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Physical Review Letters

Focusing a beam of ultracold spin-polarized hydrogen atoms with a helium-film-coated quasiparabolic mirror

V. G. Luppov, William A. Kaufman, K. M. Hill, R. S. Raymond, Alan D. Krisch

Published Oct 11, 1993• 12 citations
Publisher

Abstract

We formed the first ``atomic-optics'' beam of electron-spin-polarized hydrogen atoms using a quasiparabolic polished copper mirror coated with a hydrogen-atom-reflecting film of superfluid $^{4}\mathrm{He}$. The mirror was located in the gradient of an 8-T solenoidal magnetic field and mounted on an ultracold cell at 350 mK. After the focusing by the mirror surface, the beam was again focused with a sextupole magnet. The mirror, which was especially designed for operation in the magnetic field gradient of our solenoid, increased the focused beam intensity by a factor of about 7.5.

DOI
10.1103/physrevlett.71.2405

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We formed the first ``atomic-optics'' beam of electron-spin-polarized hydrogen atoms using a quasiparabolic polished copper mirror coated with a hydrogen-atom-reflecting film of superfluid 4He^{4}\mathrm{He}4He. The mirror was located in the gradient of an 8-T solenoidal magnetic field and mounted on an ultracold cell at 350 mK. After the focusing by the mirror surface, the beam was again focused with a sextupole magnet. The mirror, which was especially designed for operation in the magnetic field gradient of our solenoid, increased the focused beam intensity by a factor of about 7.5.
A 350 mK helium‐4‐coated mirror was used to increase the intensity of an ultra‐cold electron‐spin‐polarized atomic hydrogen beam. The mirror uses the observed specular reflection of atomic hydrogen from a superfluid‐helium‐covered surface. A quasi‐parabolic polished copper mirror was installed with its focus at the 5 mm diameter exit aperture of an atomic hydrogen stabilization cell in the gradient of an 8 T solenoid field. The four‐coned mirror shape, which was designed specifically for operation in the gradient, increased the beam intensity focused by a sextupole magnet into a compression tube detector by a factor of about 7.5.
We use a concave spherical mirror to focus at 18-mm-diam beam of H atoms down to 0.5 mm. The mirror consists of a fused-quartz substrate polished to optical precision and coated with a liquid${\mathrm{\ensuremath{-}}}^{4}$He film to obtain high reflectivity. The temperature dependence of the focused beam intensity enables us to study the influence of the dynamic surface roughness on the reflection of the H atoms. Both zero-point fluctuations and thermal excitations turn out to be of importance. A monolayer of 3He^{3}\mathrm{He}3He does not significantly affect the results.
We studied the low-energy limit of atomsurface scattering and the phenomenon of quantum reflection by using hydrogen atoms and the surface of liquid helium. Measuring the specular reflectivity of a liquid-helium-covered hemispheric mirror for a beam of hydrogen atoms we obtained information on elastic and inelastic scattering phenomena. At 100 mK, reflectivities larger than 90% were achieved; this can be used to demonstrate atomic beam focusing. Approximately three-fourths of the atoms escaping from a small orifice in a cell were reflected back into the cell. The dominant inelastic processes at work here are surface absorption and direct inelastic scattering over small angles under excitation of surface ripplons (quantized capillary waves). Helium-covered surfaces maybe useful for atomic-beam manipulation of atomic hydrogen.
Results from a novel variation on the Stern–Gerlach method for magnetic separation are presented. A superconducting solenoid lens for spin selection and focusing of a cold atomic hydrogen beam has been built and tested at Brookhaven National Laboratory. The solenoid consists of three coils connected in series, with the current in the outer two coils counter to the current in the middle coil, resulting in a large magnetic field (up to 5.2 T) with strong gradients. Beam density enhancement (focusing) of up to a factor of 10 was achieved. Excellent qualitative agreement exists with ray-tracing calculations. Quantitatively, the experimental results show a lens performance which is a factor of at least 3 better than what these calculations predict.
Two ultra-cold methods of atomic hydrogen electron spin separation for polarized jet targets and sources will be described and compared. Both of them use a helium-film coated cell at 300 mK located in a high magnetic field. The first one, microwave driven extraction, uses a 212 GHz microwave system and 7.5 T magnetic field with a high uniformity ({Delta}B/B{approx} 10{sup {minus}5}). The second method employs the gradient of a high magnetic field for the state separation and beam formation. Experimental results from the two methods will be presented.