The structure of para-xylene and meta-xylene adsorbed on Rh(111) studied by scanning tunneling microscopy

Scanning tunneling microscopy (STM) was used to study the adsorption of para- and meta-xylene on the Rh(lll) surface. Para-xylene appears in the STM images as a diamond-shaped feature with the long axis oriented along the [11 (2) over bar] and equivalent directions. Ordered domains with three orientations differing by 120 degrees are formed. The images of the meta-isomer have the appearance of triangular shapes and do not form any ordered structure. Individually, however, they are oriented with one side of the triangle parallel to the compact [1 (1) over bar 0] directions. The internal structure in the molecular images can be discerned. By a combination of symmetry and packing considerations, it is possible to attribute the brightest part of the molecular image as due to the methyl groups. When the two isomers are coadsorbed, they exhibit complete mixing, and the individual components can be differentiated by their shape. At low coverage, both xylene molecules adsorb preferentially at the step edges, whereas on the terraces, they are mobile in the time scale of the STM imaging. Coadsorption with CO produces a mixed layer, and the xylene molecules become immobile. (C) 1998 Elsevier Science B.V. All rights reserved.