An experimental study of transport properties and phase transitions near a surface

The effective potential due to a surface (an interface or a wall) can strongly affect the phase transitions of an adsorbed film. This thesis contains a study of three separate cases where the effect of the surface is important: the solid Hydrogen/vacuum interface at low temperatures, Argon films near the triple point, and Xenon on NaF, aimed at achieving a better understanding of surface thermodynamics.; The surface mobility of thin solid Hydrogen films has been studied. Solid Hydrogen provides a unique opportunity to study the mobility of the free surface of a quantum solid. A 'hole-burning' technique has been used to study the mobility of both {dollar}rm Hsb2{dollar} and {dollar}rm Dsb2{dollar} films for temperatures between 1.6 K and 5 K. Even at low temperatures where the transport through the vapor is negligible, the solid Hydrogen (or Deuterium) films were found to remain mobile. The transport is thermally activated with an activation energy of 19 K for Hz and 38 K for {dollar}rm Dsb2.{dollar} The time dependence of the regrowth has been analyzed. The analysis shows that the surface transport is not due to simple diffusion.; The second set of experiments involve a study of the effect of the bulk triple point on the wetting properties of an adsorbate-substrate system. The wetting properties of light molecular gases (Argon and Methane) on a Gold substrate were studied using a quartz microbalance technique. This study extends the range of reduced temperature more than two orders of magnitude beyond previous investigations and provides the first clear evidence for the existence of a layered, solid/liquid film structure. The existence of the layered film structure and the transitions that go with it are explained with the help of a detailed model that accounts for the surface free energies of the films. The model shows that, depending on a delicate balance of the surface energy costs and their temperature dependences, one can have either purely a single-phase film (solid or liquid) or a layered film.; The third study in this thesis involves experimental attempts at observing a continuous wetting transition, predicted by theorists in 1995 in a system comprised of Xenon on Sodium Fluoride. The wetting behavior was studied using the quartz microbalance technique. A Sodium Fluoride surface was prepared onto the quartz microbalance using vapor deposition. It was found that Xenon wets this NaF surface even below the predicted wetting temperature. The 'wet' film thickness of Xenon was found to be inexplicably large. A study of the prepared NaF film showed that its surface was extremely rough. The excessive roughness accounts for the 'wetting' of NaF by Xenon due to capillary condensation.