IR spectroscopic study of the displacement of an SF₆ monolayer on graphite by Xe

We report a study of displacement by xenon of a monolayer of sulphur hexafluoride initially condensed on a graphite surface. Earlier work showed that, below 112 K, Xe displaces SF₆ almost completely in a first-order transition. Working at higher temperatures, we show that this system has a simple eutectic-like phase diagram, at least for SF₆ not too dilute. In our experiment, both adsorbates are in equilibrium with their respective vapors in a cold cell. In our infrared reflection-absorption spectroscopy measurements, the SF₆ coverage on the surface is monitored by the frequency shift due to dynamic dipole coupling of the collective mode of the strong SF₆ ν₃ vibrational resonance. Simulations relate this frequency shift to the SF₆ areal density. Below T ≈ 134 K, with increasing Xe pressure, a small amount Xe dissolves in the solid SF₆ monolayer preceding its displacement by a solid predominantly Xe monolayer in a first-order transition. Above 134 K, there is a weaker first-order transition to a mixed liquid monolayer, followed by continuous increase in Xe concentration. If the initial SF₆ monolayer is near its melting line, the melting transition on adding Xe appears to become continuous.