Systematic Investigation of Intermolecular Interactions in NEXAFS Spectroscopy

Abstract

Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy can be used to study molecular packing and order in organic materials, but only if the spectroscopic effects of intermolecular interactions are well understood. This work aims to contribute to an improved general understanding of the roles of intermolecular interactions on NEXAFS spectroscopy by studying the effects of Rydberg quenching on the degree of Rydberg-valence mixing in saturated molecules and π-π interactions of unsaturated molecules. The effects of π-π interactions were systematically studied by using paracyclophane (PCP) molecules, in which the benzene/benzene separation distance can be systematically varied through the length of the spacer between benzene rings. The effects of Rydberg quenching on the degree of Rydberg-valence mixing in saturated molecules were systematically studied as a function of different crystalline polymorphs (orthorhombic and monoclinic) and chain lengths of n-alkane single crystals. The effects of π-π interactions with varied spacing between co-facial benzene rings in PCPs are observed and these intermolecular effects can be used to study molecular packing and order in unsaturated materials. This work explores the strengths and significance of the effects of π-π interactions on NEXAFS spectroscopy as a function of benzene-benzene separation distances. The effects of Rydberg quenching on the degree of Rydberg-valence mixing to the NEXAFS spectra are not significant between different n-alkanes crystalline polymorphs. However, linear dichroism effects were observed for these different n-alkane crystalline polymorphs. For a given a crystal structure (orthorhombic or monoclinic), the relative intensities of the two C-H peaks (287-288 eV) and the energy of the C-H band (287-288 eV) changed when X-ray linear horizontal polarization was aligned along the principal axes (X,Y) of individual crystals. In addition to the observed linear dichroism, the room temperature NEXAFS spectra of orthorhombic alkanes becomes broader as the alkane chain length decreased. This broadening of NEXAFS spectra is believed to be the result of increased molecular disorder and nuclear motion at room temperature. Nuclear motion effects refers to the energetically accessible molecular conformations present at the experimental temperature. In summary, this work is a significant contribution to the development a more comprehensive understanding of the influences of intermolecular interactions on NEXAFS spectroscopy.