Date: Fri, 27 May 94 12:53:02 EST
Subject: Re: chemical shift in x-ray spectra
From: elshaw@MIT.EDU (Libby Shaw)

To Alfred Kracher:

I have a background in Auger electron spectroscopy and X-ray
photoelectron spectroscopy, where we look at chemical shift all the
time, so I'll take a shot at an introductory explanation of how chemical
shift works.

Auger electrons and characteristic X-rays have very similar origins. In
fact, there's a trade-off in how an atom relaxes--the higher the atomic
number of an element, the more likely it is to emit a characteristic
X-ray rather than an Auger electron as a result of a vacancy in a
particular core shell. The kinetic energies of Auger electrons and
characteristic X-rays both shift as a result of chemical bonding between
atoms. (As do the energies of X-ray induced photoelectrons, but that's
slightly off the subject.)

One way to visualize how chemical shift happens is to take the case of a
metal atom chemically bound to an oxygen atom. You can think of the
metal's chemically bound valence electrons as spending more time with
the oxygen atom than with the metal atom, due to the oxygen's higher
electronegativity. The total negative charge of the electron cloud
around the metal atom is thus reduced, and the metal's core electrons
are consequently bound more tightly to the positive metal nucleus.

When an incoming high energy electron or photon knocks out a core shell
electron from the metal atom in this metal-oxygen molecule, the increase
in core shell binding energy caused by the metal-oxygen bond reduces the
kinetic energy of the resulting Auger electron or characteristic X-ray
(compared with that from a metal atom bonded to another metal). This
reduction can be quite noticeable. For instance, the principal Auger
electron peak in an electron energy spectrum from pure Si has a kinetic
energy of 1619 eV, while the same peak from SiO2 has an energy of 1606
eV.

Good fundamental explanations of the atomic relaxation process and of
chemical shift in the kinetic energies of photoelectrons and Auger
electrons are given in a number of textbooks on surface analysis, for
instance "Practical Surface Analysis" by Briggs & Seah.

Yours VT,
Libby Shaw
CMSE Analytical Facility
MIT Center for Materials Science and Engineering