Energy levels and X-ray spectra from singly-ionized atoms

Soon after the discovery of X-rays, attempts were made to investigate the spectral characteristics of the radiation. Crystal diffraction had not yet been discovered, but by studying the absorption of the radiation by successive layers of material it was established that the radiation consisted of a continuous spectrum (discussed in the previous chapter), and also a monoenergetic component, or ‘line’ spectrum (soon to be found to consist of several lines) the penetrating power of which depended markedly on the target element. This ‘characteristic’ radiation has been the subject of much study, and can be understood in considerable detail from the Bohr theory of the atom as elaborated by Sommerfeld and others. The development of wave mechanics added to this understanding and enabled a detailed interpretation of the fine structure of spectra to be built up. The classification of the orbital electrons into shells and the designation of individual electrons by means of quantum numbers are well-known features of the Bohr theory. Fig. 3.1 shows how the orbital electrons may be grouped together into the K, L, M etc. shells of progressively decreasing binding energy, and it is seen that (with the exception of the K shell) each shell consists of several sub-shells of slightly different energy.