25 Jun 2013

X-ray absorption spectroscopy and its applications to LFT

Five Slides About

Submitted by Karen McFarlane Holman, Willamette University

This series of (not five) slides introduces X-ray absorption spectroscopy (XAS), specifically XANES (X-ray absorption near-edge structure).  There is background in basic theory, the general technique including synchrotron radiation sources, and two specific examples from the literature that apply XANES spectra to (1) oxidation state and effective nuclear charge of sulfur in various compounds such as sulfates, and (2) measurement of energy levels in MO diagrams of coordination compounds (i.e., LFT).  Point (2) is analogous to showing PES peaks alongside MO diagrams for diatomics.  This is a fun slide series with some cool animations!  Note that you can extract some of the slides and re-vamp them to teach other techniques such as EDS.


Office presentation icon XAS for LFT3.ppt2.6 MB
Learning Goals: 

After assimilating the information provided in these slides, the students will:

1.     Understand the fundamental ideas of synchrotron radiation, XAS, the K-edge of an element, and the pre-edge region of an XAS spectrum.

2.     Recognize the parallels between XAS and other basic spectroscopic techniques such as UV/Vis.

3.     Identify the unique features (utility) of XANES.

4.     Compare XANES spectra for different compounds and correlate oxidation states and effective nuclear charge with the K-edge energy and/or pre-edge features.

5.     Identify pre-edge features in XANES spectra as indicators of metal-ligand mixing and determine information regarding the electronic structure of the d-manifold. 

Note that there is a lot more information that can be determined from XANES spectra regarding orbital energies, quantitation of M-L mixing, covalency, and site symmetry, especially when spectral analysis is done alongside DFT.  Students who have a strong background in symmetry and quantum would be able to go into significantly more depth than what is provided here, but these slides are only meant to serve as a basic introduction.  I wrote up notes for the instructor, so if you don’t know XAS, you’ll learn some cool stuff too!

Implementation Notes: 

These slides are meant to introduce XAS to your students.  You could go through them in 15 minutes but to really have fun with it, I would take 30.  From here, they should be able to make some interpretations of spectra - comparing oxidation states, the degree of M-L orbital overlap, etc.

I tried to make good notes... please let me know if any of the slides are unclear or if you'd like anything expanded upon, etc.

Time Required: 
15-30 min
Evaluation Methods: 

You'd want to follow up with an in-class assignment or problem set to assess their understanding.  I hope to create one or both for submission to VIPEr, and hopefully others will as well!

Evaluation Results: 

My students have done fairly well on exam questions after teaching this level of introduction.  The questions have never been overly difficult, but I do have the sense that they have a fundamental understanding of the technique.  As I teach it more, I hope to have more to report in this section.

Creative Commons License: 
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