A complete structural study of the coverage dependence of the bonding of thiophene on Cu(111)

The influence of coverage on the structure and bonding of thiophene within chemisorbed overlayers on Cu(111) has been investigated. Normal incidence X-ray standing wavefield absorption (NIXSW) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS) were used to determine the adsorption site and orientation of thiophene. In the low coverage phase the thiophene molecule forms a pi -bonded species, adopting an atop adsorption site with a Cu-S separation of 2.62 +/- 0.03 Angstrom with the ring inclined by 26 +/- 5 degrees, On increasing coverage, thiophene undergoes a compressional phase transformation to a more weakly bound sulfur lone-pair-bonded species. In this new phase the thiophene still occupies an atop site but the Cu-S separation has increased by 0.2 Angstrom and the ring is now inclined at 44 +/- 6 degrees to the surface.