MOLECULAR-ORIENTATION IN ORGANIC MONOLAYERS PROBED BY UPS AND MIES (METASTABLE INDUCED ELECTRON-SPECTROSCOPY)

A self-assembled monolayer of hexadecanethiol (HDT) on an Au(111) subs trate and an adsorbed monolayer of octane on a Pt(111) substrate as re presentatives for films of standing and lying alkanes have been charac terized by LEED and XPS and further investigated by UPS and MIES. Sinc e MIES is a perfectly surface sensitive method the spectra yield infor mation about the molecular orbitals that are exposed to the surface. T hus, this technique is ideally suited to contribute to the question of orientation of the molecules and the outermost molecular groups. Refe rring to the well investigated hexadecanethiol film on Au(111) and oct ane film on Pt(111) as reference systems we achieve information about the structure and molecular orientation of the yet unknown hexadecanet hiol layer on Pt(111). Because of its larger information depth UPS all ows us to monitor the intensity due to the substrate density of states near the Fermi-edge. It can be shown that this quantity is strongly a ffected by the chemisorption of sulfur atoms, if hexadecanethiol is ad sorbed on a platinum substrate. Nevertheless, the changes of the inten sity at the Fermi-edge can be used to trace the destruction of hexadec anethiol films at elevated temperatures. In contrast to the system HDT /Pt(111) the thiol film on the Au(111) substrate totally desorbs above 490 K.