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Physics Today

Stable spin-polarized atomic hydrogen

Bertram M. Schwarzschild

Published Jun 1, 1980• 2 citations
Publisher

Abstract

The strong attraction between hydrogen atoms of opposite electron spin would seem to make it impossible to have stable monatomic hydrogen gas, except in interstellar space. But the force between hydrogen atoms of parallel spin is predominantly repulsive. Isaac Silvera and Jook Walraven of the University of Amsterdam have taken advantage of the absence of bound states for spin-polarized hydrogen to produce a monatomic hydrogen gas that showed no sign of reverting to ordinary H2 after almost an hour in a high magnetic field at low temperature, despite its very considerable density—more than 1016 atoms/cm3.

DOI
10.1063/1.2914111

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We review the recent activity in the theoretical description of spin-polarized atomic hydrogen and its isotopes at very low temperatures. Spin-polarized hydrogen is the only system in nature that remains stable in the gas phase even in the zero temperature limit due to its small mass and weak interatomic interaction. Hydrogen and its heavier isotope tritium are bosons, the heavier mass of tritium producing a self-bound (liquid) system at zero temperature. The other isotope, deuterium, is a fermion with nuclear spin one making possible the study of three different quantum systems depending on the population of the three degenerate spin states. From the theoretical point of view, spin-polarized hydrogen is specially appealing because its interatomic potential is very accurately known making possible its precise quantum many-body study. The experimental study of atomic hydrogen has been very difficult due to its high recombination rate, but it finally led to its Bose-Einstein condensate state in 1998. Degeneracy has also been observed in thin films of hydrogen adsorbed on the 4He surface allowing for the possibility of observing the Berezinskii–Kosterlitz–Thouless superfluid transition.

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THEORETICAL ISSUES CONCERNING THE STABILITY OF ELECTRON SPIN-POLARIZED HYDROGEN

W. C. Stwalley, Yea-Hwang Uang, R. F. Ferrante +1 more

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.).

Le Journal de Physique Colloques•Jul 1, 1980•4 citations
The strong attraction between hydrogen atoms of opposite electron spin would seem to make it impossible to have stable monatomic hydrogen gas, except in interstellar space. But the force between hydrogen atoms of parallel spin is predominantly repulsive. Isaac Silvera and Jook Walraven of the University of Amsterdam have taken advantage of the absence of bound states for spin-polarized hydrogen to produce a monatomic hydrogen gas that showed no sign of reverting to ordinary H2 after almost an hour in a high magnetic field at low temperature, despite its very considerable density—more than 1016 atoms/cm3.
The authors succeeded in conserving gas in monatomic state for several hours at the ultimately permissible temperature in a strong magnetic field. Under such conditions hydrogen atoms unite themselves relatively slowly, therefore gas may be considered as stable. As it follows from experimental results, monatomic hydrogen remains gaseous up to 0.08 K - the ultralow temperature at which the investigations have been carried out. These data at least preliminarily confirm an assumption that atomic hydrogen remains gaseous at absolute zero temperature, as it is predicted by the quantum theory.
Atomic hydrogen (H) has recently been created in a long-lived state referred to as spin-polarized atomic hydrogen (H↓). A moderate density gas (n≃1017 atoms/ cc) has been produced at temperatures T≃400 mK. H↓ is predicted to be the only atomic substance that remains gaseous to T = 0 K; current experiments are consistent with this picture. Since the H↓ atoms behave as composite bosons the gas is expected to have a Bose-Einstein condensation (BEC) and related superfluidity at a sufficiently high density or low temperature. H↓ is expected to display spectacular static and dynamic properties. Since the gas is polarized, the dominant electronic magnetization, M⃗e, is proportional to the atomic density. In an inhomogeneous magnetic field M⃗e will also be highly inhomogeneous. BEC will be spatially localized and the static M⃗e should be an identifying feature of this state. The low-lying excitation spectrum should not only feature electronic and nuclear spin-transitions, but should also have magnetic translational modes of the gas. The experimental technique for stabilization of H↓ as well as many of the predicted magnetic properties are discussed.
It was a complete surprise when Isaac Silvera and his colleagues at the University of Amsterdam observed the existence of a remarkably stable gas of hydrogen atoms in 1979. They found that a gas of the so-called “atom of atoms” could be confined for several minutes in a cryogenic test cell close to absolute zero, provided that the walls were coated with a film of liquid helium. Until then, if researchers wanted to study monoatomic hydrogen for any length of time they had to use a discharge source to replace those hydrogen atoms that recombined to form molecular hydrogen. But most important, Silvera's experiments marked the first experimental investigation of a dilute quantum gas.
We review the recent activity in the theoretical description of spin-polarized atomic hydrogen and its isotopes at very low temperatures. Spin-polarized hydrogen is the only system in nature that remains stable in the gas phase even in the zero temperature limit due to its small mass and weak interatomic interaction. Hydrogen and its heavier isotope tritium are bosons, the heavier mass of tritium producing a self-bound (liquid) system at zero temperature. The other isotope, deuterium, is a fermion with nuclear spin one making possible the study of three different quantum systems depending on the population of the three degenerate spin states. From the theoretical point of view, spin-polarized hydrogen is specially appealing because its interatomic potential is very accurately known making possible its precise quantum many-body study. The experimental study of atomic hydrogen has been very difficult due to its high recombination rate, but it finally led to its Bose-Einstein condensate state in 1998. Degeneracy has also been observed in thin films of hydrogen adsorbed on the 4He surface allowing for the possibility of observing the Berezinskii–Kosterlitz–Thouless superfluid transition.
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.).