Co-Adsorption of CF$_4$ and SF$_6$ on Graphite*

We have used infrared absorption (IRRAS) and ellipsometry concurrently to study adsorption of CF$_{4}$ on graphite (HOPG) precoated with a monolayer of SF$_{6}$. Over the temperature range studied, 50 to 120 K, multiple layers of CF$_{4}$ adsorb on top of the SF$_{6}$. At the higher temperatures, $>$85 K, partial but not complete displacement of the SF$_{6}$ is observed. At the lower temperatures, $<$75 K, no displacement is seen, including cases starting with partial monolayers of SF$_{6}$. Data on spectral shifts will be interpreted to give information on compression and mixing of components.