Observation and calculation of internal structure in scanning tunneling microscopy images of related molecules

The scanning tunneling microscope has been used to distinguish among a series of related molecules on Pt(111) by details of their observed structures. The electronic isomers naphthalene and azulene can be recognized, as can three structural isomers of monomethylazulene, as well as dimethyl- and trimethylazulene. The number and position of substituents affect the diffusion rates, sticking coefficients, and orientations. A simple methodology based on extended Hückel theory, including the substrate, generates electron and hole density plots which show very good agreement with the data. © 1993 The American Physical Society.