Inner-shell ionisation

Most ionisation experiments have concentrated on the valence electrons of the target. However, experiments and calculations for the Is shell of neon and the n=2 shells of argon have been reported by Zhang et al. (1992). Fig. 10.8 shows an example. Here the 2p orbital of argon 

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Wendin, G., Ohno, M. Proceedings of the 2nd International Conference on Inner Shell Ionisation Phenomena, Freiburg, Germany, March 29/April 2, 1976... 

F. Combet-Famoux (1972) Proc. Int. Conf. Inner Shell Ionisation Phenomena Atlanta vol 2 University of Georgia Press, Atlanta, page 1130... 

Quantum theory states that orbiting electrons of an atom must occupy discrete energy levels in order to be stable. Bombardment with ions of sufficient energy (usually megaelectronvolt protons) produced by an ion accelerator will cause inner-shell ionisation of atoms in a specimen. Outer-shell electrons drop down to replace inner shell vacancies, however only certain transitions are allowed. X-rays of a characteristic energy of the element are emitted. An energy dispersive detector is used to record and measure these X-rays. Only elements heavier than fluorine can be detected. 

PIXE is a method using X-ray emission for elemental analysis. A high energetic proton beam excites emission of characteristic X-rays from the sample atoms due to inner-shell ionisation. PIXE is not a true nuclear technique, since the ionisation of the atoms by the ion beam and the subsequent emission of characteristic X-rays are purely atomic electromagnetic (rather than nuclear) processes. Methods and data for using K and L lines of X-rays, produced by ion beams (mostly proton beams) are well established and thick or thin samples can be analysed with an absolute precision of 10 % or better. 

Inner shell ionisation of the atoms with subsequent emission of characteristic X-rays. 

The approximation involves neglecting exchange terms for electrons of the ion in orbitals /z). These terms have factors such as (k //), where k is the momentum of one of the external electrons. Such terms are essentially zero for k > 4 a.u. in the case of valence electrons. This gives 400 eV as the minimum total energy for symmetric ionisation. Correspondingly-higher energies are needed to probe inner-shell structure. 

The systematics of the double-ionisation thresholds turn out to be very important in determining the properties of doubly-excited spectra. These are most prominent for elements lying close to local minima, which is why alkaline-earth elements play a special role (see chapter 7). Another important issue is the existence of crossing points between the curves for double ionisation and for ionisation from an inner shell. This is further discussed in section 7.14. 

Fig. 1.3. Double-ionisation potentials as a function of atomic number. Some inner-shell thresholds are shown as dashed lines. Note the degeneracies or crossing points between inner-shell and double ionisation, which give rise to a variety of effects discussed in chapter 7.

Centrifugal barriers have a profound effect on the physics of many-electron atoms, especially as regards subvalence and inner shell spectra. One aspect not discussed above is how energy degeneracies arising from orbital collapse can lead to breakdown of the independent electron approximation and the appearance of multiply excited states. Similarly, we have not discussed multiple ionisation (the ejection of several electrons by a single photon) enhanced by a giant resonance. Both issues will be considered in chapter 7. 

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. 

Even within the independent electrom approximation, it is obvious that there must exist inner-shell excitation spectra, and that their energy must extend well above the first ionisation potential. This arises from the simple fact that one can choose which electron is excited it does not necessarily have to be the valence electron, and the inner electrons, being more strongly bound, require photons of higher energy to excite them. Since the valence electron extends furthest out from the atomic core, one is tempted to think that it is always the easiest electron to excite, both because it can more readily interact with an external field (higher transi-... 

The outermost d subshell spectra of Ga [348], In [349] and Pb[350] all have the characteristic that they straddle the double ionisation thresholds. As a result, many of the inner-shell transitions are quenched, while the probability of photo double-ionisation is enhanced. This situation has been discussed [351] in terms of diagrams such as those of fig. 7.8 in which the ionisation potentials for double-ionisation and single-ionisation from an inner-shell threshold are plotted as a function of atomic number. One looks for crossing points, where mixing between the two becomes particularly strong. 

It is a characteristic feature of inner-shell excitation in free atoms that the series limits are broadened by core-lifetime or Auger effects (see section 8.32), which truncate the series beyond the first few members, giving rise to intrinsically unresolvable upper members and a lowering of the ionisation threshold below the series limit. This purely atomic effect, which results in a quasicontinuous region below the onset of the true continuum associated with a deep inner shell excitation threshold, allows an internal density of states to be constructed, whose properties (illustrated in fig. 11.1) have been studied both theoretically and experimentally for free atoms [596]. 

Screening or shielding occurs when an outer electron Is Insulated from the pull of the positive nucleus by electrons in inner shells. The 1st Ionisation energy of sodium is for the removal of the 3s electron. Sodium has a nuclear charge of +11, but the 3s electron is shielded by the 10 electrons In the 1st and 2nd shells. The second electron that is removed comes from the 2nd shell (it is a 2p electron). This electron Is shielded from the nucleus only by electrons which are in a lower shell, which in this case is by the two Is electrons and so it is held much more strongly by the nucleus. It is also nearer to the nucleus, which makes it even harder to remove. 

The first electron removed has a low 1st ionisation energy, when compared with the rest of the data. It is very easily removed from the atom. It is therefore likely to be a long way from the nucleus and well shielded by inner electron shells. 

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