X-Ray Spectroscopy

In the X-ray absorption-spectra according to this view we find the different energy-levels pictured, which correspond to the energy-output necessary to remove one or other of the electrons of the atom. In consequence we speak of the K-level, the L-levels, etc. of the atom. Or if we wish to indicate the parts of the elec tronic atmosphere, the K-shell, the L-shell, etc., each of these shells may of course include more than one electron. For in stance it is probable that the K-shell contains two electrons, the L-shells eight electrons, the M-shell eighteen electrons and so on.

After one electron has—by absorption of energy—been thrown out from its place within the atom its place will soon be filled up by an electron which through the attraction of the positive nu cleus falls from some of the outer shells into the empty place. This means a diminishing of the energy of the atom, the super fluous energy being sent out as an X-ray wave. In this way we understand the emission of, for instance, the Kline in the fol lowing manner. One of the electrons belonging to the K-shell has by absorption of energy been removed, its place is taken up by an electron from the L-shell, accompanied by emission of the K a-line. As after this process there is an empty place in the L-shell this may again be filled say by an M-electron with emission of an L-line and so on.

It is immediately seen that this picture of the absorption and emission-process is in conformity with the connection between the two kinds of spectra discussed in the preceding paragraph.

As to the structure of the electronic shells of the atoms some general conclusion may be drawn from the empirical knowledge of the X-ray spectra, which, as just mentioned, may be inter preted as coming from the different layers of the electronic atmosphere of the atom.

First the fact that the X-ray spectra of all the elements show a very big resemblance to each other and change from element to element mainly by moving toward higher frequencies must mean that the general structure of the electronic layers within the atom is the same for all elements. The increasing of the frequencies is readily understood by the increasing positive charge of the nucleus and the accompanying strengthening of the attractive forces acting on the electrons.

Secondly the appearance of the distinct and widely separated groups : K, L, M . . . indicates that the electronic atmosphere of the atom consists of distinct layers or shells with decreasing energy-contents beginning with the innermost shell, the K-shell followed by the L-shell, the M-shell and so on.

Thirdly all these shells or groups of electrons (except the K-group) are divided into sub-groups as indicated by the existence of more than one absorption-edge or energy-level within every main group.

Further information regarding the structure of the atom is obtained by considering the completeness or rather lack of com pleteness of the X-ray spectra of the different elements. If one

starts with the heaviest element, uranium (with atomic number 92) and proceeds towards the lightest elements it is found that successively and systematically a number of spectral lines (and consequently also absorption-edges) vanish. That means the successive peeling off of electronic shells whereby the number of possible transitions between the groups is reduced. The remain ing lines and edges of every atom consequently show directly the number of layers and furnish us with a means of mapping out the electronic structure of the different elements. If we confine ourselves to the inert gases the empirical study of the X-ray spectra suggests the existence of the following shells filled with a number of electrons as given in the table on p. 200.

X-ray Spectra and Chemical Constitution.

The most striking difference between the ordinary optical spectra and the X-ray spectra is the fact that the former show a marked resem blance between elements with same chemical character and change considerably from one group of chemically analogous elements to another whereas in the X-ray spectra no indication of the chemical nature of the elements is to be found. This is readily understood by the picture of the atom and its relation to the spectra which has just been given. From this interpretation the optical spectra arise from the surface of the atom which is also the seat of the chemical bounds, while the X-ray spectra are given off from the inner p.rts of the electronic atmosphere of the atom. But of course it is to be expected that a small influence may be seen if an atom emitting an X-ray is acted upon by neighbouring atoms. This is a question of the sensitivity or accur acy in the methods of studying the X-ray spectra. By increasing the precision of the measurements it has been possible to detect a number of such effects as just mentioned; for instance it has been shown by Bergengren and Lindh that the K-absorption-edges are a little displaced when the atom (Phosphorus, Sulphur and Chlorine especially have been studied) enters in different chemical combinations. The valency of the atom seems to be one of the main factors in this respect so that by investigations of the kind valuable information regarding the valencies may be obtained.

But the line-spectra are also in a certain degree influenced by the chemical bonds as shown by Lundquist, Backlin, Ray, etc.

Finally we may in this connection point out the service that the X-ray spectroscopy has rendered as a method of chemical analysis and for the detection of new elements. On the suggestion of Bohr that the element with atomic number 72 may be sought in minerals containing its homologue Zirkon, Coster and Hevesy succeeded in identifying this new element by its X-ray spectrum. Berg, Noddack and Tacke gave convincing evidence of the unknown elements 75 and 43 by X-ray methods. (M. S1E.)