Solid Hydrogen Surfaces

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The adsorption of weakly bound films on substrates is an established topic with many applications. H₂ films are unusual because the topmost layers appear to remain fluid as temperatures decrease to far below the critical temperature of H₂ [a] .

Albrecht et. al. [a] report detailed results from their studies of wetting phenomena on solid H₂. Most of their data on hydrogen films were obtained using the surface plasmon (SP) resonance technique (ATR; attenuated total reflection method), which provides very accurate information about both the thickness and the roughness of the physisorbed material. Using condensation from H₂ gas it was found that the deposited film is smooth up to a thickness around of 12 A. Beyond that value, the SP resonance width started to broaden considerably. This dependence was interpreted as the build-up of a smooth, homogeneous film up to a limiting thickness d₁ of 3 to 4 monolayers, beyond which the condensing molecules aggregate to form bulk crystallites. Such a behavior is known during common film deposition as Stranski-Krastanov growth.

Apparently at a transition temperature around 3 K the diffusion processes are fast enough for a surface film to relax towards its quasi-equilibrium dewetted state within a few minutes. Their analysis yielded an Arrhenius-like behavior, t ∝ exp(-ΔE/k_BT) where ΔE @ 23 K [a] for the case of H₂. The energy ΔE can be associated with the activation energy for surface diffusion. The activation energy for bulk diffusion is so high that this mechanism is negligible in the temperature range considered here. No dependence on ortho/para-concentration was found.

Triple point wetting with its rapid variation of the film thickness can easily affect practical experiments with hydrogen in films. Higher temperature accretion studies of H₂ crystal growth and H₂ mixtures will thus normally experience surface film effects at temperatures over about 3 K.

Further experimental work has been done on this mobile surface film using a hole burning technique [b] . This work evaporates a well of H₂ above a heater element, measuring the filling of this well by reevaporation after varying intervals. The driving force being the attractive forces of stronger binding to the subtrate for deeper layers. They found the surface diffusion to have a diffusion constant of D0 = 2 x 10^-4 m²/s +/- 10% with an activation energy of 43 K +/-15%. They posit a surface in equilibrium with a two-dimensional vapor above the surface, and this vapor-surface interaction provides a diffusion mechanism.

The hydrogen surface in equilibrium with the vapor is by no means like a normal solid surface, where there is also motion of surface atoms as a result of a variety of effects. Hydrogen has a zero-point energy that gives each molecule a fluidity even at zero temperature. In is not clear at this writing what this means for the time dependent morphology of a perfect solid hydrogen surface, much less a real surface.

a) U. Albrech, R. Conradt, S. Herminghaus , and P. Leiderer, "Wetting Phenomena in films of molecular hydrogen isotopes," Low Temp. Phys., 22, (2), 117 (1996).

b) E. Bloss and A.F.G. Wyatt, "Surface Diffusion of Solid Hydrogen," J. Low Temp. Phys., 119, 5/6, 743 (2000).

Please email Steve Bates at thought@Thoughtventions.com to discuss this research.

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Last updated: July 2015