what is meant by absorp(b)tion edge in xrays ? Explain it in terms of physics?

Question

Please explain it because i have to take a seminar on it.

Answer 1

This means the x-ray absorption coefficient jumps suddenly to a large value as you increase the x-ray energy. It is due to the minimum energy required to excite a prominent particular electronic transition in the target atoms being achieved.

Answer 2

At the lower x-ray energy levels (eV to KeV), the main mechanism for absorption of x-rays by atoms is the photoelectric effect. The probability (cross-section) for this drops off very rapidly as a function of energy (it's proportional to E^-3)

However, as the x-ray energy increases, every now and then it gets to the point where it's high enough to eject electrons from increasingly "deep" atomic shells. Starting from very outer shells, and progressing through M, L, and then finally K-shell, as the x-ray energy crosses that point, the absorption probability suddenly increases. These are the absorption edges that you're talking about.

Obviously, the last edge is the "K" shell edge. After that, the cross-section continues to fall and then eventually flattens out.

Note that there is 1 K-shell edge, 3 L-shell edges, 5 M-shell edges etc (corresponding to the various angular momentum sub-shells, which have slightly different energy levels).

Answer 3

K-edge extended X-ray absorption fine structure (EXAFS) spectra of Fe in varying environments in a suite of well-characterized silicate and oxide minerals were collected using synchrotron radiation and analyzed using single scattering approximation theory to yield nearest neighbor Fe-O distances and coordination numbers. The partial inverse character of synthetic hercynite spinal was verified in this way. Comparison of the results from all samples with structural data from X-ray diffraction crystal structure refinements indicates that EXAFS-derived first neighbor distances are generally accurate to ±0.02 Å using only theoretically generated phase information, and may be improved over this if similar model compounds are used to determine EXAFS phase functions. Coordination numbers are accurate to ±20 percent and can be similarly improved using model compound EXAFS amplitude information.
However, in particular cases the EXAFS-derived distances may be shortened, and the coordination number reduced, by the effects of static and thermal disorder or by partial overlap of the longer Fe-O first neighbor distances with second neighbor distances in the EXAFS structure function. In the former case the total information available in the EXAFS is limited by the disorder, while in the latter case more accurate results can in principle be obtained by multiple neighbor EXAFS analysis.
The EXAFS and XANES spectra of Fe in Nain, Labrador osumulite and Lakeview, Oregon plagioclase are also analyzed as an example of the application of X-ray absorption spectroscopy to metal ion site occupation determination in minerals