Ionizing fast electron

A connnon feature of all mass spectrometers is the need to generate ions. Over the years a variety of ion sources have been developed. The physical chemistry and chemical physics communities have generally worked on gaseous and/or relatively volatile samples and thus have relied extensively on the two traditional ionization methods, electron ionization (El) and photoionization (PI). Other ionization sources, developed principally for analytical work, have recently started to be used in physical chemistry research. These include fast-atom bombardment (FAB), matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ES). 

The hydrocarbon molecule RH is ionized under the action of fast electron (or 7-photon). The formed electrons are retarded due to collisions with molecules and become solvated electrons c7- Excited molecules of the hydrocarbon are produced due to recombination of solvated electrons c and positive ions RH+ [223,224],... 

Such highly ionized species have been detected for Cl-37 produced by the EC decay of Ar-37 in gaseous phase ((>). In solids, however, such anomalous states are not realized or their life time is much shorter than the half-life of the Mossbauer level (Fe-57 98 ns and Sn-119 17-8 ns) because of fast electron transfer, and usually species in ordinary valence states (2+, 3+ for Fe-57 and 2+, 4+ for Sn-119) are observed in emission Mossbauer spectra (7,8). The distribution of Fe-57 and Sn-119 between the two valence states depends on the physical and chemical environments of the decaying atom in a very complicated way, and detection of the counterparts of the redox reaction is generally very difficult. The recoil energy associated with the EC decays of Co-57 and Sb-119 is estimated to be insufficient to induce displacement of the atom in solids. 

The primary process, leading to the production of element specific compositional information is the ionization of an inner shell in any of the atoms of the sample by the fast electrons of the TEM. The primary process of ionization is studied by EELS. Since an atom, ionized in an inner shell is unstable, de-excitation takes place within 10 " s after the ionization. There are two alternative ways of de-excitation. 

In the past, PTRC screening was mainly based on gas chromatography-mass spectrometry (GC-MS) [116]. The choice of GC-MS was based on a number of good reasons (separation power of GC, selectivity of detection offered by MS, inherent simplicity of information contained in a mass spectrum, availability of a well established and standardized ionization technique, electron ionization, which allowed the construction of large databases of reference mass spectra, fast and reliable computer aided identification based on library search) that largely counterbalanced the pitfalls of GC separation, i.e., the need to isolate analytes from the aqueous substrate and to derivatize polar compounds [117]. 

The dominating method of ion formation in metabolic flux analysis is electron impact. It might be supplemented in the future by novel methods, such as matrix assisted laser desorption and electrospray. Additional techniques such as chemical ionization, fast atom bombardment or inductively coupled plasma ionization are only of minor importance and not further discussed in this context. 

ICRU Report 55 [36], and references therein. Fast electrons predominantly interact via the coulomb force with the bound electrons of the medium resulting in ionization, i.e., leading to formation of free electrons and residual positive ions. The residual ions can be left in any of a wide variety of final states ranging from their ground ionic state, to states resulting from simultaneous ionization and excitation, and/or dissociation of molecular constituents of the target material. The interaction of a primary electron gp with the diatomic molecule AB can precede via many channels, e.g., ionization can occur by any of the following reaction pathways... 

For collisions involving fast electrons, most of the relevant reactions given by Eqs. (9.1)-(9.13) occur with the primary and secondary electrons leaving the target molecule promptly, in about 10 sec. One the other hand, autoionization and dissociative ionization channels can result in a secondary electron being delayed relative to the primary, and in the case of resonant electron attachment, there may be a measurable delay in the exit of the primary electron. These processes are described in considerable detail by Mark et al. [19]. 

It should be emphasized that the data shown in Fig. 3 do not provide information on the fate of the excited or ionized molecule these data constitute a sum over final molecular states. Generally, measurements of excitation and ionization cross sections for fast electrons do not provide information on subsequent target relaxation modes. However, this information is often available from separate measurements that focus on state-selected partial cross sections and molecular fragmentation [19]. 

Kim and Rudd [39] have provided an excellent review of the theoretical techniques used to describe ionization of atoms and molecules by fast electrons and that have been... 

The relative success of the binary encounter and Bethe theories, and the relatively well established systematic trends observed in the measured differential cross sections for ionization by fast protons, has stimulated the development of models that can extend the range of data for use in various applications. It is clear that the low-energy portion of the secondary electron spectra are related to the optical oscillator strength and that the ejection of fast electrons can be predicted reasonable well by the binary encounter theory. The question is how to merge these two concepts to predict the full spectrum. 

Ionization of DNA s solvation shell produces water radical cations (H20 ) and fast electrons. The fate of the hole is dictated by two competing reactions hole transfer to DNA and formation of HO via proton transfer. If the ionized water is in direct contact with the DNA (F < 10), hole transfer dominates. If the ionized water is in the next layer out (9 < r < 22), HO formation dominates [67,89,90]. The thermalized excess electrons attach preferentially to bases, regardless of their origin. Thus the yield of one-electron reduced bases per DNA mass increases in lockstep with increasing F, up to an F of 20-25. This means that when F exceeds 9, there will be an imbalance between holes and electrons trapped on DNA, the balance of the holes being trapped as HO . At F = 17, an example where the water and DNA masses are about equal, the solvation shell doubles the number of electron adducts, increasing the DNA-centered holes by a bit over 50% [91-93]. 

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