Why Core Electrons for XPS analysis?
Why not the valence electrons ?
If we look into the XPS spectra, there are peaks only from the core electrons like 1s, 2s, etc... NOT from the valence electrons ...
Three main reasons:
1. The core electrons are less affected by the environment as compared to the valence electrons and the calculated Binding Energy (BE) of the core electrons is the EXACT calculation to reveal about the elements present in the sample.
2. The x-rays sources energy in XPS are in the range (0 - 1400 eV) and most elements core electrons BE ~1100 eV, so there is a matching of core electron BE and x-rays source energy. It means XPS is design for core- electrons emission.
3. The core-electrons have higher cross section as compared to the valence electrons. Therefore, there is a higher probabilities for x-rays to interact with the core electrons than the valence electrons.
Let's discuss in details:
First, the core electrons are tightly bound to the nucleus having high BE and mostly influenced by the nucleus and there is a little affect from the nearby orbitals or nearest atoms. But this change in BE is very small from 0 - 10 eV, called chemical shift and the observed BE of these core electrons in XPS provide the exact information for elemental analysis. The BE of these core-electrons are the characteristic of particular element.
Whereas valence electrons are affected a lot by the environment. For instance, the valence electrons can make chemical bonds with the nearest atoms and roaming freely so there is a significant variation in the BE and can NOT be used for elemental analysis.
Secondly, In XPS, the common x-rays sources are Al Kα ~1486.6 eV and Mg Kα ~1253.6 eV. When these high energy x-rays interact with the sample, the x-rays energy are matching with the core-electron BE and the probability (cross section) of core electron emission is higher. Therefore, XPS is design for core electrons emission because most elements core electron BE ~1100 eV.
While for valence electrons, there is lower cross section (less probability for photoionization). Therefore, the XPS peaks from these valence electrons are not clear and distinct and can NOT be used for elemental analysis. Here, for valence electron spectroscopy, ultraviolet photoelectrons spectroscopy (UPS) are widely used.
To sum up, on the basis of aforementioned discussion, we have proved that XPS is truly a core electron spectroscopy. In fact, there are peaks from the valence electrons with low intensity relatively to the core electrons peaks.
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13 июл 2024
