XPS fundamentals
The basic principles of x-ray photoelectron spectroscopy (XPS) are explained here, beginning with the photoelectric effect. In XPS the sample is illuminated with soft (1.5kV) x-radiation in an ultrahigh vacuum. The photoelectric effect caused by the x-rays lead to the production of photoelectrons, the energy spectrum of which can be determined in a beta-ray spectrometer. This energy spectrum permits us to determine the composition of the sample.
We measure the kinetic energies of the emitted photoelectrons, EK. Knowing the photon energy hν we can plot the binding energies of the photoelectrons EB on a spectrum using the Einstein equation: EK = hν - EB
Because the binding energies of the electron orbitals in atoms are known, the positions of the peaks in the spectrum allow us to identify the atomic composition of the sample surface. Measurement of the relative areas of the photoelectron peaks allows the composition of the sample to be determined quantitatively. This sample is representative of the composition of the surface of the inner wall of the TdeV tokamak.
" XPS is known for its weakly destructive nature and for its universal applicability to solid samples - be they metals, ceramics or polymers. "
Because the photoelectrons are strongly attenuated by passage through the sample material itself, the information obtained comes from the sample surface, with a sampling depth on the order of 5-10 nm. Chemical bonding will frequently have an effect on the binding energy of the electron orbital and give rise to an observable chemical shift in the kinetic energy of the photoelectron. These binding energy shifts can be used to extract information of a chemical nature (such as atomic oxidation state) from the sample surface. For this reason, XPS is also known as Electron Spectroscopy for Chemical Analysis (ESCA).
In this example the C1s envelope has been resolved into five components. The spectrum was obtained from a sample of polystyrene exposed to an oxygen plasma, and displays components representative of the various types of carbon-oxygen bonds introduced into the sample surface.
XPS is known for its weakly (usually non-) destructive nature and for its universal applicability to solid samples - be they metals, ceramics or polymers. Its main drawbacks are its relative lack of sensitivity (~1% detection limit) and limited spatial resolution.
