Auger neutralization

The technique of INS is probably the least used of those described here, because of experimental difficulties, but it is also one of the physically most interesting. Ions of He" of a chosen low energy in the range 5-10 eV approach a metal surface and within an interaction distance of a fraction of a nanometer form ion-atom pairs with the nearest surface atoms. The excited quasi molecule so formed can de-excite by Auger neutralization. If unfilled levels in the ion fall outside the range of filled levels of the solid, as for He", an Auger process can occur in which an electron from the va-... 

Oliphant and Moon theoretically considered the possibility of electron emission by resonance ionization of metastable atoms near a metal surface. Shekter [122] investigated the Auger-neutralization of ions on a metal surface. Hagstrum [124, 125] carried out an generalized analysis of metastable atoms with a metal surface. 

The results of work [ 135] are of specific interest. The work surveyed the influence of the nature and structure of adsorbed layers upon the mechanism of deactivation of He(2 S) atoms. It has been shown that on a surface of pure Ni(lll) coated with absorbed bridge-positioned molecules of CO or NO, the deactivation of metastable atoms proceeds by the mechanism of resonance ionization with subsequent Auger-neutralization. With large adsorbent coverages, when the adsorbed molecules are in a position normal to the surface, deactivation proceeds by the one-electron Auger-mechanism. The adsorbed layers of C2H4 and H2O on Ni(lll) de-excite atoms of He(2 S) by the two-electron mechanism solely. In case of NH3 adsorption, both mechanisms of deactivation are simultaneously realized. Based on the given data, the authors infer that the nature of metastable atoms deactivation on an adsorbate coated metal surface is determined by the distance the electron density of adsorbate valance electrons is removed from the metal lattice. 

The basic physical processes involved in Auger neutralization are illustrated in Fig. 3. One electron tunnels into the ground state of the ion from the level in the metal labeled 1 while a second electron is simultaneously excited from... 

Fig. 3. Auger neutralization of an ion at a metal surface presented schematically. E,(e ) is the kinetic energy of an electron observed outside the metal. ()> is the work function and E[ the ionization energy. ( [ < Ej where Ej is the energy needed to ionize an atom in free space.) 3 is the distance of the ion from the surface. This figure is similar to one originally published in Ref. ...

Fig. 4. Potential energy versus distance from the surface. Data is appropriate for He and tungsten. E, is the ionization potential for helium and ( > is the work function of tungsten. E (e") is the kinetic energy of an emitted secondary electron. The symbol He + nej implies a system composed of an helium ion and n conduction electrons in tungsten. The lower potential curve results from an Auger neutralization process where both electrons were originally at the Fermi level. (The figure is similar to one published in Ref. )...

A solid surface is bombarded by low-energy ions that undergo Auger neutralization, ejecting Auger electrons from the surface these electrons are energy analyzed... 

An alternative mechanism to resonant neutralization is Auger neutralization. The latter process involves a two-electron reorganization whereby one electron from the solid tunnels across to the ion while a second electron is excited out of the conduction band of the metal. If the second electron has sufficient energy, it may be ejected from the surface and is referred to as an Auger electron. 

Very recently it has been possible to measure the very small fraction of ions that survive Auger neutralization in collisions of He with Ag surfaces at grazing incidence [42-44]. These experiments reveal a number of survivals of 10 -10 which are many orders of magnitude larger than the... 

It has been commonly accepted that, for He+, Hagstrum s Auger neutralization model holds in the LEIS regime, namely for ion kinetic energies of the order of 1 keV. Our analysis for the He/Al system... 

Fig. 2.5 De-excitation mechanisms resonance ionization RI with a subsequent Auger neutralization AN left Auger de-excitation AD right...

Depending on the electronic structure of the surface the de-excitation of metastable helium atoms occurs either by resonance ionization (RI) with a subsequent Auger neutralization (AN), or by Auger de-excitation (AD). These mechanisms are schematically shown in Fig. 2.5 and explained below in more detail. 

Figure 3.25 Potential energy diagrams representative of (a) resonant charge transfer (RI = Resonant Ionization of a neutral atom and RN = Resonant Neutralization of a positive ion), (b) Qnasi-resonant charge transfer (qRN = quasi-Resonant Neutralization of a positive ion) and Auger charge transfer (AN-Auger Neutralization of a positive ion). The dashed arrows represent electron transfer from populated to vacant electron levels, whereas the horizontal lines represent the allowed electron levels, otherwise referred to as stationary states.

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