Spin-polarized atomic hydrogen

Introduction

Spin-polarized atomic hydrogen (spH) is the remarkably stable gaseous phase of monatomic hydrogen (H) which can be created and studied at low temperatures (typically below 1K). The existence of this phase was established experimentally in 1979 by Isaac Silvera and his group at the University of Amsterdam [1]. Ordinarily, atomic hydrogen is a very reactive gas, with the atoms eagerly recombining to form molecular hydrogen. However, by polarizing the electronic spins of the atoms, the lifetime and density of the gas are greatly extended to the point where it is essentially stable. The key ingredients of the experiment were a high magnetic field (8T) and low temperature (0.3K) to polarize the electronic spins of the atoms and a liquid helium film coating the walls to avoid the catastrophic adsorption which would occur on any other material surface. It is a remarkable property of spH that it is a gas when in thermal equilibrium at any temperature (even absolute zero). The reason for this is that the interaction potential between spin-polarized atoms (the so-called triplet potential) has only a shallow attractive well, similar to the interaction between two helium atoms. The low mass of the hydrogen atom gives a large quantum mechanical zero point motion in this potential and prevents the existence of a many-body bound state.

The hydrogen experiments in Amsterdam marked the first experimental investigations of a dilute quantum gas. Presently spH is studied by a number of groups worldwide.

Bose-Einstein condensation

The most interesting and challenging properties and phenomena to study are those associated with quantum degeneracy of the gas: Bose-Einstein condensation (BEC) in three dimensions and its analog, the Kosterlitz-Thouless (KT) transition in two dimensions. These quantum phase transitions will occur when the mean distance between atoms is comparable to the thermal de Broglie wavelength of the atoms.

The quest for BEC in spH is presently concentrated in two directions. In the first approach, the gas is compressed to high density by liquid helium. In the second approach spH is cooled to ultralow temperatures without surfaces in a magnetostatic trap. Both approaches are presently only one order of magnitude short of the critical density for BEC at the operating temperature.

Compression experiments

In a high magnetic field, the thermal equilibrium orientation of the electronic spin of the H atoms is antiparallel to the magnetic field. In this state the atoms are attracted to regions of strong magnetic field and are hence called high-field-seekers. Working with high-field-seeking spH, very high densities are possible (exceeding 1E19 atoms/cc, and comparable to the density of air). The gas can be compressed as a gas bubble in liquid helium or by a strong field gradient (magnetic or electric needle). The gas is in contact with the liquid helium walls. At high densities the gas becomes nuclear-spin polarized by chemical reaction and is particularly stable. Recombination, on the surface of the liquid helium, is slow: it requires a collision of three H atoms and relies on the weak spin-dipole interaction to flip an electron spin. Unfortunately, it is not slow enough. Despite clever schemes to enhance the cooling of the gas, overheating due to recombination (raising the gas temperatures to above 0.5K) has so far prevented attainment of BEC [2]. The KT transition, on the other hand, seems possible in a suitably designed apparatus.

Magnetic trapping experiments

The big advantage of using a magnetostatic trap is wall-free confinement, which means that the gas temperature can be very much lower than that of the walls. However, only H atoms in the low-field-seeking states (electron spin up) can be trapped, which makes the gas unstable against spin relaxation in pair collisions. This limits the density to relatively low values (typically less than 1E14 atoms/cc). Progress towards BEC in trapped H is steady, basically determined by the evolution of in situ diagnostic techniques. In our own research we developed optical spectroscopy at the Lyman-alpha transition of the H atom (wavelength 122 nm) for this purpose.

Other studies

It is noteworthy that even at densities too low for quantum degeneracy, the extreme quantal nature of H is evident in transport properties, and gives rise to interesting phenomena such as spin waves [3].

Quantum sticking has been studied with the system of H atoms and the free surface of liquid helium.

Hydrogen atom gases have also been studied at low temperatures in low magnetic fields (here the atoms are not spin polarized and the densities are limited). The interest here is to understand the chemical process of recombination at low temperatures, and the interaction of H with the liquid helium wall coating. Hyperfine magnetic resonance has been used to advantage by the group of Walter Hardy [4]. This technique has recently been used to study atomic deuterium and its mixtures with atomic hydrogen [5].

An important facet of research on atomic hydrogen at low temperatures has been the development of the cryogenic hydrogen maser [6]. This device promises to have a much higher long-term time and frequency stability than the important room-temperature hydrogen maser. Already this device has led to a confrontation of theory and experiment over the physics of low temperature spin-exchange collisions.

There is activity in particle physics laboratories and accelerators to produce spH as a target with high density of polarized proton spins [7]. The high reflectivity of helium films for H atoms has been exploited for atom optics in this context [8].

Review articles on spH

The article by Greytak and Kleppner is an excellent introduction to atomic hydrogen and, more generally, to the properties of a weakly-interacting Bose gas. The comprehensive review by Silvera and Walraven presents the state-of-the-art to 1985 and gives a broad discussion of the theoretical and experimental aspects of recombination and relaxation. The paper by Silvera and Reynolds gives a brief overview of the status of the field in 1992.

Other References

[1] I.F. Silvera and J.T.M. Walraven, Phys. Rev. Lett. 44, 164 (1980).
[2] P. Arvela, A.V. Frolov, S. Jaakkola, A. Ya. Katunin, I.I. Lukashevich, M. Mertig, A.I. Safonov and E. Tjukanov, Proceedings LT-20; T. Tommila and M.W. Reynolds, Hydrodynamic code for atomic hydrogen, Report TURKU-FL-R16, Department of Physics, University of Turku, 1993.
[3] N.P. Bigelow, J.H. Freed, and D.M. Lee, Phys. Rev. Lett. 33 1609, (1989).
[4] W.N. Hardy, M. Morrow, R. Jochemsen, and A.J. Berlinsky, Physica B 109 - 110, 1964 (1982).
[5] M.E. Hayden and W.N. Hardy, J. Low Temp. Phys. 99, 787 (1995).
[6] G.P. Kochanski, J.M. Doyle, T.J. Greytak, and D. Kleppner, Phys. Rev. A 34, 1602 (1986); M.D. Hurlimann, W.N. Hardy, A.J. Berlinsky, and R.W. Cline, Phys. Rev. A 34, 1605 (1986); R.L. Walsworth, I.F. Silvera, H.P. Godfried, C.C. Agosta, R.F.C. Vessot, and E.M. Mattison, Phys. Rev. A 34, 2550 (1986).
[7] T. Roser, D.G. Crabb, W.A. Kaufman, R.S. Raymond, J.A. Stewart, B. Vauridel, and G.R. Carst, Nuc. Inst. Meth. in Phys. Res. A 301, 42 (1991).
[8] V.G. Luppov, W.A. Kaufman, K.M. Hill, R.S. Raymond, and A.D. Krisch, Phys. Rev. Lett. 71, 2405 (1993).

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