Thioethylpyrrole monolayers on gold. A spectroscopic study in ultrahigh vacuum

The adsorption of 3-(2-thioethyl)pyrrole (3-TEP) and 1-(2-thioethyl)pyrrole (1-TEP) on gold has been studied in ultrahigh vacuum with infrared reflection absorption spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed desorption mass spectroscopy. Depositions were made from liquid phase at atmospheric pressure, from gas phase in ultrahigh vacuum: and from gas phase at atmospheric pressure. Both molecules were found to chemisorb to the gold surface regardless of the deposition method. Most of the 3-TEP, however, broke up, losing the pyrrole ring as it adsorbed on the surface. Both monolayers were sensitive to oxygen and oxidized quickly if stored in air. Deposition of 1-TEP from solution or vapor in air gave a higher desorption peak temperature than deposition from gas phase in UHV. The masses of the desorbing fragments that were detected from the heated monolayers were smaller than those seen upon introducing 1-TEP gas into the UHV system, revealing that a partial decomposition of the adsorbed molecules occurred prior to or at desorption. Some samples were electrochemically cycled to positive potentials to investigate the stability of the monolayers. A large fraction of the 1-TEP and 3-TEP stayed on the surface when cycled to 0.8 V, but some oxidation, fragmentation, and desorption were seen at higher potentials.