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The Journal of Chemical Physics

The Ramsauer–Townsend effect in molecular systems of electron-spin-polarized hydrogen and helium and their isotopes

T. K. Lim, Sigurd Yves Larsen

Published May 1, 1981• 21 citations
Publisher

Abstract

From arguments based on the De Boer quantum parameter and accurate phase shift calculations, we predict that the Ramsauer–Townsend effect occurs in numerous molecular systems involving electron-spin-polarized hydrogen and helium and their isotopes. Our work raises prospects of more precise determination of the two-body potential parameters of these systems as well as exciting quantum behavior in their macroscopic states.

DOI
10.1063/1.441751

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From arguments based on the De Boer quantum parameter and accurate phase shift calculations, we predict that the Ramsauer–Townsend effect occurs in numerous molecular systems involving electron-spin-polarized hydrogen and helium and their isotopes. Our work raises prospects of more precise determination of the two-body potential parameters of these systems as well as exciting quantum behavior in their macroscopic states.
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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.
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 a helium–hydrogen mixture at T = 300 K, the polarization degree of hydrogen spins, both electron and nuclear, was determined to be as high as (5 ± 1)%. The role of the main processes giving rise to the formation of spin-polarized H atoms has been discussed. It is shown that an effective transfer of angular momentum from the preliminarily optically oriented He * (2 3 S 1 ) atoms to the H atoms is not due to the direct chemi-dissociation of the H 2 molecules by the He * atoms alone, or due to Penning ionization of these molecules followed by the ion-molecular rearrangement reaction H 2 + + H 2 → H 3 + + H alone, but to their summary action. The relative importance of the two mechanisms depends on the rotational temperature and total gas pressure.