Time evolution analysis of a 2D solid-gas equilibrium: a model system for molecular adsorption and diffusion

The adsorption of sub-phthalocyanine molecules on Ag(111) has been studied by means of scanning tunneling microscopy (STM). The molecules are observed in different two-dimensional (2D) phases of adsorption which coexist in th ermodynamic equilibrium. In the condensed phase the molecules form well-ord ered islands with a honeycomb pattern. In the gas phase single molecules ca n be discriminated in single scan lines by characteristic tip excursions wh ich occur randomly. The energy barrier for surface diffusion as well as the condensation energy to form 2D islands is estimated and discussed. (C) 200 1 Published by Elsevier Science B.V.