I'm attempting to fit Cl2p XPS peak using casaXPS, where I expect it to exist in two different chemical states. Hence, the fitting includes a pair of doublets (total of four peaks). The constraints that one would naturally apply in this case are;
- Position of doublets. Pos(Cl2p1/2) = Pos(Cl2p3/2)+1.6 eV
- Area of doublets. Area(Cl2p1/2) = Area (Cl2p3/2) * 0.5
I was wondering whether the FWHM should be constrained to be equal for the doublet of the same chemical state?
Any information of a range of expected FWHM of Cl2p peaks is also appreciated.
If you are confident on the resolution of your instrument and have some Cl based standards to compare with (e.g. NaCl, FeCl3, KCl etc) and have some idea of the FWHM then constraints could be applied.
It would be somewhat misleading to give ranges of FWHM here as it depends on your analysis conditions such as pass energy, any charging from ineffective neutralisation etc. However, that said for my instrument (Kratos Axis Ultra DLD, operating at 20 or 40 eV pass energy I see Cl widths between 1.0 and 1.5 eV)
Personally I find the constraints you have mentioned work perfectly and the FWHM of the peaks are typically equal. Any significant deviations would perhaps suggest two states of Cl etc
Personally I find the constraints you have mentioned work perfectly and the FWHM of the peaks are typically equal. Any significant deviations would perhaps suggest two states of Cl etc
Dear Mansour,
In addition to the helpful points made above, I would like to briefly comment on the need of putting constraints on the FWHM of doublets.
If you observe (given the spectrometer's resolution) shoulders appearing in your doublets OR an unusually higher FWHM than usually reported OR large values of asymmetry in your lineshapes then this is can be a clear sign that you are dealing with more than one chemical environment. Oxides would, for sure, have a larger FWHM (because, in general, oxides are comparatively insulating). I attach here a wonderful report on the observed oxidation of gold. The Au 4f profiles were found to have higher binding energy components besides the main 4f peaks. The higher energy components (corresponding to gold oxide) were fitted with a larger FWHM as compared to the main peaks.
Exceptions: Certain effects however lead to an unreal increase in FWHM. I have observed that sputtering of organic samples leads to a FWHM of C1s (one-component) of ~2 eV starting from ~1.3 eV. Also, if your sample is not properly charge-neutralized, not just the core peaks would be shifted to higher binding energy, you would see a broadening as well.
Good luck and happy fitting,
Ahmad
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