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Physica Scripta

A note on the spin-polarization of hydrogen atoms in a helium–hydrogen gas mixture

G. V. Klement’ev, Victor A. Kartoshkin, Sergei P. Dmitriev, V. D. Melnikov

Published Aug 1, 1996• 0 citations
Publisher

Abstract

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.

DOI
10.1088/0031-8949/54/2/010

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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.
In the collision of spin-polarized atoms, the transmission of electron polarization between colliding particles is possible. A similar situation occurs in a collision, for example, between alkali atoms in the ground state. If one of the colliding particles is in an excited state and its energy is sufficient for ionization of the collision partner, then along with the elastic process (spin exchange), an inelastic process (ionization) is also possible, as a result of these processes polarization can also be transferred to the partner. Simultaneously occurring elastic and inelastic processes at the collision between two helium atoms are possible if these atoms are in the metastable (23S1) state. The paper discusses the interaction between spin-polarized metastable atoms of helium-3 and helium-4 isotopes under conditions of optical pumping of atoms. Complex interaction potentials of the atoms under consideration were constructed and on their basis, the spin exchange and chemo-ionization cross-sections were calculated. Using these cross-sections, expressions for the transfer of orientation and alignment from one atom to another were obtained.
The general results recently obtained for the Boltzmann collision term in a gas of atomic hydrogen are applied to the study of longitudinal spin diffusion in high magnetic fields. This phenomenon can be described as a diffusion of concentration in a four component reactive mixture (the reaction being here the electronic transfer between nuclei). A numerical discussion of all the coefficients of the diffusion matrix between 100 mK to 100 K is given in the text, together with a discussion of the nature of the hydrodynamic modes in such a situation. The variations of the diffusion coefficients with the internal state of the gas are important for temperatures lower than 500 mK or higher than 10 K, when the spin exchange process is not negligible compared to the direct process. Finally, the relative importance of these two processes determines the nature of the hydrodynamic modes in the system.
We study the polarization dynamics of the molecular rotation and the nuclear spin in the presence of magnetic fields, in the hydrogen isotope molecules, H2,D2{\mathrm{H}}_{2},\phantom{\rule{0.16em}{0ex}}{\mathrm{D}}_{2}H2​,D2​, and HD. If only the rotational or the nuclear angular momentum is polarized, polarization is transferred from one to the other and back in a oscillatory fashion, due to hyperfine quantum beats. Application of magnetic fields is shown to increase the frequencies of these oscillations and reduce their amplitude so that the polarization exchange can be strongly suppressed. We derive the time-averaged polarization as a function of the (static) magnetic field. Finally, we show, using continuous lasers, molecular beams, and static magnetic fields, that the molecular rotation polarization can be transferred and maintained in the nuclear spin, providing highly nuclear-spin-polarized molecules.
In this thesis we describe the results of two sets of experiments on spin-polarized atomic hydrogen at temperatures below 1 K using a 115 GHz heterodyne ESR spectrometer. First, we have made measurements of the rates of the two-body recombination processes in spin-polarized atomic hydrogen gas and spin-polarized atomic deuterium gas in a 41 kG field. In the second set of experiments, we examined the ESR absorption line associated with the spin-polarized atomic hydrogen atoms adsorbed on surfaces of liquid helium. In hydrogen, a comparision of the measured recombination rates in 41 kG with the corresponding rates measured in different magnetic fields showed that an additional recombination process is activate for the conditions of these experiments. We demonstrate that this is due to a resonant recombination process via the (v,J) = (14,4) level of molecular hydrogen. In spin-polarized deuterium, we found that the recombination rate are much larger than the analogous rates in hydrogen, and therefore that atomic deuterium gas is much less stable than the hydrogen system under comparable conditions. We report the first observation and study of doubly spin-polarized deuterium, a gas in which both the deuteron and electron spins are aligned. At temperatures near 0.1 K, the absorption line of doubly spin-polarized H atoms changes drastically as a peak appears on both sides of the peak associated with the bulk atoms. We have shown that these new peaks are due to the atoms which are adsorbed onto the surface of the liquid helium film that coats the walls of the microwave cavity. The geometry dependent non-zero average magnetic dipolar field due to aligned spins on a planar surface results in a shift of the position of the ESR absorption lines. The lineshapes of the side peaks are very unusual and are best described as ramp-like. Even after extensive attempts to explain this lineshape, we still do not know what mechanism is responsible.
Nuclear spin polarization has important applications across various fields, including physics andmedicine. It also offers several advantages, such as cross-section enhancement, in the five-nucleonfusion reactions, namely the D–T and D–3^33He reactions. This work presents a theoretical studyof spin dynamics in the hyperfine regime for selected atomic and molecular systems and exploresalternative techniques to generate or enhance nuclear spin polarization in these systems. Particularemphasis is placed on coherent spin manipulation strategies and the role of hyperfine interactions intransferring angular momentum from electronic (in atoms) or rotational (in molecules) to nucleardegrees of freedom.