On to the question. I have attached a plot of a MCA spectrum collected with a vortex silicon drift detector. The spectrum is actually the average of several spectra, all collected in the post edge region of the Co L-edge. The spectra were averaged to reduce noise. The three peaks result from fluorescence from carbon, oxygen, and cobalt. Low-energy shoulders on the Co and O peaks are also observed. These can be seen as the regions of the spectrum that are not well reproduced by the fit. The main reason I included the fit in the plot is to illustrate the presence of these shoulders, particularly in the oxygen florescence, where the additional intensity is not so obvious.

I am writing to see if anyone has any suggestion as to what the origins of these peaks might be. They are not due to additional elements, as they appear at the same incident energies as the main florescence peaks, i.e. the Co shoulder appears at the same incident energies as the main Co peak, and the O shoulder appears at the same incident energies as the main O peak. It is possible that the peaks result form other transitions. Considering Co, the main peak is due to L3/L2-M4 transitions, and the shoulder is in a position that could be consistent with L3/L2-M1 transitions. However, by fitting the peaks with Gaussians, one finds an area for the shoulder that is about 25% of the area of the main peak. This is significantly larger than what one might expect from tabulated transition strengths like those given in Hephaestus.

To summarize, does anyone know what these shoulders might result from if not lower energy transitions? If they are low energy transitions, why would the relative transition strengths differ from tabulated values?