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Physics Bulletin

A stabilised gas of atomic hydrogen

Isaac F. Silvera

Published Nov 1, 1981• 0 citations
Publisher

Abstract

We, at the Natuurkundig Laboratorium der Universiteit van Amsterdam, have recently succeeded in creating a gas of atomic hydrogen in a special long-lived state called spin polarised hydrogen, H↓ (see I F Silvera and J T M Walraven 1980 Phys. Rev. Lett. 44 164). Gases with densities up to almost 10 17 cm -3 can exist for periods of minutes up to hours depending on the conditions.

DOI
10.1088/0031-9112/32/11/019

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We, at the Natuurkundig Laboratorium der Universiteit van Amsterdam, have recently succeeded in creating a gas of atomic hydrogen in a special long-lived state called spin polarised hydrogen, H↓ (see I F Silvera and J T M Walraven 1980 Phys. Rev. Lett. 44 164). Gases with densities up to almost 10 17 cm -3 can exist for periods of minutes up to hours depending on the conditions.
Some years ago Kastler pointed out that the equilibrium constant for alkali metal atom dimer formation could be suppressed by polarizing the electrons so that the atoms interacted via the weak triplet potential. That basic idea has now been realized with atomic hydrogen, but using a polarization technique involving high magnetic fields and low temperature rather than optical pumping. By achieving high polarization, the gas can be stabilized at densities a few seconds over 10^18 atoms cm-3, and for minutes or hours at lower densities.
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.
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.
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.