Critical point of the second layer of CF4adsorbed on graphite

Ellipsometric measurements of adsorption isotherms of C${\mathrm{F}}_4$ on highly oriented pyrolytic graphite in the neighborhood of the critical point of the second layer are reported. We find $T_c\mathrm{}\left(2\right)=99.9\mathrm{}\pm 0.6$ K, a result that requires reinterpretation of published heat-capacity data in this neighborhood. Earlier isotherm measurements on C${\mathrm{F}}_4$ were not competitive with heat-capacity data in locating $T_c$ due to a combination of larger heterogeneity broadening of isotherm steps, too few scans to adequately separate the background, and the method of analysis, but they are not inconsistent with the present results.