IMAGING STRUCTURAL DETAILS IN CLOSELY RELATED MOLECULAR ADSORBATE SYSTEMS

Monolayer and submonolayer coverages of naphthalene, azulene and methy lazulene adsorbed onto Pt(111) have been imaged with molecular resolut ion. Comparisons of the adsorbate images of azulene with those obtaine d from its isomer, naphthalene, reveal that binding for the two closel y related species is quite different. Naphthalene occupies an adsorpti on site on the Pt(111) surface with three equivalent orientational min ima, while azulene exhibits no clear orientation with the metal substr ate lattice. At less than saturation coverage, naphthalene undergoes d iscrete 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 m agnitude faster. Substitution of a single methyl group onto azulene sl ows the molecular diffusion to an intermediate rate and enables imagin g of the distinct adsorbate asymmetry for two methylazulene isomers. C oadsorbing naphthalene with azulene or methylazulene allows direct com parison of localization characteristics as well as internal structure and orientation.