Section 6.3.1.1. True or photoelectric absorption

In photoelectric absorption, X-ray photons disappear completely. The absorption of each photon results in the ejection from the atom of an electron that carries excess energy away as kinetic energy. The corresponding linear photoelectric absorption cross section is occasionally termed the cross section for `fluorescence'.

Excitation of an electron from a low to a higher bound state also occurs. Such an electron can be excited only if the photon energy exceeds the gap to the nearest unoccupied level. For this reason, varies abruptly in the manner shown in Fig. 6.3.1.1.

Figure 6.3.1.1| top | pdf | Idealized diagram showing the variation of the photoelectric absorption coefficient σph with wavelength λ.

The probability of ejection of an electron is largest for a photon energy just sufficient for excitation. It is small if the energy greatly exceeds that required. With increasing atomic number Z, the absorption edges shift to shorter wavelengths. The ratio of the value of σ_ph for λ just below and just above the edge decreases with increasing Z, especially for the K edge.

The natural width of the resulting core-vacancy state sets a lower limit to the sharpness of the absorption edge (James, 1962). In some cases, such as the K edges for certain metals, the edge is substantially less sharp than that limit (Beeman & Friedman, 1939). Natural level widths are tabulated by Krause & Oliver (1979).

The wavelength of the absorption edge for a given element shifts slightly with changes in the chemical environment of the absorbing atom. There is also fine structure in the absorption coefficient that depends, especially on the short-wavelength side, both on chemical composition and on temperature. The range of the larger effects in the fine structure is of the order of 10⁻³ Å for X-ray wavelengths of approximately 1 Å. This corresponds to a photon-energy range of tens of electron volts, whereas X-ray photon energies are of the order of 10 keV. Smaller effects in the fine structure cover a far greater range. These are observed in extended X-ray absorption fine structure (EXAFS) spectra up to 1 keV from the edge (see Section 4.2.3 ). However, these small terms are of limited relevance when measuring X-ray diffraction intensities.