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AIP conference proceedings

Atomic hydrogen: Gas and surface collisions for T→0

Jook T. M. Walraven

Published Jan 1, 1990• 1 citation
Publisher

Abstract

Hydrogen atoms are very well suited to study elastic and inelastic collisions at very low energies. Atom‐atom collisions are being studied in the gas phase, where the lowest temperatures are achieved in magnetic traps. Atom‐surface collisions are studied by observing the reflection of a beam of H‐atoms from the surface of liquid Helium.

DOI
10.1063/1.39232

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Hydrogen atoms are very well suited to study elastic and inelastic collisions at very low energies. Atom‐atom collisions are being studied in the gas phase, where the lowest temperatures are achieved in magnetic traps. Atom‐surface collisions are studied by observing the reflection of a beam of H‐atoms from the surface of liquid Helium.
This review examines the numerous studies performed in recent years on atomic processes involving cold hydrogen atoms in the gas phase and at a surface. The term cold atom denotes the range of relative collision energies E < 10 meV (correspondingly, ultracold atoms have E 1 meV). This corresponds to the temperature range of tens (or units) of kelvins. Three main fields of study in which extensive experimental programs have been realized are treated: the study of collision processes involving hydrogen atoms, which are of astrophysical interest; the study of elastic atom-molecule scattering at ultralow energies; and studies on the problem of condensed hydrogen. The fundamental technical means of realization and the results obtained from the studies are discussed.
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.
We propose experiments with atomic hydrogen gas at ultra-low temperatures T < 100 μ K when the thermal energy of atoms is comparable with the changes of their potential energy in the Earth gravity field. At these conditions we suggest implementing a gravitational spectroscopy for studies of quantum properties of ultra-cold atomic hydrogen and its interactions with matter and gravity, similar to experiments with ultra-cold neutrons (Nesvizhevsky et al. Nature 415 , 297 2002 ). A magnetic trap used for reaching the Bose-Einstein Condensation (Fried et al. Phys. Rev. Lett. 81 , 3811 1998 ) can be used for cooling a large number of H atoms below 1 mK. Evaporative cooling over the trap barrier allows effective cooling of the vertical degree of freedom of the trapped atoms. Releasing these ultra-slow atoms from the trap onto the cold surface of superfluid helium will allow studies of quantum bounces and stationary gravitational states of H atoms in the potential well created by this surface and the field of Earth gravity. Experimental study of properties of gravitational quantum states of hydrogen and quantum reflection of ultracold hydrogen from surface would be of major importance for designing similar experiments with antihydrogen, which are currently prepared in CERN.