AVS 47th International Symposium
    Surface Science Thursday Sessions
       Session SS2+VT-ThM

Paper SS2+VT-ThM6
Experimental and Theoretical STM Imaging of Xylene Isomers on Pd(111)@footnote 1@

Thursday, October 5, 2000, 10:00 am, Room 209

Session: Adsorption and Desorption Phenomena II
Presenter: A. Loui, University of California, Davis
Authors: D. Futaba, University of California, Davis
J.P. Landry, University of California, Davis
A. Loui, University of California, Davis
S. Chiang, University of California, Davis
Correspondent: Click to Email
We have performed both theoretical and experimental studies of the three isomers of xylene, C@sub 6@H @sub 4@(CH@sub 3@)@sub 2@, adsorbed on Pd(111). Simulated scanning tunneling microscope (STM) images, calculated using a relatively simple computational method based on extended Hückel molecular orbital theory (EHT), showed no observable differences in surface features between occupied and unoccupied states images, nor between adsorption on the various high symmetry binding sites. The calculated binding energies suggest that xylene molecules prefer to bind in particular orientations on hollow and bridge sites. The computed images for xylene on Pd displayed almost identical surface features and symmetries as those calculated on Rh(111).@footnote 2@ Using STM and low-energy electron diffraction (LEED), we imaged the characteristic surface structures and investigated the molecular ordering for the three isomers on Pd(111). Experimental STM images showed each of the isomers exhibiting the characteristic geometries predicted by EHT. Both meta(m-)xylene and ortho(o-)xylene are characterized by an overall triangular shape, with m-xylene nearly equilateral. Para(p-)xylene shows a distinct diamond shape. M-xylene molecules adsorbed to upper step edges, usually with one side of the molecule aligned with the step edge and parallel to the close-packed directions of the Pd lattice. Only the p-xylene isomer showed long range order, forming a hexagonal structure with the long axes of the molecules parallel to each other. Translation and rotation of p-xylene molecules between consecutive scans were observed for the saturated Pd surface, causing disorder in previously ordered regions. @FootnoteText@ @footnote 1@ Supported by NSF CHE-95-20366 and UCOP CLC. @footnote 2@ D. N. Futaba and S. Chiang, Surf. Sci., 448, L175 (2000).