Chemistry Reference
In-Depth Information
spectrometer. The consequence can be peaks shifted in energy which will complicate
their identification. A hot filament electron source is usually applied to compensate
the charge of electrons leaving the surface. Nevertheless, the measured spectrum
should be calibrated to at least two peaks with well-known positions.
While the path length of the x-ray photons inside the target is several micrometers,
that of the electrons is some nanometers, only. Thus, XPS is restricted to a surface
depth of maximal 10 nm in polymers. Electrons leaving the surface without energy
loss produce the peaks in the spectra. The background is formed by the electrons that
undergo inelastic loss processes before emerging, see Figure 7.11 [29-32].
7.2.2 A
NALYSIS OF
XPS S
PECTRA
The analysis of an XPS measurement starts with the identification of the peaks
in the spectrum. The first step is to define an approximation to the background
signal. The background results from electrons which are scattered in the surface
material. There are several background algorithms known for XPS spectra, which
are more or less useful approximations, Linear, Shirley, or universal cross-section
Tougaard backgrounds are used in most cases. Linear backgrounds provide good
results for polymers [29]. Energy loss processes occur above an energy threshold and
the linear approximation fits well. On the other hand, spectra of metals exhibit sharply
rising backgrounds under the peaks. In these cases, an approximation due to Shirley
is used quite often. Tougaard announced three-parameter universal cross sections
for materials with sharp rising backgrounds such as metallic aluminum where the
approximation according to Shirley failed, too [33].
Not all peaks in XPS data are due to the ejection of electrons by a direct interaction
with the incident photons. The most important additional peaks are Auger peaks,
satellites, and plasmons. An Auger process includes the decay of a more energetic
electron to fill the vacant hole created by the x-ray photon, combined with the
emission of an electron with an energy characteristic of the difference between the
states involved in the process. Satellite peaks can be generated by nonmonochromatic
x-ray sources. These peaks are shifted to higher kinetic energies. Energy shift and
proportion to the main peaks in the x-ray spectrum are characteristic for the anode
material and well known. Another source of peaks in the background signal is due
to resonant scattering of photoelectrons with other electrons in the surface region. In
the case of insulating samples, there is a fairly sharp peak at 20-25 eV higher binding
energy than the main peak. This effect is much stronger in metals. Energy loss lines
due to interaction with conductive electrons are obtained in well-defined distances to
the main peak and are characteristic for each metal. The energy difference between
the main peak and the loss peak is called plasmon energy.
Some photoelectric processes lead to ions in excited states. In this case, the
kinetic energy of the photoelectrons is reduced resulting in a peak with a few eV
higher binding energy. For carbon in aromatic compounds, the typical shake-up
process involves the π
transition.
Additionally, valence lines and bands are present in the spectrum region between
the Fermi level and 10-20 eV. Molecular orbitals and solid state energy bands are
→
π
∗
