686 Chapter 12. Radiation Spectroscopy
photoelectric absorptions. The mechanism is graphically depicted in Fig.12.1.11(b).
Here, the vacancy created by the photoelectron transition to the continuum is shown
to have filled by an electron in the M shell. The excess energy carried by this elec-
tron is given off in the form of a fluorescence photon. This photon has the ability
to knock off an electron from the L or M shells. The figure shows transition of
an L shell electron to continuum after absorbing the fluorescent photon. Such an
electron is called Auger electron. The energy of these electrons is fairly low and
therefore they can get absorbed near their generation site. Only the ones produced
near the surface of the material manage to escape and get detected. Auger electron
spectroscopy is therefore quite a challenging task. In a very simplified case, for the
Auger electrons produced on and near the surface of the material, one can assume
that the intensity of these electrons is related to the absorption coefficient of x-rays
through the relation
μ(E)∝IA
I 0
. (12.1.11)
whereIAandI 0 represent the intensities of Auger electrons and the x-ray photons
respectively. μis the absorption coefficient of the material for the x-ray photons.
As with fluorescence spectroscopy, here also this relation does not take into account
the damping of fine structures due to self absorption of Auger electrons.
As a final note, it should be mentioned that that here we are dealing with ab-
sorption of radiation and not its scattering. In the techniques that use scattering
as the basis for spectroscopy, the material should have crystalline structure. This
limits the applicability of such methods to a narrow band of materials that can be
crystallized. As opposed to that, x-ray absorption spectroscopy can be used for vir-
tually any material in any form, thus making it a highly desirable materials research
tool.
C.2 X-rayPhotoelectronSpectroscopy(XPS)
The XAFS measurements we studied in the previous section mostly involve detection
of transmitted x-ray photons or fluorescence photons. Only in a very limited number
o f cases one resorts to the technique involving detection of Auger electrons. No
matter what technique we use, main process of x-ray photon absorption is always
photoelectric effect. During our discussion on XAFS we did not talk about these
electrons at all. However, for spectroscopic purposes these electrons can play an
important role. To explain this, the reader is referred to the basic definition of the
photoelectric effect, which states that this process involves total absorption of a
photon by a bound electron such that the electron assumes the energy given by
Ee=Eγ−Φ, (12.1.12)whereEγis the incident photon energy and Φ is the work function. This work
function refers to the binding energy of the most loosely bound electron in the
atom. Since there are other electrons as well, which can absorb photons, we can
write a generalized photoelectric equation as
Ee,i=Eγ−Eb,i, (12.1.13)whereEe,irefers to the energy of the photoelectron having a binding energy of
Eb,i. This implies that if one measures the energies of the emitted electrons, the