Date: Tue, 31 May 94 10:35:02 PDT
Subject: Re: chemical shift in x-ray spectra
From: tonygr@EAGLE.MIT.EDU (Anthony Garratt-Reed)

Libby Shaw has described the essentials of the causes of chemical shifts in
Auger/XPS analysis. I would like to comment a little about how these effects
translate into X-ray emission (EDS or WDS - here I will simply say EDS, but
that must be read to include WDS) spectroscopy.

This discussion is quite long - please delete it if you are not interested.

To use Libby's example, silicon in the ground state has 2 K electrons, 8 L
electrons and 4 M electrons. (I will ignore the sub-states these electrons
actually occupy). Both Auger/XPS and EDS start with an atom with a
vacancy in the K shell. The energy required to create this vacancy is
equal to the absorption edge in electron or X-ray absorption spectra, i.e.
1838eV. X-ray emission comes about by one of the L electrons jumping
into the K shell, leaving an ion with an L shell vacancy, and radiating the
excess energy as an X-ray. The energy that would be required to create
this ion from the ground state is given by the L absorption edge which is at
98 eV. One might expect that the energy of the observed X-ray would
be the difference between these two energies, that is 1740eV, and
indeed that is what is measured.

In the case of the Auger/XPS analysis, the principal line that Libby mentioned
is the KLL line, in which one L electron drops into the K shell, giving the excess
energy to another L electron, which is ejected from the atom. How can we
predict its energy? It should simply be the K absorption energy less the
energy required to create TWO L vacancies. (because we leave behind
an atom with two vacancies in the L shell). We know the energy required
to create one vacancy - the L absorption energy as above. What is the
energy required to create the second? We might expect it to be twice
the energy required to create the first (because we are now separating an
electron of charge -e from an ion of charge +2e), but in fact it is not that
much because of an effect called, in physics textbooks, screening.

Screening can be thought of as the M electrons giving up some of their
potential energy and moving closer to the ion core, so it is as if the ejected
L electron started off from further out from the nucleus, and therefore
requirs less energy than one would expect to become free. In fact it
only requires a little more energy to create the second L vacancy than it did
the first. Hence one would expect that the KLL auger electron would have an
energy equal to 1838-(98*x) where x is a number a little greater than 2.
Indeed, this is what is found, the actual energy (as mentioned by Libby) is
1619 eV, corresponding to x=2.23.

What happens when we change the environment of the atom (for example
we put the Si into SiO2)? Well, three things might:

1) The whole atom could become more tightly bound, i.e. all the energy
levels might become deeper. Since the X-ray energy depends only upon the
difference between the levels, it remains unchanged. However, the Auger
electron energy would be changed by an amount equal to the change in
binding energy of the atom.

2) The L-shell could change in energy with respect to the K-shell. In this case
the X-ray energy would change, but we note that the energy of the Auger
electrons would change by more than twice the amount.

3) The chemical bonding can change the configuration of the M electrons
in such a way that their ability to screen the charge is changed. This will
have minimal effect on the X-ray because the effect of the change of the
screening is virtually the same for the first K or L shell vacancy, but will
change the energy of the second L vacancy. In particular if the M
electrons are partially bound to the oxygen atoms, they will have less
screening effect and the factor x in the expression above could become
greater. I believe that in the case of the Si and SiO2 cited by Libby that
this is the predominant mechanism generating the chemical shift. We
note that the energy of the Auger electron goes down to 1606 eV,
corresponding to a vale of x of 2.36.

We can see from this discussion that, while shifts in X-ray energies are
certainly possible, we would not expect them to be as large as those
observed in Auger/XPS. Further (although I havn't emphasised it above)
one can easily get chemical shifts in Auger/XPS lines which do not involve
the outer shell electrons (as in the case of the silicon/silicon dioxide) but the
only X-rays that show a significant shift are those directly involving the outer
shell electrons. In the case of the silicon these would be the K-beta
X-rays (which are weak and, in EDS, not resolved from the K-alpha) or
the L X-rays, which are very weak, and, at about 97eV, are not resolved
from the system noise in most spectrometers. All examples of X-rays
involving outer-shell electrons are either very close to stronger, core-level
transitions or at very low energy.

Hope if you got through tis that it helps!

Tony Garratt-Reed