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Abstract

The system di-tert-butyl nitroxide (DTBN) adsorbed on a thin film of γ-Al₂O₃(111) grown on a NiAl(110) single crystal has been studied with various surface science methods, including TPD, XPS, NEXAFS, and ESR line-shapes analysis. In the monolayer regime, two strong chemisorbed species with adsorption energies of 120 and 150 kJ/mol, respectively, are found after adsorption at 40 K. One species is ESR active and reveals distinct dynamic behavior above 200 K through cw ESR line-shape analysis. The second species is oriented with an angle of about 70° between the surface normal and the N−O axis of the molecule, as the NEXAFS data show. Missing of the π* resonance and the ESR inactivity suggest a direct participation of the half-filled π* orbital located at the nitrogen atom in the bonding mechanism. Above 200 K, both adsorbed species exchange with each other. This could be followed by monitoring the temperature-dependent ESR intensity. Adsorption at room temperature leads to the formation of a third unstable species which is ESR active but shows a different ESR spectrum in comparison with the other ESR-active species. The accessibility of the nitrogen atom for bonding is discussed on the basis of high-quality ab initio calculations.