibrium even though the magnetic fields are large compared to the critical fields of the atoms.