Structure and Dynamics of a Submonolayer Film Adsorbed on Solid Surfaces

Abstract

The equilibrium configuration of a submonolayer film adsorbed on a solid surface is analyzed via a model classical Hamiltonian. In this model, the adsorbed atoms or molecules are represented by point particles interacting through connecting springs and the substrate is simulated by a two-dimensional periodic potential. The resulting structure of the film is found to be either commensurate or incommensurate with the periodic potential, depending on the force constant of the springs, the amplitude of the periodic potential, and the degree of mismatch between the length of the unstretched springs and the lattice parameter of the underlying periodic potential. Next, the lattice-vibration spectrum of the film is studied. It is found that when the film structure is incommensurate, the excitation spectrum starts from zero energy. In the commensurate structure the phonon spectrum always starts at a finite frequency. It is noted that this difference between commensurate and incommensurate behavior has an important influence on the thermodynamic properties of the film. Qualitative comparison with experimental data is made.