Electron capture energetics

Fig. 7.11. Energetics of (a) resonance electron capture, (b) dissociative electron capture, and (c) ion-pair formation. Adapted from Ref. [75] by permission. John Wiley Sons, 1981.

Electron affinity, energy between the most stable state of an anion in an electronic state and the most stable state of the neutra molecule. The maximum in the absorption spectra of negative ions. Formation of an anion by the impact of energetic electron beams. Electron capture detector. 

When argon or other inert gases are used to decelerate the electrons, the efficiency of the overall electron capture process is much enhanced" " "". Under these conditions, chlorine-substituted phenols were found to undergo significant condensation reactions involving the molecular radical anions M , or the corresponding [M — H] ions, and the neutral precursor molecules. This reaction furnishes condensation products reminiscent of those formed from polychlorodibenzodioxins (PCDDs) under energetic neutral reaction conditions " . 

Temporary anion states may be broadly classified either as shape resonances or core-excited resonances (4). The former are well described by a configuration in which the impacting electron attaches to an atom or molecule in one of the originally unoccupied orbitals. In the latter, electron capture is accompanied by electronic excitation, giving rise to a temporary anion with a two-particle-one-hole (2p-lh) configuration. One can further distinguish core-excited resonances into those In which the resonance lies energetically below its parent state and those in which it lies above. The former are referred to as Feshbach resonances and the latter as core-excited... 

Besides the generators described above there are X-ray sources based on radioactive materials to provide the excitation of the sample. The advantage of using these materials is that an isotope can be selected to provide a mono-energetic beam of radiation that is optimized for the specific application. One method consists to select a radionuclide that is transformed by internal electron capture (lEC). This mode of decomposition corresponds to the transition of one level-K electron into the nucleus of the atom. For a nuclide X, the phenomenon is summarized as follows ... 

Electron-Capture Dissociation BCD is applicable to ESI-produced multiply charged peptide ions [101,102]. It is most conveniently implemented in an FT-ICR instrument. Multiply protonated peptides [M - - nH]"+, capture a low-energy (<0.2 eV) electron to produce an odd-electron ion [M - -nH] " +, which dissociates rapidly via an energetic H transfer to the backbone carbonyl group to form c and z sequence-specific ions ... 

As mentioned before, electron capture is the alternative process to positive beta decay. For lower decay energies, electron capture often comes out on top from the competition, due to its less demanding energetic condition (see Sect. 2.4.2 in Chap. 2). 

O Table 14.1 shows a nuclide chart corresponding to charged particle reactions with Mo. Both (d,n) and (d,2n) reactions are energetically allowed (have energy requirements, Q, that are met by the bombarding particle) and lead directly to Tc. For example, the Q values for Mo (d,n) Tc and Mo(d,2n) Tc are —3.25 and —4.74 MeV, respectively. The Q value for Mo (d,2n) Tc is —3.42 MeV. In addition, (d,p) reactions with Mo and °°Mo produce Mo isotopes that subsequently decay to Tc. Therefore, the bombardments produced many different radioactive products, but after a few months 61-day Tc and 90.1-day Tc dominate the activity. Note that both of these are isomeric levels. One, " Tc, decays primarily (96%) by electron capture (EC) and subsequent y-ray emission. The other, Tc, decays by a highly converted 96.6 keV transition, which presumably produced the slow electrons reported (Perrier and Segre 1937). 

Positive radical ions can be formed rather easily by bombarding neutral molecules with energetic electrons. The formation of negative radical ions, i.e., radical anions, is more difficult since radical anions have very low ionization potentials and easily lose their extra electrons to regenerate the parent molecules. Such a species can therefore be formed by electron capture, i.e., by the process... 

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