100 years of XAFS papers, #1-#2
Gerald Seidler
University of Washington Professor of Physics and co-founder @ easyXAFS LLC | X-ray Absorption Spectroscopy, Materials Science
XAFS was first observed in 1920, independently, by Fricke and by Hertz. To celebrate the pre-centennial in 2019, the easyXAFS twitter stream is going to celebrate 99 great XAFS papers. Since many of you aren't on twitter, I'm cross-posting on linkedin. Please enjoy, and let your students know that they can learn much about the history and present practice of XAFS by watching the twitter feed or these posts. We'll start here in 1920, and some time before Christmas we'll end in deep outer space.
Paper #1: FIne Structure!
From https://twitter.com/easyXAFS/status/1080550051619827713
The year is 1920, and Hugo Fricke has published a paper in the Physical Review https://doi.org/10.1103/PhysRev.16.202 … where he reports observing "complex structure" at the K-shell absorption edge of several elements ranging from phosphorus to chromium.
The figure shows a comparison of Fricke's data on V2O5 and modern data from bealine 13-BM of the Advanced Photon Source. The small amplitude of Fricke's pre-edge is partially due to his ~3 eV energy resolution, but also indicates some contamination of a higher symmetry compound, such as V2O3.
The introduction of Fricke's paper is prescient in its statement about the importance of high energy resolution only then just available due to Siegbahn's work: "It may therefore be expected that Siegbahn's method will lead to the detection of many effects which are caused by the action of the outermost electrons from which effects a calculation of the arrangement of the electrons can be made.”
Paper 2: Before spectrum, there has to be spectrometer.
From https://twitter.com/easyXAFS/status/1080883543008374784
Before a first XAFS spectrum, there had to be a high-resolution x-ray spectrometer. Manne Siegbahn wrote about three spectrometers in Phil. Mag. in 1918-19 that refined the periodic table, led to the 1924 Nobel Prize and gave us the first XAFS spectrum. In “Precision measurement of the x-ray spectra,” Siegbahn Phil. Mag. 37, 601 (1919), there is a small vacuum spectrometer with 100-1000x finer resolution than prior instruments.
The key innovation is central to the history of all x-ray spectrometry:
The important detail here is derived from an earlier observation of the Braggs that also later led to the so-called ‘Bragg-Brentano’ geometry for powder diffraction. The key point in the figure below (from Bragg&Bragg 1913) is the angle QPO is the same as the angle QR’O.
As long as the crystal is tangent (or nearly) to the indicated circle, the scattered wavelength will be properly selected by Bragg’s law together with the small size of entrance and exit slits, even when there is *no collimation* of the incident beam.This was likely the first x-ray spectroscopic application of the idea of the Rowland circle. It certainly seems to have led the many attempts to improve x-ray spectroscopy by learning from Rowland’s famous work on curved optical gratings. More on this in future posts! For present purpose, the important point is that Siegbahn’s spectrometer combined higher flux (an early use of metal x-ray window) with a design that had both higher resolution and higher accuracy than prior gadgets. And, hence, Fricke could take the first K-shell XAFS.