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Physical Review Letters

Stability of Spin-Aligned Hydrogen at Low Temperatures and High Magnetic Fields: New Field-Dependent Scattering Resonances and Predissociations

William C. Stwalley

Published Dec 13, 1976• 262 citations
Publisher

Abstract

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 ${\mathrm{D}}_{2}$) 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.

DOI
10.1103/physrevlett.37.1628

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Physical Review Letters•Sep 8, 1986•21 citations
Spin-aligned hydrogen (H \ensuremath{\uparrow}) and deuterium (D \ensuremath{\uparrow}) are generally predicted to be stable for HT\ensuremath≳106\frac{\mathcal{H}}{T}\ensuremath{\gtrsim}{10}^{6}TH​\ensuremath≳106 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.
Measurements of the two-body recombination of spin-polarized atomic hydrogen in a magnetic field of 40 kG have been extended to temperatures above 0.5 K. The rate constant for the formation of parahydrogen shows an unexpected increase with temperature, which is explained by inverse predissociation into the v=14v=14v=14, J=4J=4J=4 level of H2{\mathrm{H}}_{2}H2​. Data indicate the level is bound by 0.7\ifmmode\pm\else\textpm\fi{}0.1 K.
In sufficiently large magnetic fields, the weakly bound levels of H2 acquire finite lifetimes owing to hyperfine predissociation and become manifest as scattering resonances in spin-polarized atomic hydrogen (H↓). In this work we investigate in detail the contribution to the reaction kinetics in H↓ due to the resonance, denoted [Formula: see text], arising from the predissociation of the (v,J) = (14,4) level of H2. To characterize the formation and disintegration of [Formula: see text], we calculate the predissociation as a function of both the applied magnetic field and the dissociation energy, D, of the level. Recombination occurs when [Formula: see text] is stabilized in collisions that cause transitions to lower molecular levels. For low H↓, densities, collisions with atoms of the saturated 4He vapour dominate; we calculate the cross section in the distorted wave Born approximation and also use a coupled-channels approach. At higher densities, stabilization occurs in collisions with H↓ as well; the effect of proton exchange between the atom and the molecule is derived. Previous experiments, analyzed neglecting proton exchange, are reanalyzed including this; the dissociation energy D is not as well determined by the data as was previously thought. Prospects for future experiments are discussed.
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.
Systems of spin-aligned hydrogen isotopes are studied. They are shown to exhibit even more extreme "quantum" behavior than the helium isotopes. Spin-aligned hydrogen is predicted to be a gas at all temperatures and its Bose-Einstein condensation and possible superfluidity are discussed. Spin-aligned deuterium is predicted to show critical behavior strongly influenced by quantum mechanics. The preparation of spin-aligned hydrogen (in the presence of large magnetic fields) is also discussed.
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.
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.