K-shell photoionization

Figure 4. Calculated partial photoionization cross sections for the K-shell of N2 over a broad energy range. The dashed line represents twice the K-shell photoionization cross section for atomic nitrogen, calculated using a Hartree-Slater potential.

Some partial photoionization cross sections, derived in this way for neon, are shown in Fig. 2.11 as a function of photon energy. The uppermost curve is the total absorption cross section. At the onset of the ionization thresholds for the ejection of Is, 2s and 2p electrons this quantity shows the corresponding absorption edges (see the discussion related to equ. (2.11)). The partition of the total cross section into partial contributions cr(i) clearly demonstrates that the dominant features are due to main photoionization processes described by the partial cross sections satellite transitions from multiple photoionization processes are also present. If these are related to a K-shell ionization process, they are called in Fig. 2.11 multiple KL where the symbol KLX indicates that one electron from the K-shell and X electrons from the L-shell have been released by the photon interaction. Similarly, multiple I/ stands for processes where X electrons from the L-shell are ejected. Furthermore, these two groups of multiple processes are classified with respect to ionization accompanied by excitation, (e, n), or double ionization, ( ,e). If one compares in Fig. 2.11 the magnitude of the partial cross sections for 2p, 2s and Is photoionization at 1253.6 eV photon energy (Mg Ka radiation) and takes into account the different... 

From K-jump ratio, one can obtain K-shell to total photoionization cross-section ratio. Again there is an empirical relation, given by Hubbel (1969) between K-shell to total photoionization cross-section ratio and atomic number of the element... 

Further, K-jump ratio and K-shell-to-total photoionization cross-section ratios are connected by the relation ... 

As a result of the photoionization, a singly ionized atom is formed, which can also be produced by electron impact. The core hole (eg, in the K shell) can be filled by an electron from a higher shell (eg, the Li shell) and the energy of this de-excitation process can be released by emission of an X-ray photon (X-ray fluorescence, XRF) or can be transferred to another electron (eg, in the L2 shell), which is then emitted with a well-defined kinetic energy (Auger process). This... 

In addition to photoionization (or photoelectric) cross sections for the K shell or the various L, M,. .. subshells, the literature provides useful information on X-ray production... 

The fine-structure constant a indicates that first-order perturbation theory has been applied the linear dependence on the photon energy Eph is due to the length form of the dipole operator used in equ. (2.1), and the wavenumber k compensates the 1 /k which appears if the absolute squared value of the continuum wavefunction is used (see equ. (7.29)). The summations over the magnetic quantum numbers M, of the photoion and ms of the photoelectron s spin are necessary because no observation is made with respect to these substates. Due to the closed-shell structure of the initial state with f — 0 and M = 0, the averaging over the magnetic quantum numbers M simply yields unity and is omitted. 

A variety of theoretical models have been developed in which relaxation is taken into account (transition state models, relaxed potential models, equivalent core models). A discussion of these models is far beyond the scope of this article. Here, we will only add some comments on methods by which it is possible to separate initial and final state effects with the use of experimentally available data. These methods are based on a combination of PE and Auger electron spectroscopy. We consider an Auger transition from an initial state with a single hole in the inner shell to a final state with two holes in another inner shell i. This Auger transition is combined with photoionization processes that correspond to the photoemission of an electron from orbital k and from orbital i. This yields... 

The Auger effect is characterized by an upper level valence electron relaxation into the vacant core-level state (after the initial photoionization), followed by an ejection of another electron in the valence level. In the nomenclature of the Auger effect, for example, for the KL1L23 transition in Fig. 2, the first shell corresponds to the core level in which the initial vacancy was created (K) either via photoemission from XPS or electron impact bombardment from an electron beam, the second... 

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