Study of the displacement transition in SF6 + Kr binary films on graphite using infrared spectroscopy and ellipsometry

Abstract

The hysteretic displacement of an SF6 monolayer on graphite by Kr was studied using infrared reflection absorption spectroscopy supplemented by ellipsometry. The SF6 molecule has a strong IR-active vibrational mode (ν3) at 948.0 cm−1. The coverage and local density of the SF6 monolayer are inferred from the ν3 absorption peak area and the blue shift of the ν3 mode, respectively, the latter due to dynamic dipole coupling between SF6 molecules in the layer. Throughout the temperature range 65.3 to 110.2 K, a first order transition with large hysteresis was found in the SF6 monolayer displacement-recovery cycle driven by Kr spreading pressure. The hysteresis is asymmetric, with displacement going to completion at a sharp pressure, while the initial ~10% of recovery occurs much closer to the displacement pressure than the pressure allowing full recovery, especially above 95 K. Over the temperature range studied, an upper limit of 0.25% on the solubility of Kr in the SF6 layer was determined by the absence of a significant change in the collective vibrational mode frequency preceding displacement. Above 92 K, spectra indicate a small solubility of SF6 in the Kr monolayer, up to 4% approaching displacement of the Kr layer at T ≥ 98 K. Below 74 K the kinetics of SF6 displacement to bulk is sufficiently slow that the monolayer is readily compressed to a metastable bilayer covering half the surface, with both layers apparently 2 × 2 commensurate with the graphite substrate. The decay time constant of the confined bilayer to bulk SF6 crystallites is about 4 h at 65.3 K.