Removing multi-electron excitations
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XAS is normally thought of in terms of a single electron phenomenon. A photon goes in and a photoelectron goes out. In fact multi-body phenomena are possible and, on occasion, must be considered in the interpretation of XAS data. One such is the so-called “shake-off” effect in which the photoelectron has sufficient kinetic energy to excite a high-lying electron. For example, at around 415 eV above the uranium LIII edge, the photoelectron can excite an or N7 transition.
The cross-section of this secondary edge can be quite small. In the example of the LIIIN6,7 transition, the secondary cross section is about 3 orders of magnitude smaller than the primary LIII edge. If, in this example, you have very good data with measurable EXAFS beyond about 10.5 Å-1 , the multi-electron excitation will not be small compared to the LIII EXAFS. Other multi-electron excitations have even larger cross-sections compared to their primary excitations. For a much more complete discussion of multi-electron excitations see Iztok Arcon's Mulielectron Photoexcitations page.
ATHENA uses a relatively simple algorithm to attempt to remove the effect of the multi-electron excitation from your data. Be warned that the algorithm described here requires considerable user input and sufficient knowledge to properly evaluate the results.
This algorithm can do serious damage to your data.Without a sober, knowledgeable assessment of the results of applying this algorithm made by you, the user, this part of ATHENA should be considered fragile and dangerous.
That said, let's carry on.
The dialog shown below provides an interface to a simple, three-parameter algorithm for approximately removing the effect of the multi-electron excitation.
The multi-electron excitation removal correction dialog.
Describe basic idea -- secondary peak is a broadened reflection of the first.
Describe algorithm -- spectrum shifted and broadened then subtracted from the data.
Plot in various spaces to evaluate result.
Real evaluation will require proper fitting of data.
Make a group.
Database of energy offsets.
Removing the effect of the multi-electron excitation. The secondary excitation can be seen as the shoulder marked by the purple arrow. The red line shows χ(k) after removal.
How did we know that to be the effect of an MEE? Problems fitting + C. Hennig, Phys. Rev. B, 75, (2007) p. 035120.
