Chemistry of sulfur-containing molecules on Au(111): thiophene, sulfur dioxide, and methanethiol adsorption

The interactions of three sulfur-containing molecules (C4H4S, SO2, CH3SH) with a clean Au(111) surface have been studied with a combination of thermal desorption spectroscopy (TDS) and synchrotron-based high-resolution soft Xray photoelectron spectroscopy. The adsorption and reactivity of the three molecules on Au(111) are very different. Thiophene adsorbs molecularly on Au(111) at 100 K and desorbs completely below 330 K without further decomposition. In the submonolayer range, three different adsorption states for chemisorbed thiophene are identified in TDS. It is suggested that thiopliene preferably adsorbs on the defect sites at the lowest exposure. After the defect sites are saturated, the change from a flat-lying geometry to a tilted adsorption configuration follows as the exposure increases. Sulfur dioxide also does not decompose on Au(111). For SO2 adsorption at 100 K, in addition to the multilayer desorption feature (similar to130 K), only one distinct monolayer peak with a tail extending to higher temperature appears ill TDS. The desorption temperature difference between the SO2 monolayer and multilayer is only 15 K, indicating a weak binding between SO2 and Au. For methanethiol adsorption on Au(111) at 100 K, three desorption states appear in the submonolayer range for the parent thiol. All of them appear below 300 K. The only desorption products at higher temperature are methane or methyl radicals (similar to540 K), and dimethyl disulfide (similar to470 K). Apart from the intact methyl thiol molecule, which exists at low temperatures (less than or equal to 150 K), two inequivalent intermediate thiolates, are seen to coexist on Au(111) in the 150-400 K temperature range, with one of them existing as low as 100 K. Atomic sulfur is present on the surface from 200 to 950 K. (C) 2002 Elsevier Science B.V. All rights reserved.