At low energies or low temperature gas (plasma) the primary processes are photoionzation or excitation by particles (electron, atom, proton). Recombination takes place with emission of photons. In hot plasma new processes become viable and in fact can dominate the ionization state of the plasma. These reactions are resonant and so have significant cross-section. ------------------------------------------------------------ Auger & X-ray Fluorescence ----------------------------------------------------------- Consider an energetic electron incident on an ion/atom. The energetic electron has just the right energy to knock an inner electron. The atom/ion is now in an unstable state. An upper level electron can decay to the energy level of the knocked off electron. The resulting energy release can release an outer electron (Auger electron). With sufficient energy relesae there can be a cascade of electrons released. Alternatively the energy released can result in radiation. This is X- ray flourescence. ---------------------------------------------------------------- Autoionization & Dielectronic Combination: ---------------------------------------------------------------- An electron passing an ion can excite an inner electron to an excited state and thereby lose its kinetic energy. The decrease in velocity of the incident electron leads to rapid capture to a higher energy state. The ion has now two electrons in excited states. The inner electron can then relax by emitting radiation (dielectronic recombination) or ejection of an outer electron (auto-ionization). Dielectronic recombination is important at high temperatures since energetic electrons can excite inner electrons (there are more possibilites for this exciation compared to ordinary recombination). However, for some ions dielectronic recombination works via fine structure lines (e.g. CII->CI, SiII->SiI in CNM/WNM).
Introduction: Auger Electron Spectroscopy (Auger spectroscopy or AES) was developed in the late 1960 s, deriving its name from the effect first observed by Pierre Auger, a French Physicist, in the mid-190. It is a surface specific technique utilizing the emission of low energy electrons in the Auger process and is one of the most commonly employed surface analytical techniques for determining the composition of the surface layers of a sample. Why Auger is important: It is a resonant process and so large cross sections. Physics of AES: The basic auger process starts with the removal of inner shell atomic electron to form a vacancy. The steps are as follows: STEP 1: IONIZATION The Auger process is initiated by creation of a core hole this is typically carried out by exposing the sample to a beam of high energy electrons (typically having a primary energy in the range - 10 kev). Such electrons have sufficient energy to ionize all levels of the lighter elements, and higher core levels of the heavier elements. In the diagram below, ionization is shown to occur by removal of a K-shell electron, but in practice such a crude method of ionization will lead to ions with holes in a variety of inner shell levels.
In some studies, the initial ionization process is instead carried out using soft x- rays ( hϑ = 1000-000 ev ). In this case, the acronym XAES is sometimes used. However, this change in the method of ionization has no significant effect on the final Auger spectrum. STEP : RELAXATION AND AUGER EMISSION The ionized atom that remains after the removal of the core hole electron is, of course, in a highly excited state and will rapidly relax back to a lower energy state by one of two routes: X-ray fluorescence, or Auger emission De-excitation results in emision of photon De-excitation results in ejetion of "Auger" electron An example of Auger emission process is illustrated schematically below: Emission of auger Electron filling the hole A rough estimate of the kinetic energy (KE) of the Auger electron from the binding energies of the various levels involved. In this particular example, KE = (E K - E L1 ) - E L3 = E K - ( E L1 + E L3 ) Here the KE of the Auger electron is independent of the mechanism of initial core hole formation.
Auger transition nomenclature Auger electron Labelling: KL 1 L AVV Incident electron The 3 letters specify the energy levels implied in the process of emission of the Auger electrons KE = E K -E L1 -E * L- j, E K, E L1, and E L the energy levels mentioned in the labeling (generally different from the neutral atom, due to the presence of electron vacancies).
Notice the peaks arising from resonance Factors influencing the Auger peak area 1. Ionisation cross section 3keV incident electron beam KLL LMM MNN 10 kev incident electron beam
Factors influencing the Auger peak area (cont d). The Auger yield Competition between the Auger process and the X-ray fluorescence. The probability of occurrence of the Auger electrons increases with the decreasing of the differences between the energy levels involved in such transitions. 3. Backscattering
Aufgabe 1: Dielectronic Recombination The process, reversed to the Autoionization is known as the Dielectronic Recombination (DR). In DR process electron is captured by the ion (atom) and this capture is accompanied by excitation of initially bound electron. Autoionization (Auger decay) Dielectronic recombination Consider DR of initially hydrogen-like Argon ions (Z=18). Which kinetic energy should have an incoming electron to populate (after) capture s 5d state? What is the energy of photons emitted in subsequent s 5d 1s s transition?
Aufgabe 1: Lösung Free electron with kinetic energy T kin Initial state Final state Electron in 5d state sees charge Z eff =17 Electron in s state sees charge Z eff =18 Energy conservation: E T Ar kin 17 (1s ) 18 1 T T kin kin 5786 5 E Ar 16 18 3.1keV (s5d) 17 5 Similar, to find photon energy for the s 5d 1s s decay: E Ar 16 (s5d) 17 5 E 18 Ar 16 (1s s) 18 1 17 4.1keV
Total Recombination (radiative, diectronic) rate coffecient for CIII. The dashed line is the well known radiative recombination (emisison of photon following electron recombination). The dotted line is the rate when the incident electron excites low levels resonace lines (fine structure) and then get captured whereas the dotdashed is the capture of the electron via excitation of "core" electrons (Doptia & Sutherland Figure 5.)