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4.1.4    X-ray Photoemission spectroscopy

Interaction of a photon of energy hν with an electron in an atomic energy level of binding energy E_B occurs by transfer of all the photon energy to the electron. If hv is greater than E_B, a photoelectron is ejected from the atom with a kinetic energy given by

EKin = hv − EB

(1)

When the atom forms part of a solid, there is a work function term to be subtracted from the right-hand side of (1), but it is comparatively small and nearly constant. Measured E_Kin values thus reflect the E_B values directly, so that measurement of E_Kin provides elemental analysis.

In X-ray photoemission spectroscopy the two sources in most common use are magnesium and aluminium, the principal line energies used being the Kα_{1, 2}, 1253.6 eV for magnesium and 1486.6 eV for aluminium. Since the kinetic energies of the photoelectrons will therefore be in the range 0–1500 eV, the paths of the electrons (see section 4.5.2) will be very short, and the photoelectrons must originate at, or very close to, the true surface if they are to escape and be measured. Like Auger spectroscopy, X-ray photoelectron spectroscopy is thus a surface-specific technique.

The tables list the photoelectron kinetic energies expected for all elements from H to U, for Mg Kα, and for Al Kα, excitation, respectively, to the nearest electron-volt. Where the element exists as a pure elemental non-volatile solid, the values will correspond to that state with reasonable accuracy, but for other elements, particularly the non-metallic ones, the values must be regarded as approximate since shifts of several eV can occur between different states of chemical combination of an element. The reference point for all the values is a binding energy for the Au 4f_7/2 level assumed to be 84.0 eV.

For further reading the reader is referred to the books by Briggs and Seah (1990) and Rivière (1990). The former gives details of high accuracy calibration values and the positions for different chemical states.

References

D. Briggs and M. P. Seah (1990) Practical Surface Analysis Vol 1: Auger and X-ray Photoelectron Spectroscopy, Wiley, Chichester.
J. C. Rivière (1990) Surface Analytical Techniques, University Press, Oxford.

Table 1

Photoelectron kinetic energies, to nearest electron-volt and referred to the binding energy Au 4f_7/2 =84.0 eV, using Mg K_α excitation. The energy of the most intense photoelectron peak for each element is underlined.