The interface between a two-dimensional (2D) molecular gas and a 2D molecular solid has been imaged with a low-temperature, ultrahigh-vacuum scanning tunneling microscope. The solid consists of benzene molecules strongly bound to step edges on a Cu{111} surface. Benzene molecules on the Cu{111} terraces move freely as a 2D gas at 77 kelvin. Benzene molecules transiently occupy well-defined adsorption sites at the 1D edge of the 2D solid. Diffusion of molecules between these sites and exchange between the two phases at the interface are observed. On raised terraces of the copper surface, the 2D gas is held in a cage of the solid as in a 2D nanometer-scale gas bulb.

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In two sets of experiments built on the work of Brian Bent, we imaged benzene molecules and phenyl radicals on the Cu{111} surface at low coverage and low temperature. The experiments allowed us to see how mobile molecules on surfaces probe the electronic structure of the surface. Bare terraces of the Cu{111} surface appear extremely flat in scanning tunneling microscope images. We are thus able to image the perturbations to the electronic structure caused by steps, defects, and adsorbates. These perturbations determine the structure and dynamics of the adsorbates. Benzene forms ordered structures along step edges at even very low coverages. Adsorbed phenyl radicals form complex pairs, aligned so as to be able to couple to form biphenyl at higher temperature. We discuss the chemical consequences of such substrate-mediated interactions.

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