Near edge x-ray absorption fine-structure studies of molecules and molecular chains bonded to surfaces

Abstract

A review and future outlook are given of the capabilities of the near edge x-ray absorption fine-structure (NEXAFS) technique for the study of simple and complex molecules bonded to surfaces. NEXAFS resonances arise from K-shell excitations to final states which are highly localized on the core excited atom. The localized nature of the measured excitations allows the determination of the orientation, hybridization, and bond lengths of specific functional groups (e.g., C–C, C–C, or C–O) in molecules ranging from diatomic to complex polyatomic species. Breakdown of this localized excitation picture is found when the molecular orbitals of two or more molecular units or functional groups interact. As examples we discuss interactions between π and/or σ states in aromatic hydrocarbon rings and conjugated π orbitals in complex acids which lead to a splitting of NEXAFS resonances. NEXAFS spectra of Langmuir–Blodgett chains reveal a pronounced polarization dependence which allows the determination of the macroscopic chain orientation on the surface.