Multilayer adsorption of xenon, krypton, and argon on graphite: An ellipsometric study.

Abstract

We present ellipsometric measurements of multilayer adsorption of xenon, krypton, and argon on highly oriented pyrolytic graphite along numerous isotherms spanning the coverage range from completion of the first layer to about twelve layers and the temperature range from below the melting of the top layer to above the bulk adsorbate melting point ${\mathit{T}}_{\mathit{m}}$. The three adsorbates have very similar phase diagrams, and all show reentrant first-order layering. The top layer of three-layer and thicker films disorders at 0.81${\mathit{T}}_{\mathit{m}--}$0.83${\mathit{T}}_{\mathit{m}}$. For films thicker than three layers, first-order layer condensation reappears at shifted coverages and chemical potentials in the range 0.83${\mathit{T}}_{\mathit{m}--}$0.87${\mathit{T}}_{\mathit{m}}$ to 0.92${\mathit{T}}_{\mathit{m}--}$0.94${\mathit{T}}_{\mathit{m}}$. The solid adsorbate films reach a limiting thickness of about 12 layers at saturation, but the limiting thickness increases rapidly just below ${\mathit{T}}_{\mathit{m}}$ and reaches the equivalent of about 24 layers in the liquid region. We discuss implications of these results for roughening and melting of the adsorbate (111) surfaces. Chemical potentials for layer condensation are compared to a simple Frankel-Halsey-Hill theory.