Auger broadening

After the ejection of an electron from an inner shell, relaxation generally occurs by the emission of a secondary electron. This is known as the Auger effect. It can be reasonably well described as a two-step process, leading to double ionisation, because the primary and Auger electron are usually separate. However, if the initial photoelectron is emitted with a very low kinetic energy, then the Auger electron can catch up and interact with it. This process is described as post-collision interaction or PCI. 

In photoabsorption spectroscopy, the distinction can also be made in a different way the probability that an electron on a Rydberg orbit is ejected depends on its penetration of the core, which scales as 1/n 3. On the other hand, the Auger electron has a constant probability Tauger of being ejected. Thus, if we observe a Rydberg series of autoionising resonances in photoabsorption, then the total width rn of the nth member is given by 

In X-ray spectra, when a deep core hole is excited, the excited shell occupies a very small volume and its spatial overlap with other core electrons is large, while with the orbital of the outer excited electron it is small. Under such conditions, the lifetime of the core vacancy becomes short one speaks of large core-level widths due to Auger broadening, which compete with autoionisation. This is why, in X-ray spectra near absorption edges due to inner-shell excitation, only the first few Rydberg members are observed. We return to this issue in section 11.2. 

Both autoionisation and the Auger effect are often referred to as radiationless transitions, because the initial reorganisation of the atom, takes place on very short timescales (typically 10 13s) without the emission of radiation. This does not, however, mean that no radiation at all is emitted by the atom during or after either of these processes. It is merely that 

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The explanation of this peak is as follows. Suppose that the number of conduction electrons is small, so that the Coulomb field is not screened out and that a hole in the X-ray level creates an exciton level below the bottom of the conduction band. The levels are shown in Fig. 2.13. Then an exciton absorption line should be possible. But the sudden change in field will produce excitations of electrons at the Fermi level, so that the exciton line is broadened as shown in Fig. 2.14(a). Also, we do not expect a sharp increase in absorption when the electron jumps to the Fermi level, leaving the exciton level A in Fig. 2.13 unoccupied, because of the very large Auger broadening due to transitions from the Fermi level into this unoccupied state. 

The final example in this set is the pair of elements Ge and Sn [352], for which the outermost d subshell absorption spectra lie above the doubleionisation limit. As a result of Auger broadening of the parent ion core, very few Rydberg members are observed. As already noted in section 6.8, series become rather short when the parent ion state (the core hole) which serves as the series limit is broadened by Auger processes. The resonances arising by inner-shell excitation become very diffuse, and little can be done by way of detailed spectroscopy except to observe the leading series members. 

In this section, we consider the effect of many channels, Auger broadening and the growth in radiative width. 

It is clear that, for deep-shell excitation, there is not much difference between the destruction of Rydberg states due to the finite volume available around the atom and the termination of series due to Auger broadening. Furthermore, since the ionisation threshold no longer concides with the series limit even for the free atom, one has to be careful to distinguish... 

One of the sure signs of quasiatomic behaviour is the persistence of multiplet structure in the spectrum of the solid. A very good example occurs in the 3d —> f spectra of La the triplet excited states are bound states localised in the inner well of a double-valley potential, while the singlet excited state is resonantly localised in the inner well of the slightly shallower singlet potential. All these states survive in the solid, and one thus observes the full 3Di, 3 Pi and lP multiplet structure very clearly in total electron yield spectra obtained from the surface of the solid (see fig. 11.2). There is even evidence of asymmetry in one of the resonances, which shows that continuum effects other than Auger broadening of the core states are involved, as one expects for atoms. 

These features of lines of various spectra (X-ray, emission, photoelectron, Auger) are determined by the same reason, therefore they are discussed together. Let us briefly consider various factors of line broadening, as well as the dependence of natural line width and fluorescence yield, characterizing the relative role of radiative and Auger decay of a state with vacancy, on nuclear charge, and on one- and many-electron quantum numbers. 

The decay widths are in meV, citations are given in square brackets. Experimental value for ammonia is lacking because of the vibrational broadening in the Auger electron spectrum of ammonia [65], See Ref. [44] for the details of the Fano-ADC computation. 

Before these partial quantities are discussed further, an important comment has to be made unlike the partial transition rates, the partial level widths have no direct physical meaning, because even for a selected decay branch it is always the total level width which determines the natural energy broadening. The partial level width is only a measure of the partial transition rate. Both aspects can be inferred from the Lorentzian distribution attached to a selected decay branch, e.g., Auger decay, which is given by... 

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