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Stabilization of atomic hydrogen

А.Ф. Сильвера, Yu Valraven

Published Apr 1, 1983• 0 citations
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Abstract

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.

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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.
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.
Under normal circumstances a gas of atomic hydrogen (H) is highly unstable with respect to recombination to molecular hydrogen, H2. We have succeeded in stabilizing a gas of spin polarised hydrogen (H↓) at T = 300 mK in magnetic fields up to 10 Tesla in a superfluid helium coated cell. Densities of at least 1016atoms/cc have already been achieved. H is found to have a surprisingly long life even in zero magnetic field. Evidence points to H↓ remaining in the gaseous form and not liquifying or condensing out on the surfaces to any large extent. The crucial experimental techniques used to stabilize H↓ will be discussed as well as some of the observed properties and expectations of this new Bose gas.
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.
In the strange world of low‐temperature physics, few more unusual phenomena have been discovered than monatomic hydrogen gas. According to a recent report (New Scientist, Jan. 1981, p. 204), hydrogen atoms do not tend to pair up to form molecules under special low‐temperature‐cryogenic experimental conditions. The reasons for the existence of atomic hydrogen gas at ultra‐low temperature, and for its implications, lie in an understanding of its almost pure quantum mechanical properties. Practical uses of atomic hydrogen will await more knowledge of its properties, but right now, if available in sufficient abundance, atomic hydrogen could be used in maser clocks because it emits highly defined frequencies of microwave radiation at room temperature, and atomic hydrogen gas could be used as an energy conservative fuel, as it recombines to the diatomic molecule. For every recombination between two hydrogen atoms, 4.5 eV of energy are released, which calculates to be over 1 million watts of power per second for 10 grams of atomic hydrogen recombined. The large impulse relative to its light mass could be significant in applications of atomic hydrogen as a rocket fuel. Practical applications aside, the main interest in atomic hydrogen at this moment is in the insight into quantum mechanics that study of its properties may provide. Atomic hydrogen atoms ‘abound throughout the universe,’ even though not on Earth, and thus quantum effects in space, and the hydrogen‐rich planets as well, may be elucidated by results of these studies.
Atomic hydrogen has been stabilized at temperatures of 270\ifmmode\pm\else\textpm\fi{}20 mK and in magnetic fields up to 7 T. It is believed to be gaseous. A sample displayed no measurable decrease in density after 532 sec. Covering of exposed surfaces with a film of 4He^{4}\mathrm{He}4He is essential.
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.