Triple-point wetting of molecular hydrogen on tailored substrates

Triple-point wetting is a well-known phenomenon of many simple adsorbates o n solid substrates. It implies that in the liquid phase above the triple-po int temperature, T-3, complete wetting with the formation of arbitrarily th ick films is observed, whereas below T-3 only a few monolayers of the solid phase are adsorbed at saturated vapour pressure. This effect is usually as cribed to substrate-induced strain in the solid film, which occurs due to l attice mismatch and/or the strong van der Waals pressure in the first monol ayers. Molecular hydrogen is a suitable system to investigate this phenomen on, in particular by tailoring the adsorbate-substrate interaction by means of thin preplating layers of other adsorbates, and by introducing disorder into the system by using not only the pure systems H-2 and D-2, but also m ixtures thereof. In particular the dependence of T-3 of the mixture on the mass ratio of its components is measured and deviations from the simple van der Waals law are discussed. The experiments show that triple-point wettin g is a rather dominating effect, which in contrast to expectation persists even if the system parameters are widely varied, indicating that the presen t picture of this effect is incomplete.