Mixing in a Solid Monolayer: SF$_{6}$ + C$_{2}$F$_{6}$ on Graphite*

We have studied mixing of SF$_{6}$ and C$_{2}$F$_{6}$ in a solid monolayer film on graphite by admitting one gas to the cell to a pressure between monolayer condensation and saturation, then slowly admitting the other gas. The total coverage is monitored by ellipsometry and the condition of the individual adsorbates is determined by infrared absorption spectra. In particular, shifts in absorption lines due to dynamic dipole coupling indicate the local environment of molecules of each species. At 75 and 80 K there is no detectable solubility of C$_{2}$F$_{6 }$in a dense SF$_{6}$ monolayer, then a layer of nearly pure C$_{2}$F$_{6 }$condenses on top. At 86 K there is limited solubility of C$_{2}$F$_{6}$ in dense SF$_{6}$, followed by appearance of a second C$_{2}$F$_{6}$--rich phase in the monolayer. At 90 K and above, C$_{2}$F$_{6}$ appears to replace SF$_{6}$ by continuous substitution; thus there appears to be a 2-D solid consolute critical point near 90 K. If SF$_{6}$ is added to a higher-temperature, low-density C$_{2}$F$_{6}$ layer, the C$_{2}$F$_{6}$ molecules are rapidly compressed from flat or tilted orientation to axes perpendicular to the substrate, then are continuously diluted.