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The reason for this comes down to the Auger-Meitner process itself. When we excite a photoelectron, we leave behind a core-hole, an electronic vacancy where our electron used to be. But atoms don’t like being in a state like this, because it is no longer in the lowest possible energy state, so it relaxes an electron from a higher energy orbital (typically in the valence state), and fills this core-hole. But in doing so, the electron must lose some energy, because it is now in a lower energy level, and it can do so in one of two ways. One, is to emit it as radiation – known as fluorescence, which tends to be the predominant mechanism for heavier elements. The other way, is to send this energy to a neighbouring electron, and release the energy in the form of kinetic energy as a newly freed photoelectron. But a special type of photoelectron – an Auger-Meitner electron. Because in this case, the photon energy in our photoemission process is equal to the energy lost by the relaxing electron – this is always fixed. It doesn’t matter what source energy you use to create the initial core-hole, the energy gained from relaxation remains constant – and so our outgoing Auger-Meitner kinetic energy is also a constant.

Now the naming of these peaks is a little different, we have 3 electronic states involved, and so we end up with a 3-term nomenclature. We also use X-ray notation to name these transitions, which does actually simplify the name somewhat. First, CLICK we have our core-hole, in this case a 1s orbital, or K-shell.  The next term, is our relaxing electron state, in this case a 2s, or L1 electron. Finally we have our outgoing Auger-Meitner electron, which here is a 2p, or L2 or L3

And so you might start to see how we end up with these big broad ranges of peaks! The energy separation between combinations of shells, especially when we also have our initial core-hole in a shell subject to spin-orbit coupling – as we see here with nickel, means that we end up with many configurations in a similar energy range. Here we can see core-holes from L2 and L3 shells (the 2p doublet), and relaxing electrons, and Auger-Meitner emissions, from M2,3,4, and 5 (the 3p and 3d orbitals). These regions are often more sensitive to valence chemistry than photoemission peaks, but as you can see they are not always particularly easy to work with!

So we will just finish with a quick refresher on X-ray notation, and this table will be made available to download on the page below for your reference. The letter refers to the principal quantum number – n1 is K, n2 is L, n3 is M and so on. Sometimes you might see V used as a shorthand for valence shell, for example, an LVV Auger transition, but more commonly is the full form. The subscript number then refers to the total angular momentum state. Starting with an s orbital, which has no spin-orbit splitting, we have 1 – then into our p states 2 and 3, d 4 and 5, and finally f 6 and 7.

And so you can start to see that these X-ray terms contain a lot of information about the electronic configuration of the electron in question. They are still widely used in physics, and in many spectroscopies, though for chemists, and indeed in XPS in general, we tend to use chemical notation instead.

So that is Auger-Meitner electrons, they are very useful for some quite advanced topics – and now you know the theory behind how they arise, and how we name them.