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Emission d'électrons Auger par bombardement ionique des métaux légers

Abstract : In 1925 Auger highlighted the particular mode of non-radiative de-excitation which his name is attached. Following a sufficiently energetic excitement, an atom can be leaved in an excited state with a hole on an internal energy E0 electronic level. After a relatively short time (in the order of 10-14s), a de-excitation occurs by transition of an electron from a higher energy level E1 on the deepest level. The energy released can cause emission of an X or gamma photon (energy hv = E0-E1), this is the most common mode of de-excitation of heavy atoms whose fluorescence yield is quite high. But it's also possible that a second electron placed on an energy level E2 acquire the energy released by the de-excitation and thus is ejected from the atom with ume kinetic energy equal to E0-E1-E2. This latter mode of de-excitation, said non radiative, is most likely for atoms of light elements whose fluorescence yield is low. The measurement of the energy of the ejected electron is used to identify the nature of the emitter atom and allows therefore a qualitative analysis. In common usage, we will use the name of "Auger effect" if the original hole is located on an internal level, leaving the term "autoionization" for a non radiative de-excitation in which the initial hole is on an external level. Different methods can be used to form the initial hole. Historically, irradiation argon with X-ray photons that Auger effect has been highlighted, but to reach the deepest levels of the heavy atoms must be used very hard X-rays, even gamma photons. Of course, during the X-ray irradiation, photoelectrons are emitted at the same time that the Auger electrons. The detailed study of the spectrum of electrons emitted is at the base of the superficial analysis process developed by Siegbahn et al. and known as ESCA. The electron beam also allows the formation of holes in the deeper levels, and therefore the emission of Auger electrons. Thus Harris proposed and tested an alternative method of surface analysis called "Auger Spectroscopy" and is the subject in recent years, a very fast developpernent in the design, in particular, to make the quantitative analysis. Finally, during the bombardment by ions of a relatively high energy Auger electrons are also produced, (a few keV), besides to X-rays, as a result of the creation of holes in a deep level of the target atoms . The effect was first demonstrated on the gas, then the light metals. More recently, a more refined technique allowed to observe the Auger electrons emitted by ion bombardment of the transition metals of the first series. The mechanism of the deep hole formation by ion bombardment has been studied theoretically by Joyes in the case of atoms inside the metal. During the collision, a molecular level, from two related atomic levels, has its energy to grow to reach the continuum of free states above the Fermi level. And an electron can move to an unoccupied conduction band and a hole can then remain on the carbon of a state after separation. The lifetime of the excited state with a hole in the 2p level of an aluminium atom in the metal was calculated and compared to the average exit time of a displaced particle. It follows from the calculation that the excitation time is long enough that the particle can eventually moved out of the metal, usually neutral (the speed of conduction electrons neighbours Fermi level is indeed much higher than the average rate of removal particles ejected) maintaining the inner bore. The Auger effect can then take place outside of the metal with formation a secondary ion. Of course, most of the Auger electrons are emitted when the particle is inside the metal. The mean free path of the electrons do not exceed a few atomic layers, the electrons are slowed down before they leave and participate in the secondary electron emission from the target, according to the theory widely accepted of Parilis and Kishinevskii The question that we tried to give an answer was, among other things, to seek proportion of Auger electrons from de-excitations occurring outside the metal that are the only ones to produce kinetic ions, while the Auger electron most likely to have the original de-excitations occurring inside the metal and in the immediate vicinity of the surface: the de-excitations do not result in ionization of particle movement. The study of the peak width must answer this question because the emitted electrons outside the metal will form a thin peak whose width can be calculated from the lifetime of the excited state (10-14s ). The width of this peak is equal to h/2piDt = DE = 10-20J, that is to say, it is about 1/10 electron-volts. The angular distribution of the electrons allows also to separate the contributions of internal and external de-excitations de-excitations, because the emitted electrons outside the metal must have an isotropic distribution, while those from the interior should follow a law near the law cosinus. On the other hand, only electrons from internal de-excitations can feel the influence of symmetries of the network during the Auger emission from a single crystal. So we built, by modifying an existing device already in the laboratory, an electronic analyser allowing us to study the secondary electron emission under ion bombardment in the largest angular range as possible (emission angle and azimuth angle) and energy up to a few hundred electron volts: on light metals, Auger electrons energy produced under ion bombardment varies in the area of 40 to 120 eV.
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Contributor : Patrick Viaris de Lesegno <>
Submitted on : Monday, August 19, 2013 - 11:15:10 AM
Last modification on : Wednesday, October 14, 2020 - 3:56:44 AM
Long-term archiving on: : Wednesday, November 20, 2013 - 4:14:27 AM


  • HAL Id : tel-00851914, version 1



Patrick Viaris de Lesegno. Emission d'électrons Auger par bombardement ionique des métaux légers. Science des matériaux [cond-mat.mtrl-sci]. Université Paris Sud - Paris XI, 1972. Français. ⟨tel-00851914⟩



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