An absorption edge is an abrupt change in photon absorption as incident photon energy crosses an electronic excitation threshold. For inner-shell photoionization, a new vacancy-production channel opens near the shell's binding energy. This threshold concerns incident energy; a Characteristic X-ray concerns a subsequent difference of atomic energies. They are different observations. Barkla's lecture explicitly treats characteristic emission as requiring a critical excitation condition, while discussing characteristic radiation separately from scattering.[ref]
Characteristic Röntgen Radiation by Charles Glover Barkla. Nobel Lectures, Physics 1901–1921; Barkla, Characteristic Röntgen radiation, printed p. 395, PDF p. 135: shorter-wavelength excitation and critical condition
In the stipulated X-ray fluorescence model, the K threshold is , whereas the K lines are and . An absorber made of a different material could have its own absorption edge between those line energies, preferentially removing the upper line. Actual near-edge structure and attenuation values require material data; the following deliberately chosen coefficients are not such data.
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An absorption edge is a specific wavelength or energy level at which a material starts to absorb photons, leading to a significant increase in absorption coefficient. It typically occurs in materials such as semiconductors, insulators, and certain elemental or compound materials. The absorption edge is significant in various contexts: 1. **Semiconductors**: In semiconductors, the absorption edge corresponds to the energy gap (bandgap) between the valence band and the conduction band.