Siegbahn nomenclature

Figure 13.2—Simplified schematic of an atom showing the origin, and the Siegbahn nomenclature, of some fluorescence radiation processes caused by impact of a photon having a high energy. The position of the spectral line is not significantly influenced by the chemical combination in which the atom is found. For example, the Kat line from sulphur is observed at 0.5348 nm for S + and at 0.5350 nm for S°, yielding a shift of 1 eV, which is comparable to the natural line width for X-rays.

Transitions are still designated according to Siegbahn nomenclature. Hence, for iron, the symbol FeK/32 specifies the location of the gap (K shell), the distance that separates the two energy levels (initial and final states of the electron a for 1, 3 for 2) and the relative intensity of the transition within the series (1 is more intense than 2). Kft transitions are approximately six times less intense than the corresponding Ka transitions. Cascade electronic rearrangements are often observed for heavy elements (light elements cannot have L or M transitions). For example, carbon only yields a Ka line at 4.47 nm (227 eV). H or He elements do not have X-ray fluorescence. 

One other very important attribute of photoemitted electrons is the dependence of their kinetic energy on chemical environment of the atom from which they originate. This feature of the photoemission process is called the chemical shift of and is the basis for chemical information about the sample. In fact, this feature of the xps experiment, first observed by Siegbahn in 1958 for a copper oxide ovedayer on a copper surface, led to his original nomenclature for this technique of electron spectroscopy for chemical analysis or esca. 

The selection rules applicable to optical dipole transitions also apply to X-ray transitions. The rules are AL = 1, Aj = 0, 1. Intensity rules are also the same as those applicable to optical transitions. The transitions obeying selection rules are called normal transitions. Not all transitions allowed by selection rules are observed. On the contrary, some transitions, which are not allowed by selection rules, are sometimes observed. These are called forbidden transitions. The observed lines were initially given names as per their observed line intensities. The Siegbahn notation used to name various observed lines is given in Table 1. As this nomenclature is intensity-based... 

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