Effects of lateral compression on a non-registered monolayer☆

Abstract A thermodynamic description is developed for the lateral compression observed in a non-registered monolayer as the pressure or temperature of coexisting three-dimensional gas is varied. The isothermal lateral compressibility of the monolayer is expressed in terms of structural data obtained in low energy electron diffraction studies. The limit-of-compression is determined by the onset of a second adsorbed layer. Detailed calculations of the limit-of-compression at zero temperature are performed for a Lennard-Jones pair potential model and for a model of xenon adsorbed on silver. The results show that the lateral spacing of the adsorbed monolayer at the onset of the second layer is very close to the nearest neighbor spacing of the bulk adsorbate, in agreement with observations on several non-registered systems. The calculations also predict a difference in the lateral lattice constant for the coexisting monolayer and bilayer; this has not been observed.