title: Methylthiolate on Au(111): adsorption and desorption kinetics.
abstract: Low energy electron diffraction, Auger electron spectroscopy, X-ray photoelectron spectroscopy and line of sight mass spectrometry have been used to study the adsorption and desorption of dimethyldisulfide (DMDS) on Au(111). At 300 K adsorption is dissociative, forming a chemisorbed adlayer of methylthiolate with a 1/3 ML, (sq rt 3 x sq rt 3)R30 degrees, structure. At 100 K adsorption is molecular, with dissociation to form the 1/3 ML (sq rt 3 x sq rt 3)R30 degrees methylthiolate structure occurring at 138-160 K. A physisorbed DMDS layer, with a coverage of 1/6 ML of DMDS, forms on top of the (sq rt 3 x sq rt 3)R30 degrees chemisorbed MT surface for T < or = 180 K, with multilayers forming for T < or = 150 K. In temperature programmed desorption, multilayers of DMDS desorbed with zero order kinetics and an activation energy of 41 kJ mol(-1); the physisorbed layer desorbed with first order kinetics, exhibiting repulsive lateral interactions with an activation energy which varied from 63 kJ mol(-1) (theta = 0) to 51 kJ mol(-1) (theta = 1); the chemisorbed methylthiolate layer desorbed associatively as DMDS via the physisorbed layer, the activation energy for the reaction, 2 methylthiolate --> physisorbed DMDS, exhibiting repulsive lateral interactions with an activation energy which varied from 65 kJ mol(-1) (theta = 0) to 61 kJ mol(-1) (theta = 1). The physisorbed disulfide layer explains the pre-cursor state adsorption kinetics observed in sticking probability measurement, while its relatively facile formation provides a mechanism by which thiolate self-assembled monolayers can become mobile at room temperature.
authors.affiliation: Department of Physical Chemistry, School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, UK.
authors.forename: Mark G
authors.initials: MG
authors.lastname: Roper
authors.forename: Robert G
authors.initials: RG
authors.lastname: Jones
mesh_terms: D000327:Adsorption; D004220:Disulfides / Q000737:chemistry*; D006046:Gold / Q000737:chemistry*; D007700:Kinetics; D013057:Spectrum Analysis / Q000295:instrumentation / Q000379:methods; D013499:Surface Properties; D013696:Temperature; D013997:Time Factors; D014965:X-Rays
chemical_list: D004220:Disulfides; C021181:dimethyl disulfide; D006046:Gold