Photoelectron spectroscopy of sulfur heterocycles adsorbed onto a Cu(110) surface

Abstract

Photoelectron spectra of Cu(110) after exposure to thiacyclobutane, thiacyclopentane, and thiacyclohexane, as well as of the clean Cu(110) surface, have been obtained using He(I) radiation. Comparison of the spectra of the absorbates with gas-phase spectra suggests a strong sulfur-to-copper dative bond. Sulfur lone-pair orbital stabilization energies were obtained for thiacyclobutane (1.4 eV), thiacyclopentane (1.1 eV), and thiacyclohexane (0.6 eV), and these stabilization energies are correlated with the dative sulfur-to-surface bond energies. Ring-strain changes are suggested as the reason for additional small shifts of absorbate-bond energies in the thiacyclobutane spectrum. Relaxation energies were found to decrease in the order 4- > 5- > 6-membered ring. An empirical linear relation was found between relaxation shifts and lone-pair orbital stabilization energies for monoheteroatom heterocycles bonded to Cu(110) by a lone-pair dative bond. All adsorbates studied showed marked angular dependence, suggesting nonrandom orientation on the surface. Al Kα X-ray data were used to calculate percentage surface coverages. Other sulfur-containing adsorbates are discussed, including thiophene, allyl mercaptan, and thiacyclopropane. Thiacyclopropane was found to decompose upon exposure to copper.