Since the exceptions are what’s interesting, the slow trends seen in x-ray absorption spectra are usually referred to as the background.
1.1.3 Edge The sharp rise seen in Figure 1.1 is called an edge. It occurs because at energies below the edge x-ray photons do not have enough energy to excite electrons from some particular orbital, while above it they do. This leads to a sharp increase in absorption. For example, it takes approximately 7112 eV to excite an electron in the 1s orbital of iron, leading to a sharp increase in the absorption of iron-containing materials around 7112 eV, known as the iron K edge. (Excitations of orbitals from n = 1 states are referred to as K edges, n = 2 states as L edges, n = 3 states as M edges, and so on.)
1.1.4 X-Ray Absorption Near-Edge Structure The peaks, shoulders, and other features near or on the edge are known as x-ray absorption near-edge structure (XANES). The interpretation of XANES is addressed in Chapter 6.