X-Ray Raman Spectroscopy—A New Tool to Study Local Structure of Aromatic Hydrocarbons and Asphaltenes

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are essential components in numerous natural and man made organic compounds. In asphaltenes, knowledge of the electronic structure of both its aromatic and aliphatic components is of crucial importance, when trying to understand and predict its macroscopic properties. Theoretical studies of complex PAHs have led to simple rules to describe their π electron distribution, but systematic experimental support is still needed. A novel approach to directly probe carbon type in PAHs is described here, namely the technique of X-ray Raman spectroscopy (XRS). Conventional X-ray absorption spectroscopy (XAS) has been used for many years for element specific characterization of local structure and chemistry. However, in the soft X-ray region (relevant for light elements) its application to numerous carbonaceous systems including asphaltenes encounters problems. Most of the difficulties, including surface sensitivity and restricted sample environments, stem from the submicron path lengths of soft X-rays and/or electrons. XRS provides a means for obtaining the information content of soft X-ray absorption spectra while maintaining the experimental benefits of hard X-ray techniques (˜mm path length). In the XRS process an incident photon is inelastically scattered and part of its energy is transferred to excite an inner shell electron into an unoccupied state. Under the dipole approximation, the resulting features are identical to the corresponding XAS spectrum. In the past, the extremely low cross-section of XRS has made this technique impractical, but intense new X-ray facilities and improvements in X-ray optics have helped to advance this unique spectroscopic tool. We have applied XRS to a series of PAHs and several asphaltenes. The results directly support the simple ideas contained in the “sextet-double bond” description of PAHs. Furthermore, we show that XRS can probe the geometrry of aromatic ring systems in asphaltenes, as well as its ratio of aromatic and aliphatic constituents.