IR spectroscopic study of the displacement of an SF 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 SF 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 SF 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 SF coverage on the surface is monitored by the frequency shift due to dynamic dipole coupling of the collective mode of the strong SF ν vibrational resonance. Simulations relate this frequency shift to the SF areal density. Below T ≈ 134 K, with increasing Xe pressure, a small amount Xe dissolves in the solid SF 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 SF monolayer is near its melting line, the melting transition on adding Xe appears to become continuous.