Double Auger decay

PA = participant autoionization SA = spectator autoionization PDA = participant double autoionization, SDA = spectator double autoionization CA = cascade autoionization AD = Auger decay ASU = Augur decay with shake-up DAD = double Auger decay, ICD = interchannel decay. 

AD), Auger decay with shake-up (ASU), and double Auger decay (DAD). Auger decay also can be followed by cascade autoionization. 

Higher-order processes contribute in both the excitation of the core electron and relaxation of the core hole. These multi-electron processes are significant because of the strong perturbation caused by the creation or the annihilation of a core hole. In these processes, additional electrons are excited (shake-up) or ionized (shake-off). Two electron processes, double autoionization and double Auger decay, were mentioned above. The final states reached in core hole decay may be excited states and also may autoionize. It is clear that excitation of a core electron and the relaxation of the core hole provide many paths leading to multiple-electron excited states. These states have a unique chemistry relative to the single-electron excited states produced by arc lamp, laser, or vacuum ultraviolet (VUV) excitation. 

These studies later were extended to molecules containing only elements from the first series of the periodic table (Carlson and Krause 1972). For these molecules, with only two electronic shells, an Auger cascade cannot occur. Each K shell vacancy produced by x-ray ionization of a Is electron produces a single Auger event with the total loss of two valence electrons. Additional electrons can be lost in shake-off ionization and double Auger decay, which were estimated to contribute about 20% to the the observed ion yields. 

Figure 1.3 Illustration of the two classes of two-electron processes caused by photoionization using magnesium as an example, using, on the left the model-picture of Fig. 1.1 and on the right an energy-level diagram (not to scale) (a) direct double photoionization in the outer 3s shell (b) 2p inner-shell photoionization with subsequent Auger decay where one 3s electron jumps down to fill the 2p hole and the other 3s electron is ejected into the continuum (Auger electron). The wavy line represents the incident photon (which is often omitted in such representations) electrons and holes are shown as filled and open circles, respectively arrows indicate the movements of electrons continuum electrons are classified according to their kinetic energy e.

As a first approximation, direct double photoionization will be neglected. This is often justified because the cross section for double photoionization in outer shells, and hence also the corresponding amplitude, is much smaller than the cross section for single photoionization in an inner shell. Therefore, the Auger decay has... 

Autoionization spectra resulting from specific resonances can be obtained by electron-electron coincidence measurements (Haak et al. 1984 Ungier and Thomas 1983, 1984, 1985). To associate a fr.rgmentation pattern with a particular core hole excited state and a particular autoionization or Auger decay channel, a double-coincidence experiment must be done using electron impact excitation. The energy of the scattered electron must be determined, the energy of the emitted electron must be detennined, and the ions produced in coincidence with these two events must be determined. The difficulties inherent in these kinds of experiments have been aptly summarized by Hitchcock (1989), If you can do it by photons, don t waste your time with electron-coincidence techniques. ... 

The relative populations of the Xe2+ 4d 2 levels produced in this process are governed by the symmetry of the mixing state. Once formed, the double hole states decay by a double Auger process, again involving a cascade through Xe3+ intermediate states, to final states of Xe4+. 

Here the double line describes the electron motion in the field of k and vacancies created in Auger-decay of The amplitude of such a process is given by the expression ... 

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