IMAGING STRUCTURAL DETAILS IN CLOSELY RELATED MOLECULAR ADSORBATE SYSTEMS

Monolayer and submonolayer coverages of naphthalene, azulene and methylazulene adsorbed onto Pt(111) have been imaged with molecular resolution. Comparisons of the adsorbate images of azulene with those obtained from its isomer, naphthalene, reveal that binding for the two closely related species is quite different. Naphthalene occupies an adsorption site on the Pt(111) surface with three equivalent orientational minima, while azulene exhibits no clear orientation with the metal substrate lattice. At less than saturation coverage, naphthalene undergoes discrete rotation among the three allowed orientations and translation between adjacent binding sites on a time scale of minutes, but azulene diffusion rates at room temperature are approximately two orders of magnitude faster. Substitution of a single methyl group onto azulene slows the molecular diffusion to an intermediate rate and enables imaging of the distinct adsorbate asymmetry for two methylazulene isomers. Coadsorbing naphthalene with azulene or methylazulene allows direct comparison of localization characteristics as well as internal structure and orientation.