NEXAFS study of 2-mercaptobenzoxazole adsorbed on Pt(111): multilayer and monolayer

The adsorption of 2-mercaptobenzoxazole on the Pt(111) surface has been studied, in both monolayer and multilayer regimes, by means of NEXAFS (near edge X-ray absorption fine structure). 2-Mercaptobenzoxazole represents a very interesting molecule because it contains an aromatic, benzene-like fragment as well as a heteroatom-rich part, chemically more reactive, containing N and S atoms, which can bond covalently to metals. Spectra at normal and grazing incidence angle of the photon beam with respect to the surface have been recorded. The carbon and nitrogen K-edges and the sulphur L-edge were measured. The C and N K-edges recorded for the multilayer system exhibit strong angular dependence in the pi* resonances region, indicating a clear preferential orientation of the molecules on the surface. The angle between the molecular plane and the surface has been found to be about 35 degrees. The S L-edge of the multilayer, instead, shows only a weak angular dependence. Furthermore, the C K-edge spectra of the monolayer reveal practically no change going from normal to grazing incidence. Hypotheses of random or "magic" angle orientation in the monolayer are discussed. The covalent nature of the bond between 2-mercaptobenzoxazole and Pt(111) surface has been hypothesized on the basis of the comparison between monolayer and multilayer systems. (C) 1998 Elsevier Science B.V. All rights reserved.