NEXAFS spectroscopy of Alkanes - matrix effects and linear dichroism

Full text: The C 1s Near Edge X-ray Absorption Fine Structure (NEXAFS) spectra of alkanes are complex and the subject of controversy. Alkanes are a broad class of saturated molecules, including simple hydrocarbons such as methane, ethane, propane; longer molecules such as paraffin; and polymers such as polyethylene. The evolution of the NEXAFS spectra between alkane molecules of varied chain length, branching, phase (e.g. gas versus solid) and temperature is poorly understood. While there is a satisfying description of spectral characteristics for simple alkanes in the gas phase (where the spectra are dominated by Rydberg states, there is considerable controversy over the interpretation of spectra in the condensed state. The selection of an appropriate model to interpret the linear dichroism in the NEXAFS spectra is closely related to these matrix effects. Different models (building block versus a molecular orbital model) can lead to different results for the measurement of molecular orientation by angle resolved NEXAFS spectroscopy. We have attempted address these closely interrelated questions with a coupled program of calculation and experiment, specifically: a.) Angle resolved NEXAFS spectroscopy measurements of isolated single crystals of linear alkanes, to provide an absolute measurement of the transition dipole moment for the NEXAFS spectra of linear alkanes. b.) Comparative gas and condensed phase NEXAFS spectra of alkanes to characterize the effect of phase on the spectra. c.) Ab initio Improved Virtual Orbital (IVO) calculations, using the program GSCF3, of isolated alkanes and alkanes embedded in model clusters, to characterize Rydberg