Tellurite And Fluorotellurite Glasses For Active And Passive

7. Surface properties; MDO 254
can then be used to determine changes in chemical bonding in the sample [3], such as
valence and ionicity.
Absorption of X-rays is typically 10 3 to 10 4 Å (10 2 to 10 3 nm, i.e. up to 1 µm) deep
into the sample. However, the mean free path of photoelectrons emitted due to an
incident beam of ≈ 1.5 keV (the energy from an AlKα source) is only 15 Å (1.5 nm) [3].
Therefore, XPS is confined to gathering information from the immediate surface of the
sample. Because most of the photoelectrons created are inelastically scattered before
emerging from the material, the peaks in a photoelectron spectrum are very sharp. The
main peak is a result of photoelectrons from only a few atomic layers. Those from deeper
in the sample produce a broad structure, or loss tail, extending to higher binding energies,
due to inelastic scattering [3].
The kinetic energy of emitted photoelectrons in the XPS is typically measured by
electromagnetic deflection. For optimum resolution, the deflection elements operate at a
constant, and low pass energy (e.g. 10 eV for high resolution scans), and utilise variable
retardation before photoelectrons enter the electrostatic analyser [3].
The loss tail of a characteristic peak consists of interband transitions 1 , and the
summation of excitations from the valence band, known as plasmons. Extrinsic plasmons
are a result of excitations after the photoelectric process; intrinsic plasmons occur during
photoemission. Extrinsic and intrinsic plasmons and are indistinguishable from one
another in the spectra [3]. The number of intrinsic plasmons excited during
photoemission from a sample is an important consideration, which is used for semi-
1 e.g. shake-up satellites, which are due to intra-atomic relaxation, as an outer-shell electron fills the hole of
the core ejected electron.