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Physical Review Letters

Spin-Polarized Atomic Deuterium: Stabilization, Limitations on Density, and Adsorption Energy on Helium

Isaac F. Silvera, Jook T. M. Walraven

Published Oct 13, 1980• 57 citations
Publisher

Abstract

Current limitations on the density of stabilized spin-polarized deuterium (D\ensuremath{\downarrow}) to \ensuremath{\sim} ${10}^{14}$ atoms/${\mathrm{cm}}^{3}$ are shown to arise from condensation on the $^{4}\mathrm{He}$ surface and subsequent recombination. A new technique for measuring the adsorption energy is employed to provide the first such measurement for D\ensuremath{\downarrow} on helium, yielding $\frac{{\ensuremath{\epsilon}}_{a}}{{k}_{B}}=2.5\ifmmode\pm\else\textpm\fi{}0.4$ K.

DOI
10.1103/physrevlett.45.1268

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Current limitations on the density of stabilized spin-polarized deuterium (D\ensuremath{\downarrow}) to \ensuremath{\sim} 1014{10}^{14}1014 atoms/cm3{\mathrm{cm}}^{3}cm3 are shown to arise from condensation on the 4He^{4}\mathrm{He}4He surface and subsequent recombination. A new technique for measuring the adsorption energy is employed to provide the first such measurement for D\ensuremath{\downarrow} on helium, yielding \ensuremathϵakB=2.5\ifmmode±\else\textpm\fi0.4\frac{{\ensuremath{\epsilon}}_{a}}{{k}_{B}}=2.5\ifmmode\pm\else\textpm\fi{}0.4kB​\ensuremathϵa​​=2.5\ifmmode±\else\textpm\fi0.4 K.
The energy of interacting spin-polarized atomic hydrogen (${\mathrm{H}}_{\mathrm{\ensuremath{\downarrow}}}$) on a helium surface and the density dependence of the adsorption energy have been calculated, permitting a realistic determination of the adsorption isotherms. Surface interactions are shown to allow Bose-Einstein condensation in the gas. At the Bose-Einstein condensation the surface coverage attains a maximum which is essentially independent of the isotherm temperature. Also calculated is the stability condition for surfaces in magnetic fields. Easily met requirements are found.
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.
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).
We study three-body recombination in the gas of spin-polarized atomic hydrogen adsorbed on the surface of superfluid helium at temperatures from 45to130mK45\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}130\phantom{\rule{0.3em}{0ex}}\mathrm{mK}45to130mK. The two-dimensional gas is thermally compressed to densities up to $\ensuremath{\approx}4\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ using the ``cold spot'' method which makes the three-body process the dominant decay channel in the system. We measure the loss rate and surface density of atoms directly and independently and observe the former to be proportional to the third power of the latter. The result for the surface three-body recombination rate constant at 4.6T4.6\phantom{\rule{0.3em}{0ex}}\mathrm{T}4.6T, ${L}_{s}=2.0(7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}25}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{4}∕\mathrm{s}$, significantly reduces the discrepancy between the theory and earlier measurements where the surface density was inferred from the adsorption isotherm. We also measured the three-body rate constant on a 0.1% $^{3}\mathrm{He}\text{\ensuremath{-}}^{4}\mathrm{He}$ film and found ${L}_{s}=1.3(4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}24}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{4}∕\mathrm{s}$. This larger value is attributed to an increased delocalization of adsorbed hydrogen atoms in the direction normal to the surface.
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.
Hydrogen atoms with their electron spin polarized in a strong magnetic field ("spinaligned hydrogen" ; symbolized H+) should remain a metastable gaseous quantum fluid even at absolute zero. However, there are many possible ways of destroying this metastability. At high density ≥ 1021/cm3) Berlinsky et al. have suggested that a coherent spin-wave destruction mechanism prevents stability. At lower densities (high enough for Bose-Einstein condensation, however), gas phase and surface recombination mechanisms should be of primary concern. One critical question is essentially the explosion limit for H+., i.e. at what density will a single electronic spin flip event trigger massive recombination, heating and destruction of the sample (perhaps a good "detector" for highly forbidden processes) ? The second critical question is what events will trigger such explosions assuming one is above the limit ? We shall present our current understanding of these two critical questions (including high accuracy two-body collision calculations) and their implications in experimental design (e.g. field inhomogeneity effects, helium coating of surfaces, removal of deuterium impurities, etc.).