Investigation of Nuclear Motion Effects in NEXAFS Spectroscopy
Date
2023-05-17
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
0009-0007-8617-646X
Type
Thesis
Degree Level
Masters
Abstract
Near Edge X-ray Absorption Spectroscopy (NEXAFS) is a widely used tool for the chemical analysis of organic materials. Sensitivity of this spectroscopic technique is based on spectral relationships such as functional group identity, oxidation states and electronic structures which are interpreted in terms of electronic transitions from core electrons to unoccupied levels. Recent research shows nuclear motion in the form of thermally populated vibrations, gauche defects, conformational changes, etc., contributing to the shape of specific lines in NEXAFS spectra.
Previous work on nuclear motion effects in NEXAFS focused on n-alkane systems. The work in this project expanded to study polymer and conjugated aromatic systems. Experimental NEXAFS spectra and MD-DFT simulations were examined at temperatures ranging from room temperature down to liquid nitrogen temperatures. Three molecular systems were examined: molecules with an isotopic/atomic substitution, molecules with and without low energy vibrations as well as molecules with vibrations presumed to be localized to a moiety.
MD-DFT simulations highlight nuclear motion effects in the form of lower peak intensity and peak broadening at higher temperatures due to a lower energy onset of the rising edge of the NEXAFS peak. Experimental NEXAFS spectra also show temperature dependent effects in the form of peak widths differing across temperature although these results are not consistent as a function of temperature.
The objective of this project was to see if a predictive model could be established to describe nuclear motion effects in NEXAFS spectra. A simplified model was used from previous work on nuclear motion effects in n-alkane systems to attempt to describe the effects observed in this project. The results have shown that the current models do not provide the answers for all the observations in both MD-DFT simulations and experimental NEXAFS spectra.
Description
Keywords
NEXAFS, MD-DFT, Nuclear Motion
Citation
Degree
Master of Science (M.Sc.)
Department
Chemistry
Program
Chemistry