Spectrometry of absorption of x-rays

The Spectrometry of absorption of the x-rays assistance to determine the structure of a material. It with the advantage of being selective as for the atomic species observed.

History

One owes with Maurice de Broglie the first absorption spectra of x-rays, in 1913. While varying the energy of the beam, it noted jumps in the absorption which it allotted to transitions from electrons of heart towards free layers or the Continuum. More precise experiments undertaken by Fricke and Hertz (1920) showed that on these jumps oscillations were superimposed which were allotted by Ralph Kronig (1931, 1932) to the diffusion of the photoelectron by the network.

Experimental device

Before the sample, one finds a source of X-radiation (a Synchrotron for example), a Monochromateur to select an energy and a detector to measure incidental flow. Part of the radiation will be absorbed by the sample. There exist various methods to measure absorption. The direct method consists in measuring the flow transmitted through the sample but it is not appropriate that for high energies. For weaker energies one will be able to measure the Fluorescence or the current of the ejected electrons of material.

Theory

When the X-radiation corresponds to the energy of transition from a electron since a level of the heart of the Atome towards a level of the Continuum, one observes a threshold of absorption, the electron is ejected. According to the Orbital which the electron occupied, one will speak about threshold K, L₁,… As these energies are specific to a given atomic species, measurement is selective (one will be able specifically to measure the environment of an oxygen or silicon atom, etc). The electron will then be diffused by the environment of the absorbent atom, this is why the absorption of the X-radiations is to some extent an experiment of diffusion of the electrons.

The ejected electron, if it is diffused by the close atoms (what will depend on its energy), will be able “to return” to its starting atom and to thus interfere with itself. One speaks about multiple diffusion if the electron is diffused by several atoms and of simple diffusion if it is diffused only by one atom (it makes a return ticket). As the energy of the ejected electron depends on the energy of the beam of X-radiation, one observes Oscillation S in the spectrum of absorption according to energy (oscillations of Kronig).

When energy is hardly higher than the threshold, the multiple diffusions are dominant. The analysis of this part of the spectrum names XANES. With higher energies, the simple diffusion becomes dominant, the analysis of the spectrum gives information at the interatomic distances and one speaks about EXAFS.

See too

Law of Beer-Lambert

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