Ionization of core electrons

Appearance potential methods all depend on detecting the threshold of ionization of a shallow core level and the fine structure near the threshold they differ only in the way in which detection is performed. In all of these methods the primary electron energy is ramped upward from near zero to whatever is appropriate for the sample material, while the primary current to the sample is kept constant. As the incident energy is increased, it passes through successive thresholds for ionization of core levels of atoms in the surface. An ionized core level, as discussed earlier, can recombine by emission either of a characteristic X-ray photon or of an Auger electron. 

Scanning electron microsopy (SEM) and electron microprobe analysis (EMA) — A scanning focused electron beam causes the ionization of core levels similar to scanning AES. An alternative and thus a competing process to the ejection of Auger electrons is the emission of X-ray fluorescence radiation. These X-rays... 

Recent advances in the techniques of photoelectron spectroscopy (7) are making it possible to observe ionization from incompletely filled shells of valence elctrons, such as the 3d shell in compounds of first-transition-series elements (2—4) and the 4/ shell in lanthanides (5, 6). It is certain that the study of such ionisations will give much information of interest to chemists. Unfortunately, however, the interpretation of spectra from open-shell molecules is more difficult than for closed-shell species, since, even in the simple one-electron approach to photoelectron spectra, each orbital shell may give rise to several states on ionisation (7). This phenomenon has been particularly studied in the ionisation of core electrons, where for example a molecule (or complex ion in the solid state) with initial spin Si can generate two distinct states, with spin S2=Si — or Si + on ionisation from a non-degenerate core level (8). The analogous effect in valence-shell ionisation was seen by Wertheim et al. in the 4/ band of lanthanide tri-fluorides, LnF3 (9). More recent spectra of lanthanide elements and compounds (6, 9), show a partial resolution of different orbital states, in addition to spin-multiplicity effects. Different orbital states have also been resolved in gas-phase photoelectron spectra of transition-metal sandwich compounds, such as bis-(rr-cyclo-pentadienyl) complexes (3, 4). 

Table 12.6 gives the values of ionization energies for all the Period 3 elements. Note the large jump in energy in each case in going from the removal of valence electrons to the removal of core electrons. 

Ionizing source of radiation for the ejection of core electrons. UPS makes use of ultraviolet light and Is generally used to study the valence electrons of atoms and molecules. AES Is a secondary electron process trtiere electron emission occurs because of the coulomble rearrangement Induced by a core hole created by a photon or a high-energy electron beam. 

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