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Bulletin of the American Physical Society

Surface reflection of submillikelvin atomic hydrogen from superfluid {sup 4}He: Quantum reflection in the thick film limit and substrate effects

Ite A. Yu, John M. Doyle, Jon C. Sandberg, Claudio Lenz Cesar

Published May 1, 1993• 0 citations
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Abstract

We present the results of experiments measuring the film thickness dependence of the sticking probability for H on superfluid {sup 4}He for H atom temperatures of 0.3-5.0 mK. Data is presented for 3 nm to 1 mm thick films on a silver sinter substrate. For the 0.3 mK H atoms, the sticking probability varies from 0.4 for a film 3 nm thick to 6 x 10{sup -3} for films thicker than 100 nm. In the case of the thick films the temperature dependence of the sticking probability is consistent with the observation of pure quantum reflection. Our data are in reasonable agreement with recent theoretical calculations.

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Bulletin of the American Physical Society•Apr 1, 1993•0 citations
We present the results of experiments measuring the film thickness dependence of the sticking probability for H on superfluid {sup 4}He for H atom temperatures of 0.3-5.0 mK. Data is presented for 3 nm to 1 mm thick films on a silver sinter substrate. For the 0.3 mK H atoms, the sticking probability varies from 0.4 for a film 3 nm thick to 6 x 10{sup -3} for films thicker than 100 nm. In the case of the thick films the temperature dependence of the sticking probability is consistent with the observation of pure quantum reflection. Our data are in reasonable agreement with recent theoretical calculations.
We have measured the sticking probability of atomic hydrogen on a superfluid 4He^{4}\mathrm{He}4He surface for atom energies between 100 \ensuremath{\mu}K and 1 mK. The sticking probability shows little variation with energy over this interval, and is close to the value 0.2. The studies were carried out using a magnetic trap that produced a density of 8\ifmmode\times\else\texttimes\fi{}1013{10}^{13}1013 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ at a temperature of 100 \ensuremath{\mu}K.
A study is made of the interaction between hydrogen atoms and the surface elementary excitations of superfluid "He. Calculations for the sticking probability and for the energy and effective mass of a hydrogen atom bound to the surface are presented. As a first step in the calculations, we formulate the Hamiltonian describing a hydrogen atom interacting with the surface of "He in its ground state together with the interaction coupling to the surface elementary excitations (the ripplons). The derivation is based on the association of the surface ground state with a flat surface and the excited states with a sinusoidally varying height of the surface. The interaction potential is derived by summing a *He-H atom-atom pair potential over helium atoms below the surface. The atom-atom pair potential is chosen so that the derived surface potential is a Morse potential with parameters which are fitted to the effective surface potential derived by Mantz and Edwards for a hydrogen atom interacting with N-1 helium atoms. The energy and angle dependent sticking probability, S(E,θ), and the thermally averaged sticking probability, S(T), are calculated. Results are compared with the experimental value of S(T), S(T)=0.035±0.005, measured for the temperature range 0.18<T<0.27K. The possibility that a hydrogen atom bound above the surface may exhibit polaronic behaviour is investigated. The energy of the hydrogen surface "polaron" and its effective mass are calculated using perturbation theory. The hydrogen atom is found to be weakly coupled to the surface elementary excitations, and therefore the polaronic effects are weak. The contribution to these results from virtual transition to free states is emphasized.
We investigated quantum states of ultracold hydrogen atoms in a combined potential comprising the H-He film interaction in the presence of a substrate and the Earth gravitational field. We show that the shift and width of the gravitational quantum states are determined by the complex scattering length for the H-He film-substrate potential. We demonstrate that for specific helium film thicknesses above a substrate, zero-energy resonances occur if the combined potential supports a bound state exactly at the threshold. This effect leads to a complete restructuring of the bound states spectrum. The dynamics of gravitational levels as a function of the van der Waals interaction depth controlled by the helium film thickness is analyzed. It reveals the critical thicknesses at about 6.1 and 1.8 nm, at which resonances appear in the case of the conductive substrate. With imaginary integral operators, we incorporate non-perturbatively the inelastic effects originating from the ripplon coupling. The inelastic effects show dramatic changes in the sticking-coefficient behavior near the critical points. The enhanced sticking coefficients provide a probe for studying critical phenomena and measuring atom-surface interaction parameters with unprecedented sensitivity.
The probability of sticking of spin-polarised atomic hydrogen, H down arrow , on the superfluid helium surface as a function of temperature from 73 to 526 mK has been identified to arise from its interaction with third-sound ripplons. When the number density of the spin-polarised hydrogen atoms in the bulk helium becomes equal to that of the third-sound ripplons, the rate of sticking of H down arrow on the surface of the superfluid helium becomes a linear function of temperature. The predicted linearity is in accord with the experimental measurements.
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.
The {sup 3}He-{sup 4}He mixture film provides a remarkable opportunity to study the behavior of the uncharged 2D {sup 3}He Fermi fluid which resides on the {sup 4}He. Variation of the {sup 4}He film thickness allows one to vary the strength of the substrate field experienced by the {sup 3}He. Changes in the coverage of the {sup 3}He allow one to vary the strength of the {sup 3}He-{sup 3}He interaction and thereby study the evolution of the {sup 3}He system from the nearly ideal dilute 2D gas regime to the interacting 2D regime to the advent of 3D behavior. Recent experimental work, primarily by NMR techniques, on Nucleopore substrates for 30 {le} T {le} 600mK has resulted in a detailed picture of the energetics of the states available to the {sup 3}He, the stepwise evolution of the magnetization, spin relaxation and measurement of the diffusion of the {sup 3}He as a function of coverage and temperature. The state of current knowledge of the mixture film system will be discussed including connection to theoretical progress and the prospects for future work in this area.