Auger and secondary X -ray electrons from gold

When a target of any element is bombarded with X-rays, the electrons emitted from its surface fall into two groups, sometimes called photoelectrons of the first and second kind respectively. Each group, when analysed by a magnetic field, yields a fairly complex velocity spectrum. Those of the first group, for which we shall use the convenient term, introduced by Whiddington in 1922, of “X-ray electrons”, are the electrons ejected from different X-ray levels of the element by the direct action of a quantum of the incident X-radiation. Their energies are given by a simple modification of the Einstein photoelectric equation, the characteristic critical absorption frequencies of the element taking the place usually occupied in that equation by the photoelectric threshold frequency. The electrons of the second group arise, in atoms which have lost an X-ray electron, as the result of more or less complicated internal “radiationless” rearrangements of the extranuclear structure. As the atom from which they emerge is left multiply ionized in its X-ray levels, no simple expression can be written down for the electron energies. Photoelectrons of this kind were detected in 1914 by Robinson and Rawlinson, but at that date it was not possible to account for their origin. They were investigated and interpreted by de Broglie in 1921 and in somewhat greater detail by Robinson and Cassie in 1925-6. The clearest and most unequivocal evidence of their mode of origin was, however, given by Auger’s beautiful cloud-chamber experiments, and these electrons are therefore generally known as “Auger electrons” .