Photoemission studies of organic-thiol self-assembled monolayers on gold surfaces

CD Zangmeister, SW Robey, and RD van Zee, Chemical Science & Technology Laboratory, National Institute of Standards & Technology, 100 Bureau Drive, Gaithersburg, MD 20899,

We have measured the photoemission spectra of organic self-assembled monolayers (SAMs) on Au(111) and polycrystalline, gold surfaces. Two classes of compounds were studied: n-alkane chains and oligomeric phenylene-ethynylenes. Single-photon spectra were recorded using a conventional ultraviolet (He I) light source, and single-color, two-photon photoemission spectra were recorded using a sub-picosecond, ultrafast laser. The peak positions and intensities in the spectra of n-alkane monolayers reflect electronic structure changes associated with chemisorption and changes in the structure of the SAM. Spectra of the oligomeric phenylene-ethynylenes were found to be similar to those of simple aromatic-thiol SAMs. The observed features in these spectra were assigned using He(I) polarization dependence and ab initio calculations of the isolated molecules.