Structures and thermal properties of the N2 monolayer on Pb(1 1 1).

The physisorption of N-2 molecules has long been a model system of molecular adsorption. We present a low-energy electron diffraction (LEED) study of the adsorption structures and thermodynamics of monolayer N-2 on Pb(1 1 1). The results indicate that the monolayer structure has a triangular incommensurate center-of-mass lattice, and that the N-2-substrate interaction is weaker than that observed on other metal surfaces. The N-2 monolayer undergoes a phase transition between an orientationally ordered phase (low-temperature) and an orientationally disordered phase at a temperature of 20 K. Potential energy and quasiharmonic calculations indicate that the weak N-2-Pb(1 1 1) interaction is the main contributing factor for the difference in orientational order of incommensurate N-2 monolayers on Pb(1 1 1) and other similar metal surfaces.