Quantum gases

Physisorption at ultra-low temperatures

Having a gaseous many-body groundstate, spin-polarized atomic hydrogen is very well suited to study collisional phenomena in the limit of ultra-low energies. In previous research we investigated, within the thesis project of J.J. Berkhout, mostly experimentally, atom-surface scattering phenomena, in particular surface adsorption and specular reflection at the surface of liquid helium films. The experiments were done at temperatures down to about 100 mK. It was established that surface adsorption is the dominant inelastic scattering channel at temperatures below about 0.5 K and that the adsorption probability is vanishing with temperature to yield complete specular reflection in the zero temperature (in accordance with existing theory [1]). This was demonstrated by focusing of atomic beams of H with a superfluid-He covered mirror.

we contributed to some new developments which have taken place since the publication of our experimental findings. Doyle et al. [2] observed the adsorption probability to increase with decreasing temperature for T < 20 mK. This result was puzzling as it was found to be consistent with a result obtained previously by Goldman [3] using a Van der Waals coefficient four times bigger than the known coefficient for the He-H system. We could explain this unexpected rise as a resonantly enhanced adsorption probability induced by the substrate beneath the helium film (see [4]). This conjecture received support from a more detailed calculation by Carraro and Cole [5]. Recent measurements by Yu et al. [6] have confirmed our explanation by demonstrating that the adsorption probability could be reduced by almost two orders of magnitude using bulk helium rather that a helium film, thus regaining the picture of a vanishing adsorption probability.

The progress on this topic has been very rewarding. Currently superfluid mirrors start to be applied in polarized proton sources [7].

References

[1] D.S. Zimmerman and A.J. Berlinsky, Can. J. Phys. 61, 50 (1983) 50; Yu. Kagan and G.V. Shlyapnikov, Phys. Lett. 95A, 309 (1983).
[2] J.M. Doyle, J.C. Sandberg, I.A. Yu, C.L. Cesar, D. Kleppner, and T.J. Greytak, Phys. Rev. Lett. 67, 603 (1991).
[3] V.V. Goldman, Phys. Rev. Lett. 56, 612 (1986).
[4] T.W. Hijmans, J.T.M. Walraven en G.V. Shlyapnikov: Influence of the substrate on the low-temperature limit of the sticking probability of hydrogen atoms on He films, Phys. Rev. B 45, 2561 (1992).
[5] C. Carraro and M.W. Cole, Phys. Rev. B 45, 12930 (1992).
[6] I.A. Yu, J.M. Doyle, J.C. Sandberg, C.L. Cesar, D. Kleppner, and T.J. Greytak, Phys. Rev. Lett. 71, 1589 (1993).
[7] 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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