Autoionization electron emission

Penning ionization, as has been noted earlier, occurs due to energy exchange between electrons. This means that the initial state of the colliding atoms A 4- B is autoionized and, consequently, is characterized not only by the interatomic potential, but also by a definite lifetime as related to decay with electron emission this time t(R) depends on the distance R between atoms. The parameter T(J ) = 1/t(jR) represents the width of autoionization decay. 

J.B.M. Warntjes, C. Wesdorp, F. Robicheaux, L.D. Noordam, Stepwise electron emission from autoionizing magnesium stark states, Phys. Rev. Lett. 83 (3) (1999) 512. 

Hitherto the discussion of Fig. 5.2 has neglected the possibility of non-radiative decay following 4d shell excitation/ionization. These processes are explained with the help of Fig. 5.2(h) which also reproduces the photoelectron emission discussed above, because both photo- and autoionization/Auger electrons will finally yield the observed pattern of electron emission. (In this context it should be noted that in general such direct photoionization and non-radiative decay processes will interfere (see below).) As can be inferred from Fig. 5.2(h), two distinct features arise from non-radiative decay of 4d excitation/ionization. First, 4d -> n/ resonance excitation, indicated on the photon energy scale on the left-hand side, populates certain outer-shell satellites, the so-called resonance Auger transitions (see below), via autoionization decay. An example of special interest in the present context is given by... 

An ingenious approach to the autoionization process was suggested by Tomelini and Fanfoni [32]. They examined diffraction processes in the framework of autoionization, i.e., a second-order process. More exactly, they studied only the second stage of autoionization, namely, the secondary electron emission from the intermediate state where the core hole is coherently distributed over the crystal. With such an approach it was shown that in crystals this diffraction contribution enhances fine structure as compared to the amorphous substance. However, the applicability of such a model is determined by the probability of the occurrence of the core hole distributed coherently over the crystal. More conventional... 

Deviations of the expected symmetry with respect to d = 90 for the noncoincident distributions sometimes indicate directly that the electron emission is not due to autoionization of an isolated atom in a state with well-defined parity. At high collision energies contributions from direct ionization can cause an asymmetry, whereas at low collision energies the symmetry can be disturbed by quasimolecular effects. Figure 24 shows an example for the influence of direct ionization as measured by Bordenave-Montesquieu et Electrons from He due to 30-keV He + He collisions are asymmetrically ejected, whereas at 10 keV (Rgure 22) they still have a symmetric distribution. 

There then remains the question of how the respective positive secondary ions are formed. Two mechanisms prevail in the literature. The first being the association of sputtered neutral elements or molecules of interest with cations sputtered as a positive ion. This then results in a positively charged cluster displaying a sufficient lifetime to allow its detection and the association of sputtered neutral elements or molecules of interest with a cation sputtered as a neutral excited atom. The second describes de-excitation following association via electron emission (an autoionization process) which then results in a positively charged cluster displaying a sufficient lifetime. 

The different emission products which are possible after photoionization with free atoms lead to different experimental methods being used for example, electron spectrometry, fluorescence spectrometry, ion spectrometry and combinations of these methods are used in coincidence measurements. Here only electron spectrometry will be considered. (See Section 6.2 for some reference data relevant to electron spectrometry.) Its importance stems from the rich structure of electron spectra observed for photoprocesses in the outermost shells of atoms which is due to strong electron correlation effects, including the dominance of non-radiative decay paths. (For deep inner-shell ionizations, radiative decay dominates (see Section 2.3).) In addition, the kinetic energy of the emitted electrons allows the selection of a specific photoprocess or subsequent Auger or autoionizing transition for study. 

Relaxation involving emission of more than one electron also occurs, e.g., participant double autoionization (PDA) and spjctator double autoionization (SDA). These channels leave the molecule with a +2 charge and in 2-hole and 3-hole 1-electron states, respectively. Sino relaxation of the core hole may leave the molecule with a deep valence hole or with an electron in a high energy orbital, additional autoionization, called delayed, subsequent or cascade autoionization (CA) can occur. An exiimple is given in Table II. 

Fig. 4. The scheme of the electron transitions forming the secondary electron spectra (a) Auger process, (b) emission from the core level, (c) emission from the valence state [(b) and (c) are the first-order processes], (d) exchange transition, and (e) direct transition of the second-order process (autoionization). The process of the incident-electron energy loss is not shown.

Autoionization involves the existence of an intermediate excited state which can decay by electron ejection as well as by emission of radiation or, in the case of a molecule, by predissociation. Thus, we may have... 

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