WHY DO VERTICAL STEPS REAPPEAR IN ADSORPTION-ISOTHERMS

On the basis of an extensive set of vapor pressure isotherm and diffraction measurements and computer simulations, we present a microscopic description of the mechanism responsible for the observations of Youn and Hess, Phys. Rev. Lett. 64, 918 (1990), relating to the (reentrant) layering properties of argon-on-graphite films. In the neighborhood of 67 K (and for films more than one layer thick) a transition region exists where the outermost solid layer becomes liquid by a coverage driven process involving layer promotion. Once formed, the liquid layer can be ''re'' solidified by further increases in argon coverage. This advancing solid-liquid interface results in (re)appearance of vertical steps in the isotherms.