Temperature Dependence in the NEXAFS Spectra of n-Alkanes

The near edge X-ray absorption fine structure (NEXAFS) spectra of orthorhombic single crystals of n-octacosane ( n-C₂₈H₅₈), recorded at room temperature (298 K) and at cryogenic temperatures (93 K), show distinct differences. The characteristic carbon 1s → σ*_C-H band in the NEXAFS spectrum of n-C₂₈H₅₈ is broader and has a lower-energy onset in its room temperature spectrum than in its NEXAFS spectrum recorded at cryogenic temperatures. Density functional theory simulations show that nuclear motion and molecular disorder contribute to the observed spectral broadness and are the origin of the low-energy onset of the C-H band in the room temperature spectrum.