Principle of XPS

Principle of XPS

XPS (X-ray Photoelectron Spectroscopy) or ESCA uses a soft X-ray source (1.5kV) (AlK or MgK) to ionise electrons from the surface of a solid sample.

XPS is a quantitive spectroscopic technique which analyses the average surface chemistry of a sample up to a depth of approximately 5nm. This technique quantitatively measures the elemental composition, atomic concentrations and chemical states of elements present at a samples surface. XPS can detect all elements with an atomic number greater than 3, therefore, Hydrogen and Helium are not possible to detect.

For analysis beyond the top 1-5nm an inert gas ion gun (normally Argon) can be used to sputter off the surface layers before analysis. Alternating sputtering and XPS spectral acquisition permits chemical depth profiles to be obtained.

If we consider a single atom with just one x-ray photon on the way, the total energy is hv+Ei, where hv is the photon energy and Ei the energy of the atom in its initial state.

Following the absorption of the photon and the emission of the photoelectron, the total energy is now KE+Ef, where KE is the electron kinetic energy and Ef the final state energy of the atom (now an ion).

Because total energy is conserved

hv+Ei = KE+Ef

or

hv-KE = Ef-Ei = BE

where we call the difference between the photon energy (which we know) and the electron energy (which we measure), the binding energy (BE) of the orbital from which the electron was expelled. We can see that the binding energy is determined by the difference between the total energies of the initial-state atom and the final-state ion. Example of XPS spectra

Note that the Binding Energy scale is drawn from right to left, so that the photoelectron kinetic energies measured by the spectrometer increase from left to right.The spectrum is dominated by three photoelectron peaks, corresponding to electrons originating in the 1s orbitals of the carbon, nitrogen and oxygen atoms in the sample surface. The background on which these peaks sit comes from electrons excited by the Xray Bremsstrahlung radiation at low binding energy, and from inelastically scattered photoelectrons at higher binding energy (essentially to the right and left of the C1s peak respectively.)

The O KLL structure results from the excitation of Auger electron emission. Auger electrons are emitted with a kinetic energy that is independent of the X-ray energy, so in cases where Auger peaks are superimposed on photoelectron peaks, the Auger peaks can be displaced elsewhere on the binding energy scale by changing the X-ray photon source, for exampleThe ejection of a 1s electron leaves a core hole that represents an unstable electron configuration for the resulting ion. Following the departure of the photoelectron, therefore, an electron from a higher orbital will drop down to the 1s level to fill the core hole. This liberates energy, equal to the difference in energy between the two orbitals, that can be emitted as a photon (X-ray emission, the favoured relaxation pathway in heavier atoms) or transferred to an electron in an outer shell, which is liberated as an Auger electron, named for Pierre Auger, who first figured out where these electrons came from in 1926.

The kinetic energy of the Auger electron (labelled for the orbital shells involved in the transitions) is, to a first approximation, a function of the orbital energies themselves and not of the radiation responsible for the production of the core hole. Ek(KLL) = Eb(K) Eb(L) Eb(L)

XPS peak splitting

Energy splitting of a photoelectron peak is commonly found in an XPS spectrum, because of the interaction between the spins of the electron, s, (up or down) and its orbital angular momentum, l. This interaction leads to a splitting of the degenerate state into 2 components.

XPS quantitative measurementsFor each and every element, there will be a characteristic binding energy associated with each core atomic orbital i.e. each element will give rise to a characteristic set of peaks in the photoelectron spectrum at kinetic energies determined by the photon energy and the respective binding energies. The presence of peaks at particular energies therefore indicates the presence of a specific element in the sample under study - furthermore, the intensity of the peaks is related to the concentration of the element within the sampled region.

; I Peak area, S - Sensitivity factor of element Peak Area measurement need background subtraction. S depends on x-ray flux, photoelectron cross-section, detector efficiency, electron mean free path and is tabulated for a specific XPS device.

Chemical Shifts

The exact binding energy of an electron depends not only upon the level from which photoemission is occurring, but also upon the local chemical and physical environment give rise to small shifts in the peak positions in the spectrum - so-called chemical shifts .

The presence of chemical bonding (and hence, neighbouring atoms) will cause binding energy shifts, that can be used to extract information of a chemical nature. For this reason, XPS is also known as Electron Spectroscopy for Chemical Analysis (ESCA).

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